IBM channel-attached DASD emulation

Dave Wade dave.g4ugm at
Sat Dec 26 03:54:01 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Mike Ross
> Sent: 26 December 2015 01:55
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at>
> Subject: IBM channel-attached DASD emulation
> I've recently been poking about with various bits of emulation with hardware
> interfaces... Dave's MFM emulator; various SCSI-to-USB or SCSI-to-SDcard
> devices; my Setasi RP12 Massbus disk emulator; the Sigma Seven Lisa
> widget/ProFile emulator etc.
> What about IBM channel-attached DASD?

The Channel Protocols and Electrical Characteristics of the channel are reasonably well documented. E.g.

but IBM DASD are complex in that the physical block size can differ from track to track, and can be changed by the user.

> There are various CPUs lying around in private collections and museums
> - System/360s; System/370s; System/3 Model 15s; all used channel-attached
> DASD: and working reliable disks are much rarer than the damn CPUs!

The ICL2960 at TNMOC at Bletchley used similar disks. It now has emulated disk attached. 
I don't think there are any working S/360's or even any that are complete enough to be restored.
The same goes for real S/370. There might be 30xx or 43xx boxes but those are not 370/s..

> Questions:
> 1. There are or were various 3rd party companies producing rather obscure
> emulated DASD replacement subsystems - Virtualblue and Bustech are two
> names that come to mind. Has anyone looked into the possibility of using
> them to emulate older devices that would be usable on the above vintage
> CPUs?

These seem rarer than the original DASD and less well understood. Probably as hard to keep running as the originals.

> 2. To those with hardware design experience: how big a task do you reckon it
> would be to do this as a home-brew with modern hardware - exactly as Dave
> did with his MFM emulator? Is it feasible? Do the entire thing in software - Pi
> or Arduino or FPGA - with appropriate driver electronics to drive a channel
> interface?

The later IBM channel runs at 4Mbytes/second. Not sure software could keep up.

> I've BCC'd some experts with experience - Rich Alderson; William Donzelli;
> Henk Stegeman - in the hope that they'll be able to contribute.
> What do people reckon would be the best target for emulation? 3340 springs
> to mind initially... would that work on machines as old as System/360s? It's
> about the *only* option for 5415 DASD...

I think the hard bit is getting the channel protocol working, once that’s running I think emulating many models of DASD would be possible. 
As someone else mentioned Hercules already has code that allows disk files to contain DASD images in various sector sizes. 
It may be possible to use this to talk to a real channel. Implementing some of the channel protocol in FPGA might be necessary.
But I think finding a real S/360 would be harder...
..... but some one has an FPGA implementation...

> (To digress briefly - a modern reimplementation of something like the Setasi
> Massbus disk emulator would also be very useful; Rich - weren't LCM
> working on something like that?)
> Mike
> 'No greater love hath a man than he lay down his life for his brother.
> Not for millions, not for glory, not for fame.
> For one person, in the dark, where no one will ever know or see.'


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