Foreword

This article is tutorial explaining how each part of the ZX97LITE works.

I designed the ZX97 as a discrete copy of the ZX81, which was the first PC for millions of people. In the process ZX97 turned into much more than a mere copy. While it is backward compatible with the ZX81, it is in all other respects a significant improvement over the ZX81.

In 1998, Rodney Knaap contacted me to offer to make a PCB of the ZX97 but this proved no small feat. The end result required 3 PCBs connected together with flat cables. In order to simplify the PCB for others to build I designed the ZX97Lite, a trimmed version of the ZX97 omitting the big RAMdIsk and the 8255 parallel port. For a full background on this story and details of Rodney’s ZX projects , check out Rodney Knaap's Website where you will also find my original ZX97 article and schematic.

Rodney has designed a single sided PCB for the ZX97L and also build and tested a prototype using this circuit. I have been unable to contact Rodney for a long time but I would urge you to try and contact Rodney for the latest CAD version of the ZX97Lite PCB. If you cannot get in touch with him, here are the beta version of the ZX97Lite single sided PCB (ZX97LPCB - for 600pdi printer), the ZX97Lite parts and jumper layout (ZX97Lite Layout ) and my ZX97 EPROM file (ZX97L 27C256 EPROM binary image)

While Rodney’s ZX97Lite schematic was a great improvement over my original ZX97 ASCII drawing, it is somewhat difficult to read because of the foreground and background colors, the dense layout, the bused signal lines, the IEC logic symbols, crowded pin numbers and in addition, it contains some minor errors and omissions

In order to make the ZX97L easier to understand, I have redrawn the schematic by replacing the IEC logic with ANSI symbols, reformatted the drawing to black on white , renumbered pins and completed some missing bits. The overall drawing was then “disassembled” into byte sized smaller functional subsystems.

The eight ZX97L subsystems are the following:

1) CPU and NOP Circuit

2) RAM and EPROM Memory

3) Memory Decoding

4) CHR$ Decoding

5) Time Base Generator

6) Sync Generator

7) Video Shift Register

8) I/O Ports and External Connector

I like to thank Rodney Knaap for working the original ASCII schematics of the ZX97LITE into a complete project together with graphical schematics and an elegant single sided PCB layout way back when in 1998.

To Jack Raats for encouraging the work on the original ZX97 and to the rest of all my friends in ZX land:

My warmest regards and enjoy!

Wilf

THE ZX97LITE SCHEMATIC DISASSEMBLED Rev. 2.1 - By Wilf Rigter 12 / 2003

The ZX97LITE is a simplified ZX97 designed to fit on one single sided PCB. The ZX97L is a complete ZX81 using discrete logic chips with 32K of battery backup RAM and 32K of EPROM. User selectable memory mapping allows the ZX97L to boot directly from user M/C code in RAM. The circuit draws just 50mA so can easily be run from a battery or even solar cells. The ZX97L OS supports a bank switched RAM / EPROM disk with a hot key. The ZX97LITE demonstrates this feature with bundled utility machine code programs stored in one 16K bank of EPROM DISK. The expanded RAM / EPROM DISK circuit and the 8255 parallel port of the ZX97 were omitted in order to keep the PCB layout simple. The ZX97L is virtually identical to the rest of the ZX97 and so this description therefore also applies that original circuit.

1) CPU AND NOP CIRCUIT

The CPU is the subsystem that rules them all and uses a Z80 inside. I won't go further into the details of the all Z80 pin functions and CPU operations as most are used in a conventional manner and information on this is readily available on the web. But some of the CPU control lines are used in unconventional ways related to the ZX81 video system and these will be described in detail:

1) The 3.25MHz CPU clock line is derived from the 6.5MHz video clock after dividing by 2. This is required since the CPU must load each video pattern byte into the 8-bit Video Shift Register during a NOP cycle, which takes 4 CPU clock cycles while the 8 pixels are clocked out of the VSR with 8 video clock pulses.

