31-08-2020, 08:55 PM
The IBM PC used drives with only 2 drive select lines and 2 motor enable lines.
This fact causes some confusion on how to connect up drives to a microbee.
The microbee disk controller was designed to be used with drives with 4 drive select lines DS0-DS3 with DS0 being used for A: drive and DS1 for B:
DS2 and DS3 are actually were used by special in house microbee systems to handle multiple drives formats e.g. 400K 3.5, 800K 3.5, 5.25 400K and 5.25 800K.
(At microbee we even had the 3.0" floppy disk drives like the Amstrad CPC and Sinclair ZX Spectrum working but chose to use the 3.5" drive as used by the Apple Mac.)
The 34 pin Floppy disk drive cable is an IBM PC quirk that was done to expedite PC manufacturing.
It simplified their PC manufacturing process by allowing the Drive select jumpers on the floppy disk drives all be set to Drive #A and have the drive address determined by the cable assembly.
The IBM PC floppy disk adapter supported signals two drive select signals DSA(pin14) and DSB (pin 12) with pins 10 and 16 assigned as corresponding Motor Enable signals.
IBM also shoved a peg into the connector socket pin 5 and removed the corresponding connector pin as a key to prevent insertion of the cable incorrectly.
Watch out for the variations with the microbee Floppy disk controller schematics. The DRAM coreboard uses the correct even numbered pin annotations for the disk drive control signals and odd pins as GND return pins. Some early schematics have the connections flipped with the disk drive control signals on the odd numbered pins and GND returns on the even pins.
The net result is just the position of the connector key being either on the top or bottom side. It was not uncommon to file off the connector key to make things fit.
This fact causes some confusion on how to connect up drives to a microbee.
The microbee disk controller was designed to be used with drives with 4 drive select lines DS0-DS3 with DS0 being used for A: drive and DS1 for B:
DS2 and DS3 are actually were used by special in house microbee systems to handle multiple drives formats e.g. 400K 3.5, 800K 3.5, 5.25 400K and 5.25 800K.
(At microbee we even had the 3.0" floppy disk drives like the Amstrad CPC and Sinclair ZX Spectrum working but chose to use the 3.5" drive as used by the Apple Mac.)
The 34 pin Floppy disk drive cable is an IBM PC quirk that was done to expedite PC manufacturing.
It simplified their PC manufacturing process by allowing the Drive select jumpers on the floppy disk drives all be set to Drive #A and have the drive address determined by the cable assembly.
The IBM PC floppy disk adapter supported signals two drive select signals DSA(pin14) and DSB (pin 12) with pins 10 and 16 assigned as corresponding Motor Enable signals.
IBM also shoved a peg into the connector socket pin 5 and removed the corresponding connector pin as a key to prevent insertion of the cable incorrectly.
Watch out for the variations with the microbee Floppy disk controller schematics. The DRAM coreboard uses the correct even numbered pin annotations for the disk drive control signals and odd pins as GND return pins. Some early schematics have the connections flipped with the disk drive control signals on the odd numbered pins and GND returns on the even pins.
The net result is just the position of the connector key being either on the top or bottom side. It was not uncommon to file off the connector key to make things fit.