Archaeology: MFM / ST-506 Interface Reference

The ST-506 interface relies on 2 cables: one 34-way control cable telling the drive electronics where to move to and what to do, and one 20-way data cable whose function is obvious given its name.  Drives typically use an IDC edge connector, controllers typically IDC pin connectors.  A 5.25″ floppy cable will work as a controller cable as long as Pin1 is in the right place and the cable has no twist!  I dismantled a floppy cable to add the connector to another cable, making a 2 drive control cable – ensure Pin 1 is connected correctly as if you fit the connector upside down it won’t be…

Up to 4 drives can be connected dependent upon the controller, cabling and power supply.  The control cable simply connects all drives in parallel, and a separate data cable is needed for each drive to the controller card.

There is a termination resistor on each drive, too.  This must only be present on the last drive on the controller cable.  Each drive has an ID, assigned by a jumper, from 1 to 4.  Unless you want to corrupt disks, ensure each is assigned a unique ID!

You can have drive ID 2 as the first drive in the chain without termination, so if you are adding a drive to a system you can leave the original drive alone if it’s going to be on the end of a chain: remove the terminator from the new drive, and change the ID.