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Z80 Mark III (continued)

This is the Z80 Mark III Input/Output (I/O) board. At the top is the 30-pin edge-connector that interfaces to the terminal electronics. The 40-pin 8255 chip below it handles the signals on this connector.

At top left is a chip socket used as a connector for two serial ports. These were for a keyboard and a cassette tape storage unit. The ports are driven by two 28-pin 8251A Universal Asynchronous Receiver/Transmitter (UART) chips, their supporting COM5016 baud rate generator and 1488 and 1489 RS-232 drivers. The COM5016 uses the 5.0688 MHz crystal to the right of the white 24-pin socket. Only one 8251A is installed; the other goes in the empty socket above it.

The white empty 24-pin socket is provision for an 8254 counter timer. The empty 28-pin socket below it is provision for an 8259A interrupt controller. Both of these were wired but never installed.

The rest of the chips are buffer and support chips.