There's not a whole lot here right now. I'll add more information as I collect it.
| Manufacturer | Model | Control PCB | Stepper Type | Avg Pos Time | Media |
|---|---|---|---|---|---|
| Seagate | ST-412 | ASSY 20201 | 4-Wire | 85ms | Oxide, 00253 |
| Seagate | ST-419 | ASSY 20225 | 4-Wire | 85ms | Oxide, 00252 |
| Seagate | ST-225 | ASSY 20301 | 4-Wire | 65ms | Oxide Coated |
| Seagate | ST-238 | ASSY 20527 | 4-Wire | 65ms | Oxide Coated |
| Seagate | ST-238R | ASSY 20527 | 4-Wire | 65ms | Oxide Coated |
| Seagate | ST-225 | ASSY 20301 | 4-Wire | 65ms | Oxide Coated |
| Seagate | ST-251 | ASSY 20629 | 10-Wire | 40ms | Thin Film |
| Seagate | ST-251-1 | ASSY 20938 | 10-Wire | 30ms | Thin Film |
| Seagate | ST-277R-1 | ASSY 21020, 20938-300 | 10-Wire | 28ms | Thin Film |
Unlike the earlier ST-225 and ST-412 generations, drives with this board use 5-phase steppers and can seek nearly twice as fast as the ST-412. A lot of discrete logic and analog circuitry has been integrated into several LSI chips.
The PCB schematic and layout for 20938 are available, see st251/20938. This board is used in the ST-251-1 as well as the ST-277R (-300 version).
The other boards are very similar to each other showing a clear design progression.
20629: Uses L293 H-bridge chips instead of the 11721-501. Firmware is in a separate EPROM chip.
| Part Number | Nicknames | Where Used | Description |
|---|---|---|---|
| 11721-501 | 20938 | Similar to the L293 H-bridge driver | |
| 10189-521 | SSI257 | 20629, 20938 | Steering diodes and head preamp, NE592 equivalent. |
| 10188-501 | SSI257.2 | 20629, 20938 | Head center tap driver, write current selector. |
| 11744-502 | RING DETECTOR | 20629, 20938 | Stepper motor seek settling chip - adaptive ringout. |
| 11695-502 | 20629 | Unknown | |
| 10223-502 | RET | 20629, 20938 | Retracts stepper motor on power-down. |
| 11743-501 | 20938 | Controls H-Bridge enables based on phase state. | |
| 11743-521 | 20938-300 | Controls H-Bridge enables based on phase state. | |
| 11782-501 | STEP SEQR | 20938 | 5-Phase stepper motor controller. |
| 11791 | 20938 | Spindle motor driver. Similar to the HA13406W. | |
| 10206-501 | SSI296 | 20629, 20938 | Read channel and write amplifier. |
| 11647-502 | STEP LOGIC | 20629, 20938 | MFM interface logic. |
| 80118-502 | R1512-12 | 20938 | Rockwell R6518(?) 6502-core microcontroller. |