Friday, October 2, 2015

Design choices for finishing SAC Interface Box and lots of research


Another day studiously ignoring the d****ned spring. By the weekend I should be over my snit and able to take on the fight once again.


Tape drives

I began thinking of how I would provide the tape drive connection to the 1130. I am leaning towards a laptop running Linux, on which to install Python and use a USB to parallel SCSI adapter to hook to my tape drives. I should be able to get something reasonable for a couple hundred dollars. That will let me keep the laptop out near the SCSI Interface Box and the 1130 system.

I also need to model the tape drive device adapters to match any software that might be using the tapes. I don't have any documentation for this in the FE or user manuals, since it was an RPQ (custom feature which had been developed for one customer and could be ordered by others, Request for Price Quotation) and not a standard feature in the price book.

The IBM 1800, which is a sister machine to the 1130, does support the 2401 and 2402 tape drives, with documentation for how to access them in the 1800 documentation. However, the 1800 uses them with an internal adapter at area code 14. The RPQ for the 1130 instead uses the tape drive controller and S/360 channel connectors, which go to a  360 Selector Channel installed in the 1133 Multiplexor box on the SAC.

The tape drives are addressed by the Selector Channel at area code 27 and use a different set of XIO functions that are implemented on the 1800. The 1800 would issue an XIO Init Read to read a block from tape, or an XIO Control to rewind, space, erase, etc. The 1130 issues the special Init IO function which points to 360 style CCWs (channel Command Words) and the three word long channel status word information not the 1800 DSW.

This means that if I want to meet my goal of allowing software written to control tapes on 1130s with this RPQ to run properly with my real tape drives, I have to implement the selector channel function, use area code 27 and the special XIO functions, then interpret CCWs and CSWs. I can try to interpolate this from the S/360 documentation on EXCP and 2401 drives, but there will remain some ambiguity.

I am not sure I can fully resolve this without working software to test against, and even then I might miss behaviors that the software doesn't trigger. I could easily implement these with my own area code and defined behavior, but that would mean that any software would have to be written for my devices specially. Any chance of finding a tape sort/merge program and running it to watch the tape drives spin would be thwarted by the incompatible decisions I might make.

Another hiccup I discovered is that while one of my drives has a SCSI interface, I suspect that the newly purchased one that will arrive Tuesday uses an IPI-3 interface. Yikes. No adapters I can find, no converters, no IPI controller cards - nada. I might have to do this one myself on an FPGA, which will be a major pain, particularly when I was close to a turnkey kind of solution with a Linux box that would let Python drive the SCSI tape devices.

Other device types

Other potential devices that I could implement include 2311 disk drives, which attach through a 360 channel installed in the 1133, giving me the same challenge of extracting adequate documentation. Since 2310 drives provide enough storage (five total cartridges online), I wouldn't need the extra logical drives that come from a 2311. Thus, I won't try to implement this.

The 1231 Optical Mark Reader would scan test forms such as the SAT tests and record the results - not very interesting. The Synchronous Communications Adapter (SCA) provided BSC or a similar communications protocol at 300, 1200 or 2400 baud over a Bell Dataset and phone lines. It would allow the 1130 to operate as a remote job entry and printing station, but that isn't interesting enough to me to warrant implementation.

That leaves the 2250 Graphical Display Unit was a vector graphics tube and light pen used for CAD and similar graphical work. It was pretty expensive relative to an 1130 and thus was typically installed on larger 360 systems or only when the 1130 was a remote graphics station hooked to a bigger 360. There is limited software for the 2250 that runs on the 1130, unless you are using it remotely from a 360. I might implement this just for the challenge, especially since it is fully documented unlike some of the other potential peripherals. 

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