St251 disk drive optional rom question

[Hugo Holden]

I have installed an ST251 hard drive in an IBM5155. It failed with a shorted tantalum capacitor after a few minutes, I replaced that temporarily, and another cap failed. In any case that problem is under control and I'm waiting for new caps to arrive for a permanent fix. Along the way after removing the pcb I noticed that this drive appeared to contain a rom see photo : But assorted other ST251 drives do not have this rom. I have another two drives one with and one without and I have seen photos of drives without it, just the place and pcb tracks where it goes. So clearly the rom is not required for operation of the drive and it is "optional". The question is, when this rom is present, what does it do or what was its intended application ?

 

[SomeGuy]

The way these boards use microcontrollers, they would all need ROM somewhere. If they don't have the exact same through-hole sized ROM then they probably use a smaller surface mount ROM chip, or more likely the microcontroller chip would contain a integrated ROM (PROM or EPROM). So what you are seeing is probably just the variance in parts they used over time.

 

[Hugo Holden]

I'm not sure if this is the case, comparing two pcb's one with the rom and one without: The one without the rom also has a missing (not fitted) 74LS373 IC that is connected to some of the rom pins, but every other IC on both pcb's are identical in appearance and numbering. Also the rom's connections (or the 74LS373) are not connected to any substitute parts or additional surface mount rom chips elsewhere. The circuit tracks are also identical in both versions of the pcb. So the drive can run with or without the rom (and the associated 74ls373) fitted it would appear, so it remains a mystery what these additional parts achieve when they are present.

 

[modem7]

I'm not sure if this is the case, comparing two pcb's one with the rom and one without: The one without the rom also has a missing (not fitted) 74LS373 IC that is connected to some of the rom pins, but every other IC on both pcb's are identical in appearance and numbering. Also the rom's connections (or the 74LS373) are not connected to any substitute parts or additional surface mount rom chips elsewhere. The circuit tracks are also identical in both versions of the pcb. So the drive can run with or without the rom (and the associated 74ls373) fitted it would appear, so it remains a mystery what these additional parts achieve when they are present.

Sounds like the difference between two known versions of the Seagate ST-11R controller.
1. DUAL ROM: http://www.minuszerodegrees.net/rom/photo/seagate_st11r_dual_rom.jpg
2. SINGLE ROM: http://www.minuszerodegrees.net/rom/photo/seagate_st11r_single_rom.jpg

From memory, member alecv worked out that on the single-ROM version, one of the chips was slightly different to that on the dual-ROM controller, and that chip had a PROM component.

... but every other IC on both pcb's are identical in appearance and numbering.

100% positive ?

 

[Hugo Holden]

Hi Modem 7

The question relates to the PCB that is part of the ST251 drive assembly, not the ST11 drive controller card. The IC numbers I referred to are the two variants of the pcb in the actual ST251 drive (not the controller card) that are the same regardless whether the rom is on their pcb or not.

Either type of ST251 drive (with the ROM in it or not) works with my ST11 MFM controller card, which has the single rom on it. So it still remains a mystery what the application of the rom, present on some (but not all) ST251's own pcb is. Its not likely related to the controller card which, with its single rom , or dual rom, works in both cases for both the variants of the ST251 drives.

I'm wondering if it might be there for some sort of identification purposes .

 

[SomeGuy]

If the other chips really are completely identical, it may be that any on-board microcontroller PROM was already programmed. Perhaps they used batches of "used" pre-programmed microcontollers, or perhaps the boards were programmed for one model of drive and switched to a different one by adding the ROM. It could also be that the additional chip acts as an "update", if they found some issue with pre-programmed code. While you could be right that it adds some extended functionality, I can't really imagine what they would add in that kind of drive.

 

[Hugo Holden]

Yes, I think that would explain it very well. Probably the drive pcb itself was pre-programmed for another type of drive, and the added rom (which also has the st251 label on it indicating that the rom is for that unit) converts the drive pcb to run the st251. And some of the their other pcb's already had the correct pre-programmed micros and therefore didn't need the added rom on them.`

 

[modem7]

The question relates to the PCB that is part of the ST251 drive assembly, not the ST11 drive controller card.

Yes I know. I did not even know that you had an ST-11R controller. So, coincidence. I was just using the ST-11R as an example of where a manufacturer appeared to be cutting production costs by (presumably) paying a little bit more for one of the board's existing chips, and consequently being able to eliminate two chips.