TRS-80 Model I
Keyboard unit with an external supply and monitor; the Expansion Interface adds RAM, disk, printer and a clock.
Open the TRS-80 Model I in Retro Bench- Also
- TRS-80 Micro Computer (Level I or Level II BASIC), with the Expansion Interface 26-1140/1/2
- Year
- 1977
- CPU
- Z80 @ 1.774 MHz (10.6445 MHz crystal / 6)
- RAM
- 4 or 16 KB in the keyboard unit (4096 or 4116 DRAM); up to 32 KB more in the Expansion Interface
- ROM
- Level I BASIC 4 KB or Level II BASIC 12 KB
- Video
- 64 × 16 or 32 × 16 text, 128 × 48 block graphics; seven 2102 video RAMs, MCM6670 character generator
- Sound
- None built in (programs toggle the cassette output)
- OS
- Level II BASIC; TRSDOS with the Expansion Interface and disks
Chips
| Chip | Where | Role |
|---|---|---|
| Z80A CPU | Z40 | Main CPU (8-bit, 40-DIP) |
| 4116 DRAM 16K x 1 | Z13-Z20 (4096 DRAM in 4K units) | Dynamic RAM, 16K x 1 (lower 16K on the Spectrum) |
| 2102 static RAM 1K x 1 | Z45-Z48, Z61-Z63 | Model I video RAM: seven 2102AN-4L hold the 1 KB screen at $3C00-$3FFF. |
| MCM6670P character generator (128 × 7 × 5) | Z29 | Model I alphanumeric character generator: 5 × 7 dot rows for the 64 or 128 character codes. |
| Model I BASIC ROM set (Level I / Level II) | Z33, Z34 (Level I); ROM board Z1-Z3 with decoder Z4 (Level II) | Level I BASIC (4 KB in two 2 KB parts at Z33, Z34) or Level II BASIC (12 KB on a ROM board under the CPU board). |
| WD FD1771B-01 floppy disk formatter/controller | Z34 in the Expansion Interface | Single-density disk controller in the Model I Expansion Interface, at $37EC-$37EF. |
Ports and connectors
- Expansion port (rear edge of the CPU board), 40-pin PCB edge (20 + 20), mates with AMP 88103-1
- Cassette (J3, TAPE), 5-pin DIN socket (180°)
- Video to the monitor (J2), 5-pin DIN socket (180°)
- Power in (J1, POWER), 5-pin DIN socket (180°), from the external TRS-80 power supply
- Expansion Interface: CPU input (J2) and screen printer bus (J3), 40-pin PCB edges on the EI, same numbering as the keyboard unit UNVERIFIED
- Expansion Interface: line printer (J4), 34-pin PCB edge (17 + 17), cable 600091 to a 36-pin printer plug UNVERIFIED
- Expansion Interface: floppy disk bus (J5), 34-pin PCB edge (17 + 17), Shugart SA400 bus UNVERIFIED
- Expansion Interface: cassette 0 and 1 outputs (J6, J7) and input from the TRS-80 (J8), Three 5-pin DIN sockets (180°)
- Expansion Interface: power in (J9), 5-pin DIN socket (180°), from its own TRS-80 power supply
Power rails
| Rail | Expect | Measure at |
|---|---|---|
| +5 V | 4.75-5.25 V | Z80A pin 11; left side of R4 |
| +12 V | 11.4-12.6 V | 4116 pin 8; top of R18 |
| -5 V | -4.75 to -5.25 V | 4116 pin 1; CR2 anode |
Symptoms
- MEMORY SIZE? never appears, garbage, or RAM errors: check 4116, Z80A, Model I ROMs
- Supply dead or crashes: a shorted RAM pulls down a rail: check 4116
- Lockups, spontaneous reboots and crashes with the Expansion Interface
- Keys repeat or double (keybounce)
- Key appears after the prompt by itself, or the keyboard is dead
- Garbled characters, graphics where letters should be: check 2102, MCM6670
- Random faults in clocks, video timing or decoding (74LS TTL)
- Cassette will not load (CLOAD)