2) Like the ZX81, the INT input is connected to A6. The Z80 decrements the refresh address counter and the INT input is sampled during RFSH time for an active low on A6 to interrupt the HALT condition. That HALT state occurs at the end of each normal SINCLAIR video line.

3) The NMI line and NMI service routine at ROM address 0066 is used in SLOW mode to interrupt user program execution during blank video lines to switch to video display or VSYNC execution when required. The NMI generator is controlled with OUT FE = NMI ON and OUT FD= NMI OFF to gate the HSYNC signal from the Time Base Generator to the CPU NMI input.

4) The WAIT line is used to finely synchronize the Z80 with NMI timing twice during each video frame with the NMI–CONT service routine at ROM address 02A9. The HALT and NMI lines decoded with an OR gate and inverter just like the ZX81 used a transistor and some passive parts to do this.

5) When active during the live video mode, the NOP generator (U9) forces a NOP (00) code on the CPU D0-D7 data lines while the CHR$ code is loaded into the CHR$ decoder circuit. During RFSH, the A0’-A9’ lines address the video pattern byte in EPROM or RAM to be loaded into the video shift register

6) During forced NOP, the eight resistors in RN2 isolate the Z80 D0-D7 data lines from the D0'-D7' data lines used in the rest of the ZX97L system. At all other times D0'-D7' transfer data between the Z80, memory and I/O ports.

7) The ten resistors in RN1 isolate the Z80 A0-A9 address lines from the A0'-A9' address lines used in the rest of the ZX97L system. The A0'-A9' address lines are normally used by the Z80 to address memory and I/O. During live video RFSH time, the CHR$ decoder circuit supplies the address to the A0'-A9' lines for a video pattern byte in RAM or EPROM.

a) A0'-A2' = video character line counter (character height = 8 lines)

b) A3'-A8' = CHR$ code

c) A9' = invert video or select second character set

9) The RESET line, connected to R9/C4, initializes the CPU on power up. A voltage supervisor discussed in part 8 is also connected to this line and J12 is used for connecting an external reset pushbutton.

2) RAM AND EPROM MEMORY

The original unexpanded ZX81 had 1K of static RAM and an 8K ROM for the ZX81 BASIC operating system. The RAM contains the system variables, BASIC and M/C programs, BASIC variables and the Z80 stack.

One interesting side effect of using a 1K ZX81 is that the RAM can barely contain the 24 x32 character video display file (DFILE) before running out of memory for the other RAM contents. To deal with that 1K limitation, the ZX81 DFILE is compressed to just 24 bytes of carriage returns or New Line (N/L) characters at system initialization and after a CLS. As characters are displayed on the screen each video line is expanded as required. Up to 32 displayable characters can precede the N/L character.

As an indication of how tightly the CPU is coupled to display of the ZX81 video consider the following:

The N/L CHR$(118) at the end of each of the 24 DFILE character lines is actually a HALT opcode. In effect, displaying a N/L character caused the Z80 to HALT. The Z80 INT line connected to A6 (a mentioned in part 1) wakes up the halted CPU at the end of each horizontal display line before updating the parameters for the next horizontal video line.

The ZX97L uses a low power CMOS 32Kbyte Static RAM chip with battery backup power for protecting the RAM contents during power down. The 32K EPROM contains the ZX81 OS modified to include several new BASIC keywords for direct access to the included RAMDOS directory program and the included machine code assembler and disassembler programs located in the upper 16K bank of EPROM memory. This upper 16K area is the area where the ZX97L can be expanded with a bank switch, which will provide up to 32 pages of 16K each (512K total) like the original ZX97.

The memory subsystem shows the simple battery backup system for the CMOS RAM using a 3V lithium coin battery and some diodes. The common Vcc connection instead of common GND has the advantage that all lines connected to the RAM are automatically pulled up to Vcc (rather than GND) during power down. There are two resistors elsewhere in the circuit that pull up the tristate RAMCS and ROMCS lines. An additional 10K pull up resistor on the RAM /WR line as shown in the schematic is highly recommended to protect the RAM contents during power down.

