sbelectronics / eta-3400

Remake of the Heathklt ETA-3400 Memory/IO board for the ET-3400 / ET-3400-A

Scott's ETA-3400 Memory/IO Accessory Remake

(c) Scott Baker, http://www.smbaker.com/

youtube video: https://www.youtube.com/watch?v=gP17z7haC4o&ab_channel=smbakeryt

Purpose

The ETA-3400 was a memory/IO accessory for Heathkit's ET-3400 and ET-3400-A trainers. It added a serial port, a cassette port, up to 4K of RAM, and 4K of ROM. The ROM included a terminal monitor that you could use to operate the trainer over serial, as well as a "Tiny Basic" ROM that allowed you to program the trainer in the BASIC programming language.

This project is a remake of that board, using slightly more modern components, but still in the vintage theme of the original.

Recompiling the ATF16V8

The ATF16V8 is a programmable logic device that implements the IO addressing and some control line manipulation. It's programmed in a languaged called CUPL. You'll need the WinCUPL tool from Microchip in order to recompile the PLD. The desired output is the JED file, which you can program using a programmer such as TL866 with minipro software.

Burning the ROM

I wrote a small python tool, makerom.py, that loaded the original heathkit ROMs into a 4K rom image (the way they're addressed is a little bit weird, the trainer puts them at 0x1400 to 0x23FF). I used a TL866 with minipro to burn it to a 28C64 EEPROM chip.

You might notice that roms/combined.bin is 5K instead of 4K. That's a mistake (the last 1K is all zeros), but that's also the way it was when I burned my ROM and made my video, so I left it as-is. In the future, I may look into finding a way to load more programs into the remainder of the ROM and make it addressable.

Setting up the raspberry pi zero

The raspberry pi zero is entirely optional (the board features a RS232 port that can be used with an external terminal), but offers the ability to build a terminal directly into your ETA-3400. Some instructions on setting up the Pi Zero W

# disable serial console
sudo raspi-config
  5 - interfacing options
    P6 - serial
        No login shell
        Yes serial interface enabled

# install minicom
sudo apt-get -y install minicom
sudo minicom -s
  Port - /dev/serial0
  Baud - 2400
  Bits - 7,E,1
  Flow Control - off

# as an alternative to minicom, picocom can be used
sudo apt-get -y install picocom
picocom /dev/ttyS0 --baud 2400 -p 1 -d 7 -y e

Note: You'll want to set an end-of-line delay in minicom or picocom if you want to be able to do text uploads to basic. I recommend 325 milliseconds. Otherwise, your terminal
might send the next character while basic is saving the freshly-entered line, and the first character of the next line will be lost.

Static RAM Notes

My tests of several 62256 RAM variants that I obtained. Note that I have only performed a short test on each one to verify whether BASIC loaded properly and was able to run a short program. Your mileage may vary.

• AS6C62256-55PCN (Mouser) 55ns part DOES NOT WORK correctly. 70ns part (-70PCN) is fine but hard to find.
• HM62256LP-12 (Jameco 42850). According to catalog is a 150ns part, but as received with -12 number is probably 120ns. Tested and works.
• KM62256BLP-10 (Jameco 42833). 100ns. Tested and works.
• HM62256BLP-7 (Jameco 82472). 70ns low power. DOES NOT WORK.
• MS62256L-70PC (ebay). 70ns. Tested and works.
• CY62256NLL-70PXC. 70ns. Tested and works.

Gerbers

The gerbers are present in the gerbers directory.

• gerbers/v-0.2-initial .. This was my initial order, sent to jlcpcb, to make the boards used in the video.

Resources and Acknowlegements

• The ET-3400 group is a great resource, and several people there were helpful and supportive of this project.