SBC-85 AN EXPANDABLE, OPEN SOURCE, 8085 SINGLE BOARD COMPUTER ECOSYSTEM
Decades ago I wirewrapped my first 8085 3-chip system with an 8085, 8155, 8755 and I was hooked forever. Sure there are better processors, but arguably none held onto their (mostly industrial) market share for so long. I decided to rekindle my fondness for the 8085 by updating and creating an 8085 single board computer with an eye towards expansion capabilities. The end result is the SBC-85 System which includes a card edge connector and backplane to create a 70s-80s style system akin to the MIL-MOD8 or STD bus.
The 8085 SBC has an RS232 serial port (hanging on the SID - SOD), 22 pins of I/O, timer (both courtesy of the Intel 8155), 8kB RAM, and two 2732 EPROMs one on the board and one in a ZIF.
The SBC-85 CPU board is a 100mm x 100mm 2-layer with a 120 pin edge connector (mechanically PCI). The serial SID and SOD lines, via a max232, offer a RS232 serial port for communication with a resident monitor or whatever you may decide to put in firmware. The 120 expansion bus includes power, all of the 8085 status and system signals, latched address bus, and many more unused terminals for future or other board-to-board signals.
RAM is provided by a 64-kBit 6264 and an additional 2kbit in the 8155. In addition to the additional RAM, the 8155 provides an onboard timer and 22 pins of I/O. Finally, two 2732s provide onboard EPROM, one socketed on the board and the second in a ZIF for easy swap.
Power is provided by an external regulated 5V power supply through either the barrel connector or the backplane.
SBC-85 CPU DESCRIPTION
The SBC-85 is an 8085 based Single Board Computer. As such, it contains all elements required for stand-alone operation including the processor, Random Access Memory (RAM) and Read Only Memory (ROM or EPROM). The SBC-85 also includes an RS232 Serial port with a female DB9 connector for communication. Finally, the board includes an expansion bus for insertion into a backplane.
The primary components on the SBC-85 are as follows:
U8 - 8085 Microprocessor operating at 3MHz
U6 - 2732 EPROM Base Memory (address 0x0000 thru 0x0FFF)
U9 – 2732 EPROM Expansion Memory (address 0x1000 thru 0x1FFF)
U5 – 6264 64Kbit RAM (address 0x2000 thru 0x3FFF)
U7 – 8155 RAM, I/O, Timer (address 0x4000 thru 0x4500)
U3 – MAX232 TTL / RS232 Level Shifter
Additional ‘glue logic’ includes the following:
U2 – 74LS32 Quad 2-input OR gate for address decoding
U4 – 74HCT573 Octal Latch for AD0-AD7 latching to form A0-A7
U10 – 75HCT521 Equality Comparator 8155 address decoder
In addition to the 8085 Single Board Computer, the following add-on cards are either released, in progress, or in mind
- 4 Slot Backplane * Cassette Tape Recorder Interface * Remex Paper Tape Reader Interface * Prototyping Board * Bus Monitor
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