This information applies to Commodore 1551 Disk Drive used with plus/4. Diskettes created on the 1551 are read write compatible with Commodore 1540/1541, 4040, 2031 disk drives. The 1551 is connected to the computer through the "TCBM" interface. This interface is specially created by Commodore. Bytes are transferred as parallel data with a 8-bit bi-directional port, two handshake lines, and two status lines. This handshake protocol allows for less timing constraint than slower serial interfaces. Estimated data rate over the TCBM handshake is 1600 bytes per second versus 300-400 bytes per second for the 1541 drive. The 1551 connector plugs into the host computer's expansion port and allows external cartridges to be plugged in. You can have two 1551 drives, two 1541 drives, and two printers hooked up to the computer at once as long as each peripheral is assigned a different device number. When there is a contention with duplicate device numbers, the 1551 has priority over the 1541. Memory map of the 1551: Location Purpose $0000-0001 8 bit port - 6510T $00 : cpu data direction register $01 : cpu port register bit 0: drive head stepper 0 (STP) bit 1: drive head stepper 1 (STP) bit 2: drive motor on/off (MTR) bit 3: drive LED (ACT) bit 4: write protect sensor (WPS) bit 5: density select #0 (DS0) bit 6: density select #1 (DS1) bit 7: byte latched (1 - yes, 0 - no) $0002-00ff Zero page work area, *job queue, variables $0100-01ff GCR overflow area and stack $0200-02ff Command buffer, parser, tables, variables $0300-06ff 4 data buffers, 0-3 $0700-07ff BAM resident in memory $4000-4007 6523a I/O to computer controller $c000-ffe5 16KB ROM, DOS and controller routines $ffe6-ffff JMP table, user command vectors *job queue and HDRS offset by 2 bytes - starts at $0002 TIA 6523 - Triport Interface Adapter The TIA6523 is a tri-port adapter. It has 28 pins, three data direction and three port registers. It's mapped in at $4000-$4007 in the 1551. Some drives actually contain a 6525 with a 6523 pinout. Address 6523 TIA ($4000-$4007) ------------------------------------------------------------------------------- 0 Port A Data (data sent to the computer) 1 Port B Data (connected to the shift register of the R/W head) 2 Port C Data bit 7: DAV (DAta Valid) handshake from bit #6 from $FEF2/FEC2 of plus/4 bit 6: SYNC 0 -> SYNC found 1 -> no SYNC bit 5: MODE (jumper) drive number (0 -> 8, 1 -> 9) bit 4: MODE (R/W head) 0 -> head in write mode 1 -> head in read mode bit 3: ACK (ACKnowledge) handshake to bit #7 of $FEF2/$FEC2 bit 2: DEV 1 -> drive mapped at $FEF0 on plus/4 0 -> drive mapped at $FEC0 on plus/4 bit 1: STATUS1 - mapped at $FEF1/FEC1 on plus/4 bit 0: STATUS0 - mapped at $FEF1/FEC1 on plus/4 01 Timeout during reading 10 Timeout during writing 11 End of data Data direction registers ( 1 -> bit out, 0 -> bit in) 3 Port A Direction 4 Port B Direction 5 Port C Direction (set to $1F by the drive ROM) 6 N/A 7 N/A User commands: User command Function u1 or ua Block read replacement u2 or ub Block write replacement u3 or uc Jump to $0500 u4 or ud Jump to $0503 u5 or ue Jump to $0506 u6 or uf Jump to $0509 u7 or ug Jump to $050c u8 or uh Jump to $050f u9 or ui Jump to ($fffa) reset tables u: or uj Power up vector The u1 and u2 commands are used to replace the Block-Read and Block-Write commands, ui restarts the 1551 without changing variables, uj cold-starts the 1551 almost as if it had been turned off and on again. Specifications of the 1551 disk drive: Storage: Total formatted capacity 174848 bytes per diskette Maximum sequential file size 168656 bytes per diskette Maximum relative file size 167132 bytes per diskette Records per file 65535 Files per diskette 144 Tracks per diskette 35 Sectors per track 17-21 Sectors per diskette 683 total Sectors per diskette 664 free for data Bytes per sector 256 Integrated circuit chips used: 6510T Microprocessor, I/O 6523A I/O 23128 16KB ROM 4016 2KB RAM Gate array Controller circuit Physical dimensions: Height 97 mm Width 200 mm Depth 374 mm Weight 10.5 kg Electrical requirements: Voltage US 100-120 VAC Voltage export 220-240 VAC Freq US 60 Hz Freq export 50 Hz Power used 25 Watts Media: 5 1/4 inch diskette This information was rewritten by Jan Sefcik from the Commodore 1551 Disk Drive User's Manual. The memory map and the description of the TIA were provided by Attila Grósz in 2003. Edited by Marko Mäkelä.