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