TRS-80 Model 4
A Model III with a 4 MHz Z80A, 80 × 24 video, 128 KB and TRSDOS 6.
Open the TRS-80 Model 4 in Retro Bench- Also
- Model 4 (26-1067/8/9), Model 4P portable and Model 4D; later units use the gate-array board
- Year
- 1983
- CPU
- Z80A @ 2.02752 or 4.05504 MHz
- RAM
- 16, 64 or 128 KB (4116, or 6665 64K DRAM)
- ROM
- 14 KB Model III ROM set (Model III mode only)
- Video
- 64 × 16 and 80 × 24 text from a 68045 CRTC and 2 KB video RAM; optional 640 × 240 graphics board
- Sound
- Optional one-bit sound board (port 90H), standard on the disk models
- OS
- TRSDOS 6 with BASIC (Model 4 mode); Model III BASIC and TRSDOS 1.3 (Model III mode)
Chips
| Chip | Where | Role |
|---|---|---|
| Z80A CPU | U57 (1983 board); U1 (gate-array board, Sams CSCS13) | Main CPU (8-bit, 40-DIP) |
| 4116 DRAM 16K x 1 | U77-U84 (16K version) | Dynamic RAM, 16K x 1 (lower 16K on the Spectrum) |
| 4164 DRAM 64K×1 | U85-U92 (MCM6665, 64K version; 128K adds a second row) | Main RAM (8 chips = 64 KB) |
| 2364 Mask ROM 8K×8 | U68 (MCM68A364) | BASIC and KERNAL ROMs (C64); BASIC and KERNAL (VIC-20, PET); character ROM (C128) |
| 2332 Mask ROM 4K×8 | U69 (MCM68A332) | Character ROM (C64, VIC-20, PET) |
| 9316B / 2316B ROM 2K x 8 | U70 (MCM68A316 ROM C) and U23 (MCM68A316E character generator) | 2K x 8 mask ROM: Apple II monitor and BASIC ROMs; TRS-80 ROM C and character generator |
| SY68045 CRT controller | U47 (60 Hz SY68045; 50 Hz units use a different part number) | Model 4 video timing and screen addressing for the 64 × 16 and 80 × 24 modes. Ports 88H-8BH. |
| WD FD1793 floppy disk formatter/controller | U6 on FDC board 8858160 | Single and double density disk controller on the Model III and Model 4 FDC boards, ports F0H-F3H. |
| Model 4 gate arrays (gate-array board) | Gate-array board: GA 4.1 timing U9, GA 4.4 FDC U76, GA 4.5 RS-232/interrupts U82, video gate array U17 | On the later gate-array Model 4 board they replace the PALs and much of the TTL: timing (GA 4.1), FDC support (GA 4.4), RS-232 and interrupts (GA 4.5) and a video gate array. |
Ports and connectors
- I/O bus (J2 on the 1983 board; J4 on the gate-array board), 50-pin PCB edge (25 + 25) UNVERIFIED
- Printer port (card edge), 34-pin PCB edge (17 + 17) UNVERIFIED
- Cassette (rear DIN), 5-pin DIN socket (180°) UNVERIFIED
- RS-232C (DB-25, J3 on the gate-array board), DB-25 UNVERIFIED
- 65 W supply output (TB2, Astec AA12090), 13-pin locking header (Molex 5265-12 housing), pin 3 keyed
- Monitor board input (RCA video monitor #8492002), 10-position edge on the RCA monitor board (position 5 keyed) UNVERIFIED
Power rails
| Rail | Expect | Measure at |
|---|---|---|
| +5 V | 5.00-5.25 V, under 50 mV ripple | Z80A pin 11; m4-psu pins 8, 9, 10 (TB2) |
| +12 V (V2) | 11.40-12.60 V | m4-psu pin 2 (TB2) |
| +12 V (V3) | 11.40-12.60 V | m4-psu pins 5, 6, 7 (TB2) |
| -12 V | -11.40 to -12.60 V | m4-psu pin 4 (TB2) |
Symptoms
- Computer dead (65 W Astec supply)
- All supply outputs low
- Supply smokes or trips at switch-on
- No boot in either mode: check Z80A, 2364, 4164
- No picture or wrong screen size: check 68045
- Disk errors: check 1793, Model 4 GAs