Sunday, April 16, 2017

The Glitch Works' 8085 Single Board Computer

The Glitch Works' 8085-based Single Board Computer is another one of those cool little projects using ancient technology.

This time, it is the 8-bit 8085 from Intel (although second sourced/copied by a number of other manufacturers). The 8085 is the descendent of the 8080, arguably the microprocessor that kickstarted the personal computer revolution.

The Glitch Works very generously provides the full Eagle board and schematic files for the project via GitHub.

Of course I had to grab them (the rev2 version) and send off the brd file to OSH Park for a set of PCBs.

The design is done the way I like, i.e. relatively conservative with a good use of decoupling capacitors. The ROM circuitry is very configurable: you can use a straight-up 27256-style EPROM or a 28C256 EEPROM (the pin-outs are different). There's even a power-on reset circuit.

A 40-pin connector provides for system expansion. At first glance, I thought that only the I/O ports were available, but Glitch Works pointed out that all I needed was an address latch to get the lower address lines from the data lines. Duh!

I didn't like a couple of things in the design (there's always something, isn't there?), the first is the choice of connection for power (a 0.1" dual pin header) which will require work to use with commonly available power supplies e.g. the ubiquitous 2.1x5.5 barrel plug. The second is the grounding of the 8085's SID signal line.

The latter is one of the nice things of the 8085 vs a lot of other microprocessors of the era. The SID and the companion SOD signals can be easily set up for use as a simple serial port. Of course, it will also be software driven and not very fast.

However, considering that The Glitch Works' 8085 board doesn't come with any other peripheral devices, the availability of both signals would have enabled a real, fully-functional single board computer.

As it is, I had to carefully slice the SID pad to separate it from the ground plane. Then I hacked together a little daughter to provide for an FTDI connector.

The FTDI cable also provides the 5V power supply to the board. Although with a non-CMOS 8085, we are pushing the current-limits of the FTDI cable...!

As you can see in the picture, I am using a real-life ancient ceramic 8085A. This is one that is made by NEC.

Yet another picture:

Ancient hardware is useless without useful and sufficiently ancient software. So I grabbed Donn Stewart's adaptation of Li-Chen Wang's Palo Alto Tiny Basic and adapted it for use with this contraption.

Instead of using a UART for the serial input/output, I use the SID/SOD lines to drive a baud rate of 2400.

This gives us a fully functional 8085 computer without the need for an additional peripheral card. Sweet!

The source code for the Tiny Basic adaptation can be found here. It assumes a crystal of 6.144MHz and 32KB RAM.

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