Heathkit ETA-3400

6800 cpu Microcomputer
(stuff i've discovered)

last updated November 27, 2015


The ETA-3400 turns the standard 256-bit ram 100khz 6800 cpu ET-3400 trainer
into a fully functioning 1MHZ 4K microcomputer !


Overview I acquired an ET-3400 plus a partially built ETA-3400 Memory I/O Accessory, but the previous owner never built the ETA-3400 properly...
Aim: get it all running! - in the process discovered some things..


ET-3400 mods
I followed the manual that describes converting the ET-3400 to run with the I/O accessory, this involves removing the installed 2112 ram, changing the CPU clock speed from 100khz to 1 mhz, wiring the databus down to the feature connector, and installing the pin header connector so that you can plug it into the ETA-3400.
The before photo with the back off.

And after...

Fortunately I already had some of the classic 70's rainbow flat-ribbon cable in-stock pulled from other equipment, so i was able to replicate the installation as per the instructions almost fully. The instructions called for two white wires, but i decided to differentiate the two by going with white and black which is more logical and less confusing.



How to make it work!  

Step 1: check it out

ETA-3400 Schematic

The ETA-3400 was missing all socketed IC's, and on later inspection, i found that the RS-232 and TTY jumpers were randomly linked (wrongly) along with voltage regulaotrs in the wrong places, transistors also in the wrong places...

When the ET-3400 came, it had a mysterious 40-pin chip plugged into the breadboard on it, which i later discovered was the Heathkit numbered 6821 PIA chip i needed, that was a result!

Step 2: ROMs PROMs

Turns out you can get the rom files fairly easy, but i was missing the one very crucial Address Decoding PROM. Nobody seems to have dumped this.1

ROMS U105 AND U106 (binaries)

Step 3: Make the address decoding PROM (Heathkit part number 444-28) from the manual

All is not lost, fortunately Page 57 of the ETA-3400 assembly manual gives the contents of the address decoding prom, which is in fact only 32x8 anyway.

This means its 32 bytes in size, its arrangement is nice and easy, it contains 32 8-bit wide values, which are easily converted from binary into hexadecimal for which we can put in a binary file and burn to a 32x8 PROM.

Certain assumptions were made :
- The table represents positive logic
- the MSB is on the left, the LSB on the right
- the order is from top to bottom

In the image to the left you can see the Hex values i converted the binary to, and you can see i didn't bother writing lots of FF's in pencil after the part containing the important info, but the binary file is padded correctly with FF's.

so, here's my U108 ROM FILE (binary)

STEP 4: Test operation it in-circuit

I used my fluke 9100 and did some R/W to address $0000 to see if i got pin 1 of u108 to toggle.. sure enough, i could read/write and test the entire first 1k or ram at $0000 to $03FF.

I carried on for the further addresses, R/W @ $0400 caused pin 2 to toggle, $0800 pin 3, $0C00 pin 4.. success!

I then read the contents of the Fantom II Monitor ROM at $1400 and then the TinyBASIC ROM @ $1C00, performing a CRC-32 check on those showed they could be read absolutely perfectly!

U106 = Fantom II = $1400 - $1BFF = e4142682
U105 = TinyBasic = $1C00 - $23FF = bbd6a801

STEP 5 : Fire up the terminal and have a play

As you can see, once i connected to the ETA-3400 serial port (plus fixed the mssing ROMs and all other IC's, and reparing the AC wiring, and low voltage regulators, and TTY/RS-232 jumpers!!) it all worked.

You turn everything on, hit GO 1400 on the ET-3400 and then you get a MON> prompt to play with.. typing G 1C00 launches TinyBASIC and there you have it!

STEP 6 : Test the Cassette interface

Using the SAVE command from basic, i was able to record a proper sounding waveform of data to my PC through the line-in.

Now to LOAD from the computer.. Well, something funny happened here. Not only did i find yet more components soldered in the wrong places on the Cassette IN circuit (which i fixed), but I also found that the PCB is silk-screeed with the reverse polarity of D107 to how the schematics show it... who should we believe?

Maybe my PC audio output isn't a great enough voltage to get the comparator U109 to spit out TTL data.. so that doesn't want to work :\ Maybe a real cassette tape player might actually work. Asking others, it seems like this was a common ailment...




ON THE WORKBENCH Bill Degnan's page helped me understand what i needed to do to the ET-3400 to modify it for use with the ETA-3400. Good reading...
ETA-3400 Schematic You wont be doing much without these :)





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These pages are (C) Andy Welburn 1996-2002. I cannot be held responsible if the information supplied herein results in a blown monitor/power supply/house fuse/mind. Oh yeah, have a nice day :)