IBM channel-attached DASD emulation

Henk Stegeman h.j.stegeman at
Mon Dec 28 13:09:29 CST 2015

Hi Mike,

A few corrections:

1) Some disk OEM's produced 3340 compatable diskdrives in the 70's.
The OEM manufacturer connected their equipment direct to the internal S/3
I have only seen 1 CPU at a scrapper which had this.

2) The BUS/TAG connector/cables are the same as used by IBM for 360/370
channel, but are used between the IOP and the 3340 disk string.
They do not carry 360 channel signals but special IOP --> disk signals.

3) CPU --- int chan ---> IOP ---> IDE/SCSI drive. (IOP has modified HW & FW)

Regards Henk

On Sun, Dec 27, 2015 at 10:29 PM, Henk Stegeman <h.j.stegeman at>
> Hi Mike,
> Here are the options to attach 3340 DASD emulation to an IBM System/3
> 1) direct connect to the internal channel of the S/3. Some disk OEM's did
> this.
> Requires a FPGA with at one side MST-1 interface logic and at the other
> IDE or SCSI interface.
> The availabe IBM documentation is complete enough to finish this project.
> You need good IBM HW & VHDL knowledge.

Oh interesting - I had no idea! Do you have names and product numbers
for those OEMs? Any more info? Ever seen one?

If they were sold commercially by OEMs then the project already IS
finished so to speak; it's just a question of *finding* one! Or
finding the documentation and using that as a basis for re-creating
the OEM solution.

It doesn't require any hard-to-perform or reverse changes to the S/3?
No backplane rework or wire wrap? It was just a case of plugging OEM
cards into appropriate slots?

> 2) direct connect to the BUS/TAG interface connector.
> This needs a 8+P bit stream. You have to do some reverse engineering to
> figure out what the exact format is.
> I have no idea if the HW must be implemented in a FPGA (for timing
> or if an AVR processor will do the job.
> Advantage is that you don't have to modify anything inside the S/3.

And the other advantage is that it could be part of a more
generalizable device that could be used to replace other Bus/Tag
peripherals on other IBM systems... if some of us were to start a
home-brew project that might be the best approach to take.

> 3) IBM has implemented an IOP (I/O Processor) between the S/3 CPU and the
> 3340 drives.
> This IOP is a modified version of the ones used in the IBM 370/115 & 125.
> is a powerfull multi thread capable beast.
> With small HW modification you can connect an IDE or SCSI drives to it.
> The difficult part is modifying the firmware of the IOP. This requires a
> special assembler (to be written) and very good assembler skill's.

If IBM have already implemented it then ready to use IOPs exist or at
one time existed out there in the wild... your reference to modifying
IOP firmware... do you mean to connect to the S/3 - or to attach IDE
or SCSI? Or both? Does this IOP have an IBM model number? Was it a
separate box or a set of cards and backplane that could be installed
in the 370? I hope to be getting a 370/125 next year...

Fascinating info Henk; I had no idea any of this stuff existed. I
thought the only options for S/3 5415 were real hardware 3340s or


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