Many Z80 systems use RD connected to the memory chip OE lines. In the ZX81 OE and CE are connected together so that memory contents can be read during RD and RFSH time when the video pattern are fetched.

After this little warm up exercise with the simple CPU and memory circuits, we examine how the ZX97L uses a memory mapping circuit to decode the 32K of RAM and 32K of EPROM memory into various 8K blocks in the 64K memory space.

3) MEMORY DECODER

While reverse engineering the ZX81 ULA, I used the ZX80 circuit as a guide since the ZX81 ULA chip circuit operation and the ZX81 ROM were backward compatible with the older ZX80 and so had to be designed similarly. The main difference in the circuits was the ZX81 ability for SLOW mode video operation. My objective for the ZX97L was to create a discrete TTL ZX81 with increased functionality that used fewer parts than the ZX80 and was a transparent circuit design so that its operation could be understood by anyone with some experience in TTL logic circuits.

The ZX80/81 had a primitive memory decoding circuit, which used MREQ together with /A4 and A14 to generate the ROMCS and RAMCS lines respectively. As a result, copies of the unexpanded ZX81 RAM and ROM memory images were echoed at intervals throughout the 64K memory space. This was corrected in many memory add on devices which included more complete decoding of the memory by overriding the ZX81 RAMCS / ROMCS signal lines. The ZX97L memory mapping circuit may generate the same RAMCS and ROMCS signals but in every other respect this decoder is anything but primitive.

The decoder circuit uses a 74HC251 MUX (U14) to generate the EPROM and RAM chip select (CS) signals. These 32K memory chips are decoded into a patchwork of 8K memory blocks mapped in the Z80 64K memory space.

The ZX97L memory is mapped out as follows:

0-8K = EPROM or RAM - the ZX97L BASIC firmware in EPROM or the BASIC

firmware with enhancements in write protected RAM.

8-16K = RAM - machine code programs, second video dot pattern lookup table, etc.

16-32K = RAM - ZX81 BASIC program memory just like an expanded ZX81 with 16K RAM

32-40K = RAM or EPROM - 0- 8K Shadow RAM or ZX97L OS in EPROM.

40-48K = EPROM - machine code programs, Hires files, etc.

48-64K = EPROM or RAM - 16K EPROMDISK and DFILE echo in RAM (no machine code program execution allowed) or future 512K RAMDISK.

The 74HC251 MUX (U14) uses the A13-A15 address lines together with an active low enable from the U13.B output to select one of the D0-D7 data inputs. When the inputs of U13.B (/RFSH or /MREQ) are both high, the Y and W outputs of U14 are tri-stated and R5 and R6 pull the two memory select lines to Vcc.

If either input pin of U13.B (RFSH or MREQ) is low, the MUX is enabled and if the selected data input is low then the RAMCS is active low or if the selected data input is HIGH then the ROMCS is active low. When the MUX is enabled by U13.B, the 64K memory map is translated as follows:

1) To take a simple case first with input D1 selected and connected to GND, the 8-16K-memory block is enabled in RAM. This area is traditionally used to run machine code programs like the ZXA assembler. Several utility MC programs are bundled in the 16K EPROM disk. The RAMDOS directory program, using the new keyword DIR that can be used in BASIC programs, lists, loads and runs these MC program files in the 8-16K RAM area.

2) With the D2 and D3 inputs selected and connected to GND, RAMCS is generated which enables 16K of RAM from 16K-32K for the ZX81 BASIC system, DFILE and user BASIC programs. This 16K block of RAM is just like a ZX81with a 16K RAM pack attached

3) With input D5 selected and connected to Vcc, selecting the memory block between 40K- 48K always generates ROMCS to map EPROM in that area which is used to store and run additional firmware or video pattern tables.

