Commodore 1570 User Manual page 125

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 sent under attention are
sent slow.
117