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IBM ST506 MFM Adapter

@DFFD.ADF IBM ST506 Fixed Disk Adapter

60/80 HD Cabling Schematic

Early MFM Adapter
   Card Identity
Later MFM Adapter
IBM MFM Implementation

Early MFM Adapter

P/N 72X8540, "6127874B" |

Equivalent to Model 50 MFM Adapter "6127873B".

U30 is covered with a grey-black resilient compound with a "shield" plate pushed into it (see below).

The card-edge connectors at the top are not labeled.

Card Identity

From Peter (edited):
   The IBM MFM adapter P/N 72X8540 is an early downlevel card, which has been withdrawn with ECA 002, service code 33, available from 87-06-17 / IBM Boca Raton. This adapter has been used in early PS/2 Models 8560-041 with serial No. range 8001342 - 8009651 (US production only).

The ceramic shield has been obsolete on the new redesigned MFM adapter for Models 60 & 80. These adapters were already "factory reworked" cards - the very first series had the U30 module without the shield and experienced "sudden death" due to some sensibility against electrostatic discharge. Therefore the shield. The P/N for the various cards stayed unchanged.

Source: IBM Engineering Changes Group 819 - PC-Family / PS2 Family Service Information Manual, IBM Doc.No. SR28-0280-2 / 3rd Edition Nov. 1987

Later MFM Adapter

P/N 90X8643 |

Equivalent to Model 50 MFM Adapter 90X8642.

U18 P8051AH "0649" - some samples are marked "93X3903 Y15".

IBM MFM Implementation

Charles Lasitter asks:
   I've had an inquiry about how the 72X8540 ST-506 / MFM adapter works in Model 8560 computers, and specifically I'm wondering about the usual stuff: Is this some Unique IBM flavor of ST-506, such that "Don't Bother!" is the word of the day when it comes to considering non-IBM MFM drives?

Any hope of substituting larger drives for this machine, and if so, what's the point at which it will freak out over translation issues and require exquisitely unique device drivers to step in front of the operating system and hide the messiness?

From Peter:
   From the principle the stuff IBM used there is the usual Western Digital stuff... with the major difference of a fixed BIOS-resident lookup table with fixed values and no "User Type". Back in the glorious old days we used to solve this problem with a software called "SpeedStore". You enter the BIOS type of a drive which comes closest to the one you want to install and then override the CMOS settings with the software and a boot-sector resident driver.

Another significant difference: IBM has castrated the 4-device ST-506 interface down to 2 devices with altering the device addressing a bit. All drives have to be set to "second drive" (DS1 when counting "0"-based from DS0 to DS3). The two possible drives are addressed with the motor-on and drive select lines - and a twisted cable for the first drive, which "corrects" the false addressing logic. The IBM PS/2 BIOS also and consequently supports only two MFM drives (and two ESDI as well... they repeated the mistake there again).

This part is still missing in the PS/2 Reference PDF section. I *think* I have the MFM controller HITRM or TRM anywhere... but I might be wrong. I PDFed the ESDI and SCSI controllers - which seemed the more important to me.