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µPD765 - Floppy Disc Controller (used in [[DDI-1]] and CPC 664/6128).
The ports used by Amstrad and compatible interfaces use:
* Port FA7Eh - Floppy Motor On/Off Flipflop
* Port FB7Eh - FDC 765 Main Status Register (read only)
* Port FB7Fh - FDC 765 Data Register (read/write)
The Vortex disc interface uses other ports - exactly how this works has not yet been discovered.
== IC Models used in CPC ==
More than one manufacturer made 765 compatible ICs. These are the ones known to be used in the CPC by looking at pictures of CPC mainboards.
All should operate almost identically.
* FDC9216
* SED9420C
== Accessing the FDC 765 ==
The Data Register (Port FB7Fh) is used to write Commands and Parameters, to read/write data bytes, and to receive result bytes. These three operations are called Command-, Execution-, and Result-Phase. The Main Status Register signalizes when the FDC is ready to send/receive the next byte through the Data Register.
a. Command Phase
A command consists of a command byte (eventually including the MF, MK, SK bits), and up to eight parameter bytes.
b. Execution Phase
During this phase, the actual data is transferred (if any). Usually that are the data bytes for the read/written sector(s), except for the Format Track Command, in that case four bytes for each sector are transferred.
c. Result Phase
Returns up to seven result bytes (depending on the command) that are containing status information. The Recalibrate and Seek Track commands do not return result bytes directly, instead the program must wait until the Main Status Register signalizes that the command has been completed, and then it must (!) send a Sense Interrupt State command to 'terminate' the Seek/Recalibrate command.
== Motor On/Off Flipflop ==
Writing 00h to Port FA7Eh turns all disk drive motors off, writing 01h turns all motors on. It is not possible to turn on/off the motor of a specific drive separately.
An exception are the Vortex F1-S, F1-D, M1-S and M1-D drives.(How are they different?)
== Notes ==
Usually single sided 40-track 3" disk drives are used in CPCs. For practical purposes, 42 tracks could be used — the limit is specific to the drive and some support more tracks but 42 is a good maximum. The FDC controller can be used to control 80-tracks and/or double sided drives, though AMSDOS doesn't support such formats. AMSDOS supports a maximum of two disk drives only.
== FDC Track Format ==
[[File:FDC765 Track Format.png]]
== Internal details of the chip ==
* [https://scarybeastsecurity.blogspot.com/2020/11/reverse-engineering-forgotten-1970s.html The decapped photos of Intel 8271, NEC D765 and Intel 8272 chips] prove that NEC D765 and Intel 8272 chips are identical.
* From [https://hackaday.com/2012/08/13/taking-a-look-at-decapped-ics/#comment-734991 a comment in the hackaday website]: "Internally this is a microcoded part with a primative controller of NEC’s own design. Testing microcode embedded in a part can be troublesome. The uPD765 had a few extra gates associated with the DMA Request and DMA Ack pins. Presenting a certain illegal combination here places the part into a “test” mode and allows the sequencer microcode to be output on the normal Data pins. The sequencer microcode is responsible for high level commands such as Read Track, Recalibrate, Format Track, or Write Data. There is a similar test mode for the nano-code array which serializes data at the floppy disk head."
== FDC Block Diagram ==
[[File:FDC Intel 8272A block diagram.gif]]
== Generic System Diagram ==
The Amstrad CPC and Amstrad Plus do not have a DMA controller associated with the FDC. The INT pin of the FDC is not connected either.
[[File:UPD765A System Diagram.png]]
== 3-inch Floppy Drive Interface ==
[[File:Interface FDC - Floppy drive.png]]
== FDD Block Diagram ==
[[File:Floppy Disk Drive - Block Diagram.png]]
== Datasheets ==
* [[Media:UPD765 UPD765_App_Note_Mar79.pdf| NEC uPD765 Datasheet OCRedpreliminary (1979)]] [[Media:D765 NEC.pdf]] [[Media:UPD765-NEC.pdf]] [[Media:Z765A datasheet.pdf]] - uPD765 disc controller* [[Media:Intel 8272A Datasheet preliminary (1982).pdf]] [[Media:Datasheet.hk_d8272a_2873060.pdf|Intel 8272A Datasheet (1986)]] - Licensed clone of the uPD765* [[Media:FDC9229BT Datasheet.pdf]] - FDC9229BT data separator (that usually assists the uPD765 chip)* [[Media:SED9420.pdf]] - SED9420C data separator
* [[Media:DDI Schematic.png]] - DDI-1 Schematic (disc interface for CPC464)
* [[Media:Panasonic-3 inch Floppy Drive EME-150.pdf]] Panasonic EME-150M 3inch Floppy Disk Drive Datasheet
* [[Media:NEC_FD1035_Floppy.pdf]] - NEC FD1035 3.5inch Floppy Disk Drive Datasheet (1984)
== External links ==
*[http://quasar.cpcscene.net/doku.php?id=assem:fdc Quasar FDC documentation (in french)]
*[https://64nops.wordpress.com/2021/07/04/a-la-decouverte-du-fdc/ FDC blog articles (in french)]
*[https://info-coach.fr/atari/hardware/FD-Hard.php Atari ST Floppy Disk hardware analysis]
[[Category:CPC Internal Components]][[Category:Programming]][[Category:DATA Storage]][[Category:Electronic Component]]
