Designing a Memory/IO (ETA-3400) addon for the ET-3400 Trainer

In this video, I build a memory/IO expansion for the vintage Heathkit ET-3400 trainer:

Purpose

I’ve always wanted one of these Heathkit microprocessor trainers, and finally one caught my eye on eBay and I pulled the trigger. The basic interaction with the trainer is through the onboard keypad and LED displays, but Heathkit also made an accessory that added additional ROM, RAM, serial port, and a cassette interface.

This allowed you to use a machine monitor over the serial port, and even featured a Tiny Basic interpreter in ROM that allowed the trainer to be programmed in Basic.

Design

The schematics for the ETA-3400 are well published on the web, and you can readily see exactly how Heathkit implemented the original accessory. One option would have been to faithfully reproduce the original, but that would have been a little inconvenient in comparison to some of the more modern ICs that are available now. Rather than using eight static RAMS and a pair of ROMs, we could simply use one larger RAM and one larger ROM. Rather than implementing address translation in a ROM, it could be done in programmable logic. A modern MAX202 could easily replace the handful of transistors and discrete components used in the original. While slightly newer, these choices are still in the vintage theme of the original.

Below is the schematic:

IMPORTANT NOTE: Although I designed the circuit to take either an AS6C62256 SRAM or an AS6C6264 SRAM, I had some trouble with part substitution. I recommend you stick with the recommended part, a 70ns 62256. When I tried a 55ns IC (either 62256 or 6264), behavior was erratic.

There’s a lot going on, so click the schematic to get a larger view, or download the PDF in my github repo.

The large 2×20 header is the ribbon cable from the ET-3400. It contains the data bus (D0-D7), the address bus (A0-A15), clock (02), power and ground, and several control lines. Noteworthy control lines include:

• vma.02, asserted when the CPU wants to access memory,
• R/W, asserts when the CPU wants to write (vma.02 and r/w together indicate a write; vma.o2 without r/w indicates a read)

RAM and ROM use a 62256 and 28C64 respectively. The RAM footprint I used was technically for a AS6C6246, and that *should* work, but in my prototype all I had as AS6C62256. If you’re ordering parts, I recommend the AS6C62256 just to be consistent and it’s a negligible cost difference for a more capable RAM.

The PIA, MC6821, is right out of the heathkit schematic. It’s a general-purpose peripheral interface and is used to interface to the serial port and cassette. The serial interface makes use of a MAX202 RS232 level converter, which is then plumbed out to a DB25 connector. The hex inverter interposed between the PIA and MAX202 is not needed, just leave it out and set the jumpers to bypass it.

The cassette circuitry is straight out of the heathkit schematics. It outputs audio via an RC network and voltage divider. It inputs the audio using a comparator. I have not tested the cassette interface yet, but I attempted to faithfully reproduce the heathkit circuit.

Address decoding makes use of a ATF16V8 programmable logic device. This take the upper address bits (A10..A15) and asserts the appropriate chip enable (RAMCS, ROMCS, or PIACS).

An optional feature is a raspberry pi header. A pi zero w can be added to the board and will be connected to the serial and cassette interfaces. This allows you to hide a pi inside the case and have remote SSH access to your trainer (or hook up the HDMI and USB to drive a display and keyboard). As I said, the pi is entirely optional.

Jumpers

JP1	PLD Spare. For future use.
JP2	Baud Rate. See schematic. The first three positions select baud rate. The fourth position should always be jumpered. The fifth and six positions are not used by the ROM, but could be used by a user program.
JP3	Trainer Power. Shorting this jumper enables the board to get power from the ET-3400’s expansion connector. Leave the jumper unpopulated if you’d rather use external power (via the barrel jack) instead
JP4	Pi Power. This connects 5V to the raspberry pi header. If you’re not using a raspberry pi, then don’t be concerned with it. If you are using a pi, then it makes a handy header for a power switch to turn the pi on/off (or just leave it connected all the time)
JP5	Cassette Gain. The 50mv is if you’re using the microphone input on your cassette recorder, and the 500mv position is if you’re using the AUX jack.
JP6	Max202 to DB-25. This allows you to implement a null-modem if you want. Jumper them straight across (1-2,3-4) for normal operation, or flip the jumpers (1-3, 2-4) for null modem.
JP7	PIA to Max202. This jumper block connects the serial TX and RX from the PI to the MAX202 driver chip. For normal operation it should be present (jumper 1-2, 3-4). If you’re installed the raspberry pi, then you’ll want to remove this jumper.