4) The D6 and D7 inputs are connected to /M1 which makes it a little more complex case:

The memory mapper circuit uses /M1, which is active low when machine code is executed, to differentiate between video and EPROM memory access. The ZX81 reserves the 48-64K area for the execution of ZX81 video display file (DFILE) or rather an echo of the DFILE 16-32K area. When /M1 is active, D6-7 inputs are low, RAM CS is active and an echo of RAM in 16-32K is enabled. Since MC program execution is reserved to the processing the ZX video display file echoed in this 16K memory space, direct access to the 48-64K area is restricted to storing data files and no machine code can be executed. For the unexpanded ZX97L, when /RD or /WR are active, the D6-7 inputs are high and the ROMCS output is active to enable the upper 16K of the system 27C256 EPROM. or (future) bank switched pages. In a sense, the allocation of the 48-64K memory as either RAM or EPROM is changed on the fly by the /M1 signal.

5) The D0 input logic is the most complex case. The first 8K block is used like the classic ZX81 ROM with the ZX81 BASIC operating system but can be swapped with user enhanced OS in Shadow RAM. This is great for OS development when testing new code. For example, new video patterns can be added or fast communication routines can replace the ZX tape routines. The decoder MUX D0 input is controlled by J11, which is also connected directly to D4. When D4 is high ROMCS is selected and when D4 is low RAMCS is selected. With J11 closed, the inverter U4.E output and U23.C OR gate output, connected to D0, is high and the 0-8K EPROM block, with the ZX81 OS, is enabled in 0-8K of the memory map . With J11 open, the EPROM ZX OS is swapped with an 8K RAM page which normally resides in the 32-40K area but which can be loaded with new or modified operating system firmware.

The U13.A AND gate is used to access the 0-8K in RAM only if /RD or /RFSH

are low to write protect RAM in the 0-8K area. This is an important consideration since several ZX81 OS operations routinely write to memory in the 0-8K area (let alone a runaway program randomly overwriting the OS in RAM)

After this relatively complex subsystem, we are ready to test our ZX-IQ on the circuit that converts the ZX81 alphanumeric and graphic CHR$ codes into video patterns to be displayed on the monitor screen with some small but significant improvements over Sir Clive's design.

4) CHR$ DECODER

This subsystem is closely tied to the ZX81 video design, which has mystified more than a few experienced ZX81 fans. By following along with the description of operation of this circuit, we learn much about the clever ZX81 video system and discover that Sir Clive missed some powerful video options for the ZX81 by just a few extra gates.

The ZX81 alphanumeric and graphic characters in the video display file (DFILE) consist of up to 768 (24x32) bytes of CHR$ codes. These are converted into rasterized patterns of video pixels and displayed on the monitor screen as 32 blocks of 8 x 8 light or dark pixels. For each of the 64 displayable ZX81 character codes, a rasterized graphic shape is stored in 8 contiguous bytes in a video dot pattern lookup table in the last 512 bytes of the 0-8K ZX81 ROM.

The CHR$ decoder operates in two steps:

1) The Z80 reads each character in DFILE and together with the CHR$ lookup circuit generates the address of a dot pattern byte

2) The dot pattern byte is accessed during RFSH and loaded into the video shift register to be shifted out serially at 6.5M pixels per second.

The DFILE echo above 32K is literally "executed" during the video display. Each CHR$ in the DFILE is fetched by the Z80 as an OPCODE , but executed as a NOP instruction during 4 CPU clock cycles. The CHR$ decoder circuit uses 6 bits of each fetched CHR$ code together with 3 bits of a the horizontal line counter and with part of the Z80 RFSH address to form the address to a byte in the dot pattern lookup table.

The three bits of line counter U11.A, which is clocked by each HSYNC pulse, are used for A0'-A2'of the pattern byte address. The counter increments 8 times as each row of DFILE characters is read 8 times in succession. This generates one row of video characters on the screen, with each adjacent character made up of 8 bytes of video dot pattern data in a block of pixels 8 high by 8 wide. This is repeated for each of the 24 character rows. The total number of controlled dots (screen resolution) for a ZX81 screen is 256x192. The lower 6 bits of each CHR$ code in DFILE is clocked into U12 at the end of the /RD signal and makes up A3'- A8' of the video pattern look up address. The CHR$ D6-7 bits are used, in the VIDEO circuit, discussed later in PART 6, to enable video and to invert the video pattern for that CHR$, generating a reverse foreground - background character on the screen.

