Commodore Serial Interface Information

The Serial Interface consist of two 6-pin DIN Female Connectors on each drive. The second connector is for daisy chaining to other drives and/or peripherals. The voltage interface is a serial interface at TTL levels.

There are three types of operation over a serial bus - Control, Talk and Listen. The host is the controller and initiates all protocol on the serial bus. The host requests the peripheral to listen or talk (if the peripheral is capable of talking as disk drive). All devices connected to the serial bus receive data transmitted over the bus. To allow the host to route its data to an intended destination, each device has a bus address (known as device number). Disk drive's device addresses are 8-11 (8 is normal).

Data and control signals as follows:

Pin No. Signal Direction Description
Pin 1 SRQ (Service Request) in/out Used by fast serial bus as bi-direction fast clock line. Unused by the slow serial bus.
Pin 2 GND (Ground)   Logic ground
Pin 3 ATN (Attention) out The host brings this signal low which then generates an interrupt on the controller board. The attention sequence is followed by device address. If the device does not respond within a preset time the host will assume the device addressed is not on the bus.
Pin 4 CLK (Clock) in/out This signal is used for timing the data sent on slow serial bus (software clocked)
Pin 5 DATA in/out Data on the serial bus is transmitted one bit at a time (software toggled).
Pin 6 RESET out This line will reset the peripheral upon host reset.

The 6-pin DIN connector looks like (from outside):

In detail, the 1571 serial bus supports the new FAST serial communication as well as standard (SLOW) serial communication.

The important difference between the FAST serial bus and the SLOW serial bus is the incorporation of the hardware controlled lines for the CLOCK and DATA lines. Fast serial communication is transparent to any peripheral connected to the serial bus that does not contain the necessary hardware or software to talk at fast speed.

To remain compatible with the SLOW serial bus all bytes are sent under attention are sent slow.

Source: Commodore Disk Drive 1570/71 User's Guide