Implementation

A picture of the PCB is below:

Here’s an annotated picture:

Here is a component placement diagram (click the image to get a high resolution view; there’s also a PDF in the github repo that may be easier to read):

BOM

Here is the part list:

 

Part	Value	Quantity	Description	Source
C1	0.0047uF	1.0		digikey BC2683CT-ND
C2	2.2uF	1.0		digikey 445-173575-1-ND
C3	0.047uF	1.0		digikey BC2686CT-ND
C4-C14	0.1uF	11.0	bypass capacitor	digikey 399-4264-ND
	470uf/16V	1.0	additional stability when using pi; place across the 5V and GND pins of SV1
D1, D2	1N4148	2.0		digikey 1N4148FS-ND
IC1	28C64	1.0	EEPROM	digikey AT28C64B-15PU-ND
IC2	AS6C62256	1.0	RAM	digikey 1450-1182-5-ND (use the 70ns version, not the 55ns)
IC3	6821 PIA	1.0	PIA	ebay
IC4	ATF16V8BP3	1.0	PLD address decode	digikey ATF16V8B-15PU-ND
IC5	MAX202	1.0	serial driver	digikey MAX202ECPE+-ND
IC6	MC3302P	1.0	comparator	digikey 296-17223-5-ND
J1	DCJACK 5.5×2.1	1.0	[Optional] external power	digikey CP-002A-ND (not verified, looks right)
JP1	1×3 header		[Optional] spare input to PLD	digikey, S1012EC-40-ND (cut to size)
JP2	2×6 header		Baud select jumper
JP3	1×2 header		Jumper, enables power from ET3400
JP4	1×2 header		Jumper, enables power to pi socket
JP5	1×3 header		Jumper, cassette output gain
JP6	2×2 header		Jumpers, ser output	digikey, S2012EC-20-ND (cut to size)
JP7	2×2 header		Jumpers, max202
R1, R5, R8	100K	3.0		digikey S100KCACT-ND
R2, R4, R9, R10	1K	4.0		digikey S1KCACT-ND
R3, R7, R11, R20	10K	4.0		digikey S10KCACT-ND
R6	3.3K	1.0		digikey S3.3KCACT-ND
R12	100 ohm	1.0		digikey S130CACT-ND
R13-R19	150 ohm	7.0	RasPi protection resistors	digikey S150CACT-ND
R21	0 (jumper)	1.0	a chunk of wire will do
RN1	2.2K x 6 SIP	1.0	2.2Kx6 bussed sip resistor	digikey 4607X-101-222LF-ND
SV1	1×5 RA header	1.0	[Optional] I2C header from pi	digikey S1111EC-40-ND (cut to size)
SV2 or SV3	1×4 RA header	1.0		digikey S1132EC-40-ND (cut to size, tall)
U$2	2×40 header	1.0		digikey, S2012EC-20-ND (cut to size)
X1	RASPI_BOAR		[Optional] Header for Pi Zero W
X2	DB25	1.0	DB25 connector	Digikey, AE10935-ND
	shunt jumpers			Digikey, S9337-ND
CAB1	40-ribbon		40 pin ribbon cable		ETA-3400 to ET-3400 cable
IDC1, IDC2	2×20 IDC conn		2×20 IDC ribbon cable connector		ETA-3400 to ET-3400 connectors

Instead of CAB1 and IDC1/IDC2, you can probably hack up an old 40-pin IDE cable. If you do opt to do so then you may need to shorten the IDE cable if it’s excessively long (carefully pry off one connector, move it where you want and then carefully crimp it back down)

Notes

1. I opted to plumb through several PIA lines to the raspberry pi for future use. These all go through 150 ohm resistors for protection, but in some cases there can still be interference if the pi is erroneously outputting a signal. Most notable, this happens on GPIO18, which is connected to the Cassette RX. If you’re pi is setup to output sound on GPIO18, then it may interfere with loading programs from tape. Usually “gpio -g mode 18 in” will remedy this. Alternatively, you can leave R16 off. In fact, you can probably leave R15-R19 off.
2. The raspberry pi presents quite a bit of load on the ET-3400’s power supply. When using an onboard pi, I suggest placing a 470uF capacitor across the 5V and GND pins on the expansion board. The SV1 header is an excellent place to do this.
3. Keep the ribbon cable between the expansion board and the ET-3400 as short as practical. The longer the cable, the more the potential for interference.
4. The ET-3400-A modification instructions for adding the expansion header call for adding EMI/RFI shield of some sort to the trainer. This is probably not a bad idea, particularly with the ribbon cable wrapping around under the trainer as it does.

3D printed case

I made a 3D printed case. It’s about an inch tall and sticks to the bottom of the ET-3400 with magnets:

Resources.

• My github repo at https://github.com/sbelectronics/eta-3400 is where you’ll find any source code, PLD images, and even gerbers.
• The ET-3400 group at https://groups.io/g/ET-3400 was helpful to me developing this project and has many resources including the original Heathkit schematics.