These 9 least significant address bits (LSB) select one of the 512 video dot pattern bytes. are stored and tristate isolated with U12 - (octal register) and U10.A (tristate buffer) until the RFSH and A14 are both low which then enables 9 LSB on the A0'-A8' address lines. The most significant 7 address bits of the dot pattern byte address are supplied by contents of the Z80 Interrupt Register (IREG), which is gated onto the A8-A15 lines during RFSH time. The 7 bits in the I register are therefore used to point to the start of the lookup table while 9 least significant address bits (LSB) select one of the 512 video dot pattern bytes in that lookup table.

So far this circuit functions similar to the ZX81 ULA circuit but with one significant difference:

1) In the ZX81, the pattern lookup table must be in the internal ROM because the ULA CHR$ decoder A0'- A8' outputs lines are physically connected to the only to the ROM address lines.

2) In the ZX97L CHR$ decoder both the RAM and ROM can be accessed and either can be used for the pattern lookup tables as long as they are located in 0-16K or 32-48K (ie A14 low). This was done to permit the display of true hires video which requires that CHR$ decoding is disabled and means that hires display files must be located in 16-32K or 48-64K. Given that limitation, you can locate one or more user defined 512 byte pattern lookup tables in RAM for example in the 8-16K or 32- 40K area. The first byte of the lookup table must be located on a 512-byte boundary (ie locate the table between 8192 - 8703).

All that is needed to access the new lookup table is to load the IREG with the high order 7 bits of the table starting address. Note that bit D0 of IREG (gated to the Z80 A8 line but not to the A8' line) is normally 0 but is otherwise ignored in the ZX81 CHR$ video mode.

A second enhancement provided by the ZX97L CHR$ decoder circuit gives you a choice of using 64 Characters and their reverse in a 512 byte pattern table or 128 unique characters in a 1024 byte pattern table. In the normal ZX81 CHR$ video mode there are 64 CHR$ codes with unique patterns stored in a 512 byte pattern table and uses CHR$ bit D7 to invert video for that CHR$, generating the same pattern but with reversed foreground / background colors on the screen. The details of the shift register and the parallel loading pulses are described in the ZX97L Video Circuit.

In the ZX97L enhanced CHR$ video mode D7 of the CHR$ code is combined with D0 of IREG to address a second pattern table thereby doubling the number of unique displayable characters to 128 instead the normal reversed character set.

For example, both upper and lowercase alphabet characters as a well as new symbols or new graphic game characters can be displayed. This second table of 64 character video patterns can be located for example in 8192-8703 RAM, contiguous with the existing ZX81 video pattern table in 7680-8191 EPROM.

To enable the 128 CHR$ mode, set bit D0 of the IREG to a logic 1. Now instead of simply reversing the displayed character whenever D7 of the CHR$ code is high, a second unique pattern (ie lowercase) for that CHR$ is displayed. Bit D0 of IREG and D7 of the CHR$ generate A8 and INVERT at the inputs of AND gate U25.A at RFSH time. If INVERT and A8 are HIGH, the U25.A output A9' is high and accesses the second pattern table.

Note that dot patterns in the second pattern table are displayed in reverse video but that is simple to correct by loading the second table with reversed (instead of normal) dot pattern bytes.

The Time-base Generator is next. (It's a much simpler circuit to give us all a bit of a break).

5) TIME BASE GENERATOR

Compared to the convoluted "CHR$ to video" circuit, the ZX97L time base circuit has a certain clarity. The circuit consists of a 6.5MHz CMOS crystal oscillator, which is the clock for the video shift register. The 6.5M signal is inverted with U4.B. to generate the /6.5MHz signal. A divide-by-2 D flip-flop (U5.A) generates the 3.25MHz CPU clock. A divide-by-207 (U6+U7) circuit generates the 15,750kHz video horizontal sync pulses.

