IN MEMORIAM TO MARCEL VAN KERVINCK (gigatron TTL microcomputer)

May 25, 2021 • Andreas Schniertshauer

R.I.P. Marcel van Kervinck

In memoriam to Marcel van Kervinck one of the developers of the Gigatron TTL microcomputer passed away on 25.05.2020.

Gigatron TTL microcomputer

The Gigatron TTL is a retro-style 8-bit microcomputer (without microprocessor), where the CPU is implemented by a set of TTL chips instead of a single microprocessor, imitating the hardware present in early arcades.

The original DIY kits sold by Marcel van Kervinck and Walter Belgers are no longer available due to the depletion of the stock and Marcel van Kervinck’s passing. Also Gigatron ROMs are no longer available, but you can burn one yourself after downloading from github - gigatron-rom

Specifications

8-bits system built out of 1970s TTL chips (74LS), 34 TTL ICs
No microprocessor and no complex chips
Instruction decoding with diodes
8-bit CPU Harvard Architecture, 6.25 MHz
Gigatron Control Language (GCL) Interpreter
16-bit vCPU Von Neumann Architecture
32K SRAM (upgradable to 64K), 64 Kwords EPROM
VGA 640x480 EGA 64 Colors 6-bit RGB, effective 160 x 120 x 64
4 bits audio out, 4 software channels
4 blinkenlights

RISC Instruction Set

The native 8-bit CPU instruction set has only 17 instructions:

Memory load/store: LD ST
Logical operations: ANDA ORA XORA
Arithmetic operations: ADDA SUBA
Unconditional jumps: JMP BRA
Conditional jumps: BGT BEQ BGE BLT BNE BLE
No operation: NOP CTRL

PCB and ROM

The following photo shows the unassembled black PCB and the ROM version 5, version 3 and also the prototype ROM. In front of the picture you can see a gigatron TTL microcomputer sticker.

I really love this little minimalistic system without a microprocessor, it can do 6502 emulation and can run Apple-1 code, such as WozMon. Also BASIC is running and some great games from the 80s e.g. Snake, Racer, Tetronis, Bricks, TicTacToe.

Assembly

Step 1 - Bypass Capacitors

Solder 38 ceramic 100nF capacitors (C5 to C42).
Measure resistence between +5V and GND (e.g. pins of C4), should be infinite or in the MΩ-range indicating that there is no short circuit.
Measure capacitance between +5V and GND (e.g. pins of C4), should be 38 x 100nF = 3.8µF (I measured 3.67µF).

Step 2 - Power Circuitry

Solder the power circuitry (on the upper left side and top middle of the PCB).
Power on, the power LED should light up.
Measure voltage between +5V and GND (e.g. pins of C4), it should show +5V (I measured 5.16V).

Step 3 - Clock Generator

Solder the clock circuitry on the top right side of the PCB.
Measure IC U1 pin 14 connected to +5V and pin 7 connected to GND.
Measure the average voltage of the pulse between pad CLK1 and GND, it should show a voltage between 1.5 and 2.5 volts or -1.5 and -2.5 volts (! measured 1.94 volts and 6.25 MHz).
Measure the clock pulse with an oscilloscope, it should show some kind of square wave.

Step 4 - IC Sockets

Solder all IC sockets, use a ZIF (Nullkraft) socket for the EPROM.
Measure power and GND on all sockets (without ICs attached).

Uxx	IC	GND	Power
U1	74HCT04	7	14
U2	MCP100	3	2
U3	74HCT161	8	16
U4	74HCT161	8	16
U5	74HCT161	8	16
U6	74HCT161	8	16
U7	27C1024	11,30	40
U8	74HCT273	10	20
U9	74HCT273	10	20
U10	74HCT244	10	20
U11	74HCT139	8	16
U12	74HCT153	8	16
U13	74HCT138	8	16
U14	74HCT138	8	16
U15	74HCT240	10	20
U16	74HCT32	7	14
U17	74HCT153	8	16
U18	74HCT153	8	16
U19	74HCT153	8	16
U20	74HCT153	8	16
U21	74HCT153	8	16
U22	74HCT153	8	16
U23	74HCT153	8	16
U24	74HCT153	8	16
U25	74HCT283	8	16
U26	74HCT283	8	16
U27	74HCT377	10	20
U28	74HCT244	10	20
U29	74HCT161	8	16
U30	74HCT161	8	16
U31	74HCT377	10	20
U32	74HCT157	8	16
U33	74HCT157	8	16
U34	74HCT157	8	16
U35	74HCT157	8	16
U36	62256-55	14	28
U37	74HCT377	10	20
U38	74HCT273	10	20
U39	74HCT595	8	16

Remarks:

In the manual version 20200615 on page 33 it is described that U36 has 24-pins but correct is 28-pins.
62256 32Kx8bit CMOS SRAM

Step 5 - Resistor Arrays and Blinkenlights

Solder the resistor arrays and LEDs, watch for the correct orientation.

Step 6 - Instruction Decoder

Solder 30 diodes, watch for the correct orientation.

Step 7 - Audio, Game Controller Connector, VGA Connector

Solder the connectors and the remaining resistors for VGA.

Step 8 - PCB Cleaning and Check

Clean the PCB and remove all flux and let the board dry.
Check the whole PCB for soldering errors, e.g. shorts, cold joints a.s.o.

Step 9 - Insert ICs and Power On

Insert all ICs and watch for the correct orientation.
Check again for the correct placement of all ICs before powering on.
Power on (without monitor, keyboard, controller), you should see a blinking lights sequence if all works correctly.
Power off and attach controller / keyboard and monitor and power on again.
In my case the monitor did not show anything and the keyboard did not work, try another monitor / keyboard (this both worked for me), else have fun debugging - you will learn a lot if it does not work, so don’t give up.

Updates

Update (2021-08-28)

Now (1. August 2021) FORTH and a little C compiler is also running, see Twitter - Gigatron TTL microcomputer.

Update (2021-08-31)

The following photo shows the fully assembled black PCB with ROM version 4:

The following photo shows the system menu of the ROM version 4:

Update (2021-09-04)

The adapter kit to hook up a PS/2 keyboard to the Gigatron including a version 5 EPROM upgrade is available on tindie as from 20. September 2021.

tindie - Gigatron Pluggy McPlugface PS/2 adapter kit +EPROM