# Microfloppy Control Interface The **Microfloppy Control Interface** (MCI) is the means by which floppy disk drives are controlled by a Macintosh. ## Signal Multiplexing The MCI uses the CA2-0 lines and the SEL line to select one of a set of one-bit signals from the drive, the value of which is reflected on the RD line. ### 400 KB GCR Drive | CA2 | CA1 | CA0 | SEL | RD Output | | ---- | ---- | ---- | ---- | ---------- | | Low | Low | Low | Low | !DIRTN | | Low | Low | Low | High | !CSTIN | | Low | Low | High | Low | !STEP | | Low | Low | High | High | !WRPROT | | Low | High | Low | Low | !MOTORON | | Low | High | Low | High | !TK0 | | Low | High | High | Low | Always Low | | Low | High | High | High | !TACH | | High | Low | Low | Low | RDDATA0 | | High | Low | Low | High | † | | High | Low | High | Low | SUPERDRIVE | | High | Low | High | High | Always Low | | High | High | Low | Low | SIDES | | High | High | Low | High | Always Low | | High | High | High | Low | !DRVIN | | High | High | High | High | REVISED | † TODO ### 800 KB GCR Drive | CA2 | CA1 | CA0 | SEL | RD Output | | ---- | ---- | ---- | ---- | ---------- | | Low | Low | Low | Low | !DIRTN | | Low | Low | Low | High | !CSTIN | | Low | Low | High | Low | !STEP | | Low | Low | High | High | !WRPROT | | Low | High | Low | Low | !MOTORON | | Low | High | Low | High | !TK0 | | Low | High | High | Low | SWITCHED | | Low | High | High | High | !TACH | | High | Low | Low | Low | RDDATA0 | | High | Low | Low | High | RDDATA1 | | High | Low | High | Low | SUPERDRIVE | | High | Low | High | High | † | | High | High | Low | Low | SIDES | | High | High | Low | High | !READY | | High | High | High | Low | !DRVIN | | High | High | High | High | REVISED | † TODO ### Superdrive | CA2 | CA1 | CA0 | SEL | MFMMODE | !WRREQ | RD Output | | ---- | ---- | ---- | ---- | ------- | ------ | ----------- | | Low | Low | Low | Low | * | * | !DIRTN | | Low | Low | Low | High | * | * | !CSTIN | | Low | Low | High | Low | * | * | !STEP | | Low | Low | High | High | * | * | !WRPROT | | Low | High | Low | Low | * | * | !MOTORON | | Low | High | Low | High | * | * | !TK0 | | Low | High | High | Low | * | * | SWITCHED | | Low | High | High | High | Low | * | !TACH | | Low | High | High | High | High | * | INDEX | | High | Low | Low | Low | Low | * | RDDATA0 | | High | Low | Low | Low | High | Low | INDEX | | High | Low | Low | Low | High | High | RDDATA0 | | High | Low | Low | High | Low | * | RDDATA1 | | High | Low | Low | High | High | Low | INDEX | | High | Low | Low | High | High | High | RDDATA1 | | High | Low | High | Low | * | * | SUPERDRIVE | | High | Low | High | High | * | * | MFMMODE | | High | High | Low | Low | * | * | SIDES | | High | High | Low | High | * | * | !READY | | High | High | High | Low | * | * | !DRVIN | | High | High | High | High | * | * | PRESENT/!HD | \* Don't Care ### Hard Disk 20/DCD | CA2 | CA1 | CA0 | RD Output | | ---- | ---- | ---- | ----------- | | Low | Low | Low | DCDDATA | | Low | Low | High | DCDDATA | | Low | High | Low | !HSHK | | Low | High | High | !HSHK | | High | Low | Low | † | | High | Low | High | Always Low | | High | High | Low | Always High | | High | High | High | Always High | † TODO ### Composite | CA2 | CA1 | CA0 | SEL | 400 KB GCR | 800 KB GCR | Superdrive | HD20/DCD | | ---- | ---- | ---- | ---- | ---------- | ---------- | ------------- | ----------- | | Low | Low | Low | Low | !DIRTN | !DIRTN | !DIRTN | DCDDATA | | Low | Low | Low | High | !CSTIN | !CSTIN | !CSTIN | DCDDATA | | Low | Low | High | Low | !STEP | !STEP | !STEP | DCDDATA | | Low | Low | High | High | !WRPROT | !WRPROT | !WRPROT | DCDDATA | | Low | High | Low | Low | !MOTORON | !MOTORON | !MOTORON | !HSHK | | Low | High | Low | High | !TK0 | !TK0 | !TK0 | !HSHK | | Low | High | High | Low | Always Low | SWITCHED | SWITCHED | !HSHK | | Low | High | High | High | !TACH | !TACH | !TACH/INDEX | !HSHK | | High | Low | Low | Low | RDDATA0 | RDDATA0 | RDDATA0/INDEX | † | | High | Low | Low | High | † | RDDATA1 | RDDATA1/INDEX | † | | High | Low | High | Low | SUPERDRIVE | SUPERDRIVE | SUPERDRIVE | Always Low | | High | Low | High | High | Always Low | † | MFMMODE | Always Low | | High | High | Low | Low | SIDES | SIDES | SIDES | Always High | | High | High | Low | High | Always Low | !READY | !READY | Always High | | High | High | High | Low | !DRVIN | !DRVIN | !DRVIN | Always High | | High | High | High | High | REVISED | REVISED | PRESENT/!HD | Always High | † TODO ## Signal Descriptions | Signal Name | Description | | ----------- | --------------------------------------------------------------------------------------- | | !DIRTN | Step direction; low=toward center (+), high=toward rim (-) | | !CSTIN | Low when disk is present | | !STEP | Low when track step has been requested | | !WRPROT | Low when disk is write protected or not inserted | | !MOTORON | Low when drive motor is on | | !TK0 | Low when head is over track 0 (outermost track) | | SWITCHED | High when disk has been changed since signal was last cleared | | !TACH | Tachometer; frequency reflects drive speed in RPM | | INDEX | Pulses high for ~2 ms once per rotation | | RDDATA0 | Signal from bottom head; falling edge indicates flux transition | | RDDATA1 | Signal from top head; falling edge indicates flux transition | | SUPERDRIVE | High when a Superdrive (FDHD) is present | | MFMMODE | High when drive is in MFM mode | | SIDES | High when drive has a top head in addition to a bottom head | | !READY | Low when motor is at proper speed and head is ready to step | | !DRVIN | Low when drive is installed | | REVISED | High for double-sided double-density drives, low for single-sided double-density drives | | PRESENT/!HD | High when a double-density (not high-density) disk is present on a high-density drive | | DCDDATA | Communication channel from DCD device to Macintosh | | !HSHK | Low when DCD device is ready to receive or wishes to send | ## Commands The MCI uses the CA2-0 lines and the SEL line to select one of a set of commands which is then initiated by a positive pulse on the CA3 line. | CA2 | CA1 | CA0 | SEL | Description | | ---- | ---- | ---- | ---- | -------------------------------------------------------------- | | Low | Low | Low | Low | Set !DIRTN low (toward center; track number increases on step) | | Low | Low | Low | High | | | Low | Low | High | Low | Step drive heads | | Low | Low | High | High | Select MFM mode | | Low | High | Low | Low | Turn motor on | | Low | High | Low | High | | | Low | High | High | Low | | | Low | High | High | High | | | High | Low | Low | Low | Set !DIRTN high (toward rim; track number decreases on step) | | High | Low | Low | High | Reset SWITCHED to low | | High | Low | High | Low | | | High | Low | High | High | Select GCR mode | | High | High | Low | Low | Turn motor off | | High | High | Low | High | | | High | High | High | Low | Eject disk (CA3 must be held high for ~0.5 seconds) | | High | High | High | High | | ## Drive Identification | Drive Type | SUPERDRIVE | SIDES | !DRVIN | REVISED / PRESENT/!HD | | --------------- | ---------- | ----- | ------ | --------------------------------------------- | | None | High | High | High | High | | 400 KB GCR | Low | Low | Low | Low | | 800 KB GCR | Low | High | Low | High | | Superdrive | High | High | Low | High if double-density disk present, else low | | Hard Disk 20 | Low | High | High | High |