Of all the old computer things I have, the SDK-85 is one of the most fascinating. This cool device came to me from a college course I took and was used as part of the course work as a system on which to learn simple machine language and computer interfacing. When I did the course, the SDK-85 was already well-obsolete but it was still in use by my institution and I think it honestly was a good idea because of its bare-bones approach and true bare-metal aspect. You had the opportunity to get straight to the hardware and learn about these things without much of an obstruction in your way. It was abstract, obscure, and even a little daunting at first, but massively rewarding and felt very nostalgic.

My particular SDK-85 is manufactured by URDA: University Research and Development Associates and is essentially a direct copy of the Intel SDK-85 with some subtle differences. The most important, and unfortunate, difference is that the Intel variant has a real keyboard with real keyswitches and keys. The URDA unit makes due with a cheap rubber membrane keypad that happens to be very prone to bounce and poor contact. The keypad requires regular cleaning and maintenance in order to keep it working. I lived with the keypad for a number of years but eventually migrated to a 110 baud RS232 terminal on which I can actually see more than one byte of data at a time! (I’ll write about the terminal I use later on.)

Basic specifications:

• CPU: 8085A
• Instruction Cycle: 1.3µs
• Tcy: 330ns
• ROM: 2kB
• RAM: 256B
• Parallel I/O: 38 lines
• Serial I/O: Tx/Rx via SID/SOD of CPU. 110 baud software control.
• Serial Interface: Factory 20mA current loop. (I’ve changed it to true RS232).
• Interrupts: RST7.5 - Keyboard, RST6.5, INTR
• DMA: Jumper Selectable Hold Request
• Software: Built in ROM monitor with basic accessory routines.

Some pretty pictures of the beast and its accessories.

How it looks on first boot.

This is my coding environment.

This book gets used more than its fair share as an 8085 coding reference.

And of course one must attach peripherals and make them talk!

A look at the architectural layout as a system.