Zilog Z8

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Zilog Z8

While in an electronics surplus store in May 1995 I found some microcomputer boards that were going for $1 each. A sticker on the PAL chip of one of the boards suggested that these boards came out of printers of some sort. The CPU chip was a Zilog Z8 and I was particularly interested in the chip being stamped “ROMLESS”. Most surplus boards seemed to have CPUs with on-chip ROM so that the program in them couldn’t be changed. These Z8 boards, I surmised, had their programs on a chip external to the CPU - a chip which could be replaced. An empty socket beneath the CPU labelled 27128 gave me further confidence since the 27128 is a 16K x 8 EEPROM. I decided to buy one of the boards to take home for a closer look.

While I studied the board, I put out a call to see if anyone had any information on the Zilog Z8 such as the pinout, electrical characteristics and instruction set. I heard back from Don, VE3HUR, that he had a manual containing 13 pages of data on the Z8 and, when I asked him if he could let me photocopy them, he removed the pages from his manual and sent them to me. Thanks, Don! (Note: today, a search online for “Z8 User Manual” will find .pdf versions of the manual).

At the same time, I was researching the pinout and data on the other chips on the board and gradually gathering datasheets. I was also tracing the circuit diagram of the board. Because the diagram soon became complicated, it actually ended up being eight circuit diagrams, one page for each subsystem.

I became convinced that the board had definite prospects of being useful for a number of projects and so I went back to the shop and bought 19 more of them while they were still only $1 each. There were two versions: Rev D and Rev F. The Rev D board had additional chips that turned out to be drivers wired in parallel with the drivers already on the board so as to give enhanced output power. The Rev D board also usually has a three more LEDs installed. One Rev D board has a Dallas DS1220 battery-backed RAM but, since it’s soldered onto the board and the battery is long dead, it is not of much use. Below is a photo of the two versions of the board. Click on the photo to see a larger version.

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After completing the circuit diagrams, I could see what the boards offered:

  1. •an on-board power-on LED and a socket to lead to an off-board power-on LED.

  2. •an on-board buzzer with provision for a separate off-board buzzer.

  3. •a spring-return reset switch designed to interrupt an off-board high-voltage power supply but which is easily reconfigured as a standard reset switch.

  4. •two 8-bit dip switches on the back of the board that can be read in to set any configuration data.

  5. •a UART providing differential line drivers to a 9-pin molex plug with provision for an on-board DB9 socket.

  6. •two 7-segment LEDs under program control.

  7. •from two to five LEDs under program control.

  8. •a hardware watchdog timer.

  9. •several programmable timers.

  10. •14 LED or relay driver lines.

  11. •20 stepper motor control lines.

  12. •DC motor control

  13. •8KB of RAM