Friday, November 6, 2015

Not much work time, but some research leading to potential to hook tape drives to 1130


Driver chip 3 seemed a likely suspect as it controls the six bits that are hot when I am loading a PC file in the 1130 core memory. However, I powered up the SAC