Part 1e: Circuit diagram and prototype

 Circuit Diagram

This is the original circuit diagram for the ETA-3400 using late '70's RAM and ROM IC's:

Note how many IC's are required down the right side of the circuit just for 4K RAM and 4K ROM.

This is the final circuit diagram for the ETA-3400X after some tweaking:

I have duplicated half of the original circuit. The +5V, -12V and +12V PSU rectifier and regulator, 6820 PIA, serial TTY and cassette ports are identical. Changes are with a single 128KB RAM and single 32KB ROM and the 22V10 GAL memory decoder.

I added red and green LEDs and resistors to the cassette port input and output lines to show that there was some activity on those ports. I wanted to make that as simple as possible with the least number of components to keep the pcb size down, but unfortunately it doesn't always work well.

On the lower left corner of the circuit there is an ALT jumper that goes into the GAL and that is used to select from two different memory mapping schemes. The intent is that this could be connected to a switch on the front panel or rear panel so that either configuration can be selected by the user. For now, as I don't have an enclosure it's just a jumper.

The RAM is over double the size needed, however 128KB static RAM IC's were easier to obtain so that's what I chose. I'm only using up to 48K of it in one of the memory configurations. The ROM is 32K and different sections of it are mapped to different areas of ROM depending on the ALT jumper. Also because of the way the ROM is mapped, I have to move 1K pages of memory around in the EEPROM or EPROM so that they appear in sequence in ROM for the CPU. Because the is only 15 address lines to the ROM, there is a mirror address for each location, however the Chip Select will ensure it is only selected for specific address ranges.

Also the circuit allows for smaller RAM and ROM chips. If smaller RAM chips are used there will be a problem with mirrored addresses, but that can be something to deal with later, probably in the firmware in ROM.

The PSU is also optional as the ET-3400 main unit can supply enough power through the ribbon cable to run this board. It uses far less power than the original ETA-3400 since there is only one RAM and one ROM chip. I performed tests and there is only a maximum of 25mA on the +12V and -12V lines which are only used on the serial and cassette ports, and about 200mA on the +5V supply. However if you intend to use the breadboard on the ET-3400 then you may have to consider adding the PSU section to the board if you don't initially.

Prototype

Moving on to the prototype, I was able to add the ROM, RAM and GAL on the breadboard section of the ET-3400, but that filled up the board. Initially I only added those three chips so I could test the memory decoding. It took a few attempts to get the programming correct. I was able to use the ET-3400 built in monitor, hex keypad and 7-segment display to check ROM locations to see that I had programmed the ROM correctly.

The extra circuitry was added to strip board and wired to the breadboard.

Rats nest prototype. The GAL and EPROM were a bit of a pain to remove and reprogram as I often accidentally pulled jumper wires out while doing that.

The PIA's I had were 6821s instead of the original 6820s. I'm not sure what differences there are, but they seem to work interchangeably in this circuit. The PIA and analogue serial terminal port were built on some pre-etched prototyping boards from Jaycar. Initially I did try to use another breadboard, however the 'Chinesium' ones you buy these days are crap. the jumper wires are now thinner than they used to be and the fingers in the breadboard are poorly aligned and have less grip. After a lot of intermittent issues I eventually just soldered up the parts onto the etched strip boards and that solved the reliability problem.

On the top right of the rats nest picture there is a VT100 terminal clone kit that I got from 'The High Nibble' called the VT132. After you build the kit, you just add a PS/2 keyboard (or USB if you built that version), and a VGA monitor and you have yourself a VT100 terminal clone. More information on that kit here on Dave's site:

https://thehighnibble.com/

At this stage there is no cassette port circuitry on my prototype, but I'll add that later once I get something to run.

Next I'll go into the patches to the Fantom II monitor and then Tiny BASIC...