The HSYNC pulses occur between each horizontal video line and are used by the TV/monitor to synchronize the video dot stream with the start of the electron beam scanning the next horizontal line across the phosphor screen of the TV/monitor.

The HSYNC divider consists of 8 binary counter stages (U6). Three 3 input AND gates decode the binary count of 11001111 (208 decimal) to reset the counters to zero. The last two divider stages are connected to AND gate (U7.C) which generates the actual HSYNC pulse. The HSYNC pulse width is 15/3.25= 4.6 usec and horizontal line duration is 207/3.25=63.7 usec.

There are 192 lines used to display the 24 rows of video characters. The (50/60Hz) vertical sync frequency is selected depending on a jumper setting (J4), which is described later. With a 60Hz Sync there are 262 lines total generated per vertical field and with 50Hz there are 313 lines per field. Since 192 lines are used for video display, the remaining lines are blank lines during which slow mode program execution takes place. As a result, the 50Hz slow mode user program execution speed is almost 2 times faster than the 60Hz setting.

During slow mode program execution, the HSYNC signal is gated to the Z80 NMI input and interrupts the user program every 64 usec. The NMI service routine counts remaining blank lines on the bottom of the screen until VSYNC must be generated and then counts the blank lines at the top of the screen until the live video display must be generated. The HSYNC “divide by 207” circuit is synchronized with the VSYNC pulses by holding the counter reset via U23.A when the VSYNC output is active high.

By now you may wonder how useful this rather specialized information is? Well the easiest way to a better understanding of the ZX80 and ZX81 (which I'm sure everyone here has an interest in) is through the ZX97L, which in part was designed for just that purpose. The ZX97L circuit was also used as a model of the ZX81 ULA as discussed in the extended correspondence I had with Carlo Delhez during the development of his very fine XT2 emulator.

6) VSYNC / NMI CIRCUIT

This part of the ZX97L will look familiar if you have ever looked at a ZX80 schematic. I tried to simplify it but could not without losing the transparency of logic, which makes this subsystem almost self-explanatory.

The circuit consists on an I/O address decoder and two R-S latches (U18). The decoder uses U8 and U17 to decode the following I/O addresses defined in the ZX81 operating system:

IN FE - Read keyboard port and Start Vertical Sync

OUT nn - End Vertical Sync (any I/O write - no specific address)

OUT FE - Turn NMI pulses on

OUT FD - Turn NMI pulses off

In addition, IN FE reads the keyboard anytime but to start Sync, the NMI pulses must be off (ie in the FAST mode).

The 256 I/O addresses are not fully decoded as the circuit uses the same linear decoding as the ZX81 with each address bit reserved to a RD or WR I/O address. That means that the hardware will respond to any I/O address wit

A0 or A1 low but addresses FE and FD are normally used for the sake of consistency with existing ZX81 software.

Throughout the remaining description the terms set and reset refer to forcing the state of the S-R latch outputs, which are used to generate various output signal.

IN FE

An active low on IORQ, RD and A0 generates the keyboard enable signal, which will be discussed later together with the ZX97L keyboard port. Reading I/O address FE also sets the Vertical Sync pulse but only if the NMI latch is off. That way a M/C program can read the keyboard port directly without disturbing the video if the ZX97L is in the SLOW mode.

Also note that the ZX81 uses the VSYNC latch to modulate the ULA video output pin with the tape output (mic) signal. That is what causes the familiar striped signal on the screen during tape saving and loading and also saved a precious I/O pin on the ULA. However this mixed use of the video pin also means the HSYNC signal is mixed in with the tape signal making that port difficult to use for direct serial communication.

The ZX97L has no such constraints on I/O pins, so the SEROUT output is separate from the video output, not affected by the HSYNC signal and can be reliably used as an asynchronous serial transmitter (“bit banger”) output.

