Here is the inside of IBM 5150:

enter image description here

PCB 1 sits on top of a floppy disk drive. As shown in the picture, the floppy disk drive is connected to PCB 2, where PCB 2 is plugged in one of the ISA-8 slots on the motherboard. This makes PCB 2 a 'floppy disk drive controller'.

Since the controller is PCB 2, then what is PCB 1? What is it called? What is its purpose? Is it a part of the floppy disk drive or is it something else?


closed as off-topic by Scott Seidman, Sparky256, Brian Carlton, Bence Kaulics, Daniel Grillo Jul 11 '16 at 11:26

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    \$\begingroup\$ Yes it is part of the floppy disk drive. For example, PCB 2 will set the "Direction" and "Step" signals on the interface cable, and PCB1 will interpret those signals and drive the stepper motor to move the head in or out. \$\endgroup\$ – Brian Drummond Jul 10 '16 at 20:26
  • \$\begingroup\$ Don't forget about determining head position from the tracks it's reading, setting the proper locations, and moderating read and write commands issued from the controller. \$\endgroup\$ – Daniel Jul 10 '16 at 20:37
  • \$\begingroup\$ @BrianDrummond And what is the name for PCB 1? If I understand correctly, the 'controller' is PCB 2. Is PCB 1 called controller as well? If not, what is it called? \$\endgroup\$ – Utku Jul 10 '16 at 21:01
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    \$\begingroup\$ I'd probably call board 1 a drive controller, and board 2 a floppy disk interface. \$\endgroup\$ – Peter Bennett Jul 10 '16 at 22:14
  • \$\begingroup\$ VTC. Off-topic for EE.SE. \$\endgroup\$ – Sparky256 Jul 10 '16 at 23:10

PCB1 is part of the floppy disk drive. It takes care of low level drive functions such as amplifying the analog signals going to and from the read/write heads, producing the 4 phases that operate the head stepping motor, switching power to the drive motor etc.

It interfaces to the Floppy Disk Controller/Interface card (PCB2) via a 34 way ribbon cable. The particular interface 'standard' used in the IBM PC (and been adopted by most other manufacturers) was designed by Shugart Associates.

Most of the signals in the cable are common to both drives - only the Drive Select and Motor On lines are different. This allows a single cable to be 'daisy-chained' from one drive to the other. In the cable between the drives you can see that part of the ribbon is twisted in the middle. This swaps the Drive Select and Motor On lines over so that both drives can have the same pinout but be selected as drive A or B depending on where they are plugged into the cable.

The Floppy Disk Controller interfaces between the CPU bus and the floppy drive. It takes commands and data from the CPU and operates the various interface lines that tell the drive to do things such as turning the motor on, stepping the heads from track to track, and enabling the read or write circuit. It also converts the data from parallel bytes to the special MFM serial format that is required to store it on disk.

  • \$\begingroup\$ So the controller is PCB2. Does PCB1 have a 'special name' like this? \$\endgroup\$ – Utku Jul 11 '16 at 7:36
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    \$\begingroup\$ Generally the floppy drive PCB is not considered to be a separate unit, since it is an integral part of the drive. The SA400 OEM manual simply calls it the 'drive pcb'. classiccmp.org/dunfield/r/sa400.pdf \$\endgroup\$ – Bruce Abbott Jul 11 '16 at 10:14

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