Commodore 1541 disk drive
Single-sided 170 KB GCR drive with its own 6502. Hot inside, slow on the bus, and the VIA and DOS ROM fail first.
Open the Commodore 1541 disk drive in Retro Bench- Also
- 1540/1541 long board 1540008 (schematic 1540001; Carlsen calls it 1540050); short boards 1540048 (sch. 1540049), 250442 (1541A) and 250446 (1541A-2, sch. 251748); 1541B/1541C 250448; 1541-II 340503 (sch. 355207)
- Year
- 1982
- CPU
- 6502 @ 1 MHz
- RAM
- 2 KB (4 × 2114 on the long board; one 2016/6116 later)
- ROM
- 16 KB DOS: 2 × 2364 (325302-01 $C000, 901229 $E000) or one 16K ROM 251968 (1541C, 1541-II)
- Video
- none
- Sound
- none
- OS
- Commodore DOS 2.6
Chips
| Chip | Where | Role |
|---|---|---|
| 6522 VIA | Serial VIA ($1800): UAB1 long board, UC3 short boards, U6 1541-II. Disk VIA ($1C00): UC4 long board, UC2 short boards, U8 1541-II. 1541C: UC1 and UC3 | Two 8-bit ports, timers, serial and user port |
| 6502 / 65C02 CPU | UC5 (long board 1540008), UC4 (short boards 251748), UC2 (1541C 250448), U3 (1541-II) | CPU |
| 325572-01 (64H105) 40-pin gate array | UC1 (short boards 1540048, 250442, 250446; not on the long board, which does this in TTL) | 1541 short board read/write logic: GCR shift register, byte-ready, sync detect, stepper phases, spindle motor driver |
| 251828 (64H156) 40/42-pin gate array | UC4 (1541B/1541C 250448), U10 (1541-II 340503) | Read/write, byte-ready, sync, stepper phases, serial ATN acknowledge and CPU clock select (1541B/C, 1541-II, 1571) |
| 251829 (64H157) 20-pin gate array | UC5 (1541B/1541C 250448 only; not on the 1541-II) | Address decoder (ROM, RAM, I/O selects), saddle canceller and motor-speed PLL (1541B/C, 1571) |
| 2364 Mask ROM 8K×8 | UB3 ($C000, 325302-01) and UB4 ($E000, 901229) on short boards; UAB4 ($C000) and UAB5 ($E000) on the long board | BASIC and KERNAL ROMs (C64); BASIC and KERNAL (VIC-20, PET); character ROM (C128) |
| 27128-pinout 16K x 8 ROM | UA2 (1541C, 251968-01), U4 (1541-II, 251968-03 / 355640-01) | 16K x 8 ROM or EPROM in the JEDEC 27128 pinout. |
| 6116 2K x 8 static RAM | UB2 (2016, short boards), UA3 (1541C), U5 (1541-II) | 2K x 8 static RAM. |
| 2114 SRAM 1K x 4 | UA2, UA3, UB2, UB3 (long board only) | Static RAM (ZX80 and ZX81 1K) |
| 7406 hex inverter buffer/driver (open collector) | UB1 and UD1 (short boards 251748); UA1 (1541C 250448); U7 (1541-II) | Serial bus line drivers (CLK, DATA, ATN acknowledge), stepper phase and LED drivers |
| ULN2003 / µPA2003 seven Darlington driver | U13 (1541-II only) | Seven-channel Darlington driver (stepper and motor drive) |
Ports and connectors
- Serial bus (IEC) in / out, DIN-6 × 2 (wired in parallel)
- Transformer to board (P1, internal), 4-pin header on the logic board (short board 251748)
- Power in (1541-II only), 4-pin DIN socket (CN7)
- Read/write head (P8, short board), 5-pin header, one keyed position
- Stepper motor (P7, short board), 6-pin header
- Spindle motor control board (P5, short board), 3-pin header
- Stepper motor (CN3, 1541-II), 6-pin header
- Spindle motor and write protect (CN4, 1541-II), 7-pin header
Power rails
| Rail | Expect | Measure at |
|---|---|---|
| +5 V | +5 V | 6502 pin 8; VIA pin 20; 5 V regulator output |
| +12 V | About +12 V | VR1 output; 1541-stepper pins 1, 2 (P7, stepper common); 1541-spindle pin 2 (P5) |
| +12 V | About +12 V | VR2 output |
| +12 V | About +12 V | 1541ii-power pin 4; 1541ii-stepper pins 1, 2; 1541ii-motor pin 6 |
| +12VF | About +12 V once the 6502 is running | Q1 output (power up/down write protection) |
| Unregulated | About 10 V (5 V side) and 20 V (12 V side) | CR4 and CR2 cathodes |
Symptoms
- No LEDs at power-up
- Green power LED dim or flickering, spindle runs, red LED off
- Red LED stays on and spindle runs continuously after power-up: check 2364, VIA, 6502, 325572
- Red LED flashes a code at power-up: check 6116, 2114, 2364, 27128 ROM
- DEVICE NOT PRESENT: check VIA, 7406
- SEARCHING FOR $ then the computer hangs: check 7406, VIA
- FILE NOT FOUND or read error 21/74 on every disk
- Head knocks on power-up, initialize or format
- Reads its own disks but not commercial disks, worse when warm
- Loads with the red LED flashing, or speed will not stabilize
- Spindle runs all the time but the drive works
- Motors do not run at all: check 2003
- Will not format; reads fine: check 325572, VIA