OUT nn

An active low on IORQ and WR at any address (usually OUT FF) resets the Vertical Sync pulse. This may seem primitive but is required to conform with various ZX81 video drivers (normal and hires) some of which use the IORQ WR generated during the INT acknowledge cycle to reset the sync pulse.

As described in IN FE , this signal is also used to generate the tape or serial output pulses.

OUT FE

An active low on IORQ, WR and A1 sets the NMI latch. The NMI latch controls U17.D which gates HSYNC pulses to the Z80 NMI input. When the NMI latch is turned on, the ZX97L is in the slow mode and program execution only occurs when the video is not being serviced. The NMI pulses interrupt the user program execution every 64 usec, CALL the NMI service routine to decrement a blank line counter and either returns to the user program or executes the video routines VSYNC and DISPLAY) if required.

OUT FD

An active low on IORQ, WR and A1 resets the NMI latch and blocks the NMI pulses. This occurs when the ZX97L fast mode is desired, for VSYNC, for tape

load and save (serial i/o) and during execution of the video routines. CSYNC

The horizontal and vertical sync signals are combined in the XOR gate U15.B Keep in mind that the active VSYNC signal disables the HSYNC counter and therefore the output of U15.B generates an active low OR composite sync (CSYNC) signal, which is combined with the video dot pattern.

7) VIDEO SHIFT REGISTER

This part of the ZX97L was probably the most difficult to design in terms of generating the precise timing to get a stable video image on the screen. Many parts of the ZX97L hardware, CPU and software must smoothly work together to generate the video display. This part combines that effort in the composite video output to the TV/Monitor.

DFILE ECHO DECODER

The basis of the ZX81 video display system is the execution of an echo of the DFILE characters during which the Z80 is forced to execute NOP instructions.

This video circuit uses a special DFILE echo address decoder (U19), which provides an output (NOP) when the displayable characters of DFILE are executed in the upper 16K of memory. The 74HC138 octal decoder (U19) inputs C, B and A are connected to the A14, A15 address lines as well as CHR$ data bit D7 that identifies an inverted dot pattern. The M1, CHR$ data bit D6 and HALT signals are connected to the chip enable lines.

As a result, the decoder generates two valid active low output states: Y6 or Y7, which represent non-inverted and inverted video data. When low, the Z80 ALT signal disables the decoder as the last character (N/L) of a horizontal line is executed and the CPU enters the HALT state executing NOPs until the INT line goes active low and the HALT state is exited.

The VSR is loaded asynchronously through the parallel inputs with an active low load pulse generated from the NOP signal delayed by U20.B and U21.B. The load pulse is clocked out on /Q of U21.B at the end of the last T state of the RFSH cycle, which is decoded with U13.C. The load pulse duration is just a few tens of nanoseconds of propagation delay thanks to the connection between /Q of U21.B and the U20.B preset input. The VSR load pulses, the VSR data and the invert signal are resynchronized with the (inverted) /6.5M video clock using U5.B to ensure a crisp and seamless appearance of the dot patterns on the screen. The ZX80 used some RC components in a kludge circuit to accomplish the same load pulse shaping. These are generally unstable and the video quality may deteriorate with time and temperature.

THE INVERT CIRCUIT

The Y7 output (decoded D7 bit of the CHR$ code) is delayed and latched (U20.A and U21.A) when the VSR is loaded and if high, XOR gate U15.C inverts the serial dot pattern as it is clocked out of the VSR. This INVERT signal is also used in conjunction with the Z80 I register to select the second character pattern lookup table.

BLACK ON WHITE ON BLACK

The foreground and background colors (B/W) of the display and the borders around the display are user selected with J8 and J9.

VIDEO OUTPUT

The video signal is combined with the composite sync signal using resistors R15-16 and D11. The summed signals are buffered by T1 and the VID output level is adjusted with P1. The video signal has three distinct output levels, from most positive to GND: white, black and sync

HINDSIGHT

The design of the load pulse circuit came before the addition of U15.C, which reduces the stringent timing by re-synchronizing the combined VSR data with the inverted video clock. Some simplification could be made to reduce the ZX97L chip count by one.

For a more intimate look at the ZX97L and ZX81 video timing details also read the ZX81 video tutorial.

8) I/O, POWER SUPPLY AND EXPANSION CONNECTOR

This is the last and simplest part of our ZX97L circuit disassembly and potentially the most interesting because it interfaces with signals from the outside world.

EXPANSION CONNECTOR

Rodney's schematic shows two 25-pin connectors (J1A,J1B), which are the equivalent of the 46-pin edge connector of the ZX81. These pin connectors are lined up side-by-side in dual row header, compatible with a 50 pin insulation displacement connectors on a flat cable that can be terminated with a 0.1 inch edge connector to connect to ZX81 peripherals. The order of the signals seems to be mixed up in the schematic but looks ok in the component layout but this needs to be confirmed.

5V VOLTAGE REGULATOR

The 7805 is a standard 5V/1AMP linear voltage regulator for the Vcc power supply. A better choice is a LM2930 - 5V/150mA low dropout voltage regulator allowing the use of a 6V battery instead of an 8V minimum unregulated input supply voltage. The LM2930 regulated ZX97L circuit uses 50mA, which would leave 5V/100mA for peripherals.

UNDERVOLTAGE RESET SUPERVISOR

The MC34164 is a simple way to protect the memory contents from power-up and power-down transients. The supervisory chip comes in a TO92 transistor type package. The chip output pin holds the reset line low until the voltage at the input (Vcc) is greater than 4.5V.

INPUT PORT

The ZX97L has an 8 bit input port mapped at I/O address FE of which 5 bits (D0-D4) are used for the keyboard column data, one bit (D6) for 50/60Hz select and 1 bit (D7)

for tape or serial input. The 8th bit (D5) is not used.

KEYBOARD

The keyboard is made up of 8 rows of 5 keys. Each row of keys has a common connection for the 5 pushbutton switch contacts. That common of each row is connected to J3 and is scanned by an address line (A8-A15) from the Z80 isolated with a diode (D3-D10). The other side of each row of five switch contacts is connected to J2, with the 5 keyboard columns on U16 - D0-D4. These keyboard column inputs are pulled up with 10K resistors in RP3 (not shown in Rodney’s schematic). By reading the keyboard (IN FE) with one of the 8 address lines low, the keys for that row are read in. Repeated times 8 and all keys of the keyboard are read in by the OS software.

50Hz/60Hz

This input D6 is read by the OS and the number of blank lines above and below the live video display is determined based on the jumper setting.

SERIAL IN / SERIAL OUT

Bit D7 of U19 is used here for SERIN, a logic level equivalent of the ZX81 ape input, which can be driven, for example, by a PC parallel port or another ZX97L. The ZXTAPE software is a simple PC program can be used to send ZX81 files from the PC to the ZX97L using the same protocol as the ZX81 tape LOAD signal at a rate of about 100 bits per second in the FAST MODE.

Similarly the SEROUT line is left as a logic level signal compatible with the SERIN line and two ZX97Ls can connected back to back to directly exchange information using the BASIC LOAD and SAVE commands. This arrangement is a change from the ZX81 and from Rodney's ZX97L PCB layout, which have tape recorder compatible input and output circuits. The logic level version uses fewer parts and has some changed component value. ZX purists may wish to install the additional components to make these lines tape compatible.

CONCLUSION

The ZX97L is a minimal but highly functional BASIC computer with hardware and software that is compatible with the ZX81. The simplicity of the design makes the logic easy to understand and useful as an educational aid while the low power consumption and video output makes it practical for some portable applications.

Next some ZX97L additions - a 512K byte memory expansion and an 8255 for

printing, high-speed communication with a PC and controlling the world.

The complete ZX97 schematic Redrawn By Wilf Rigter 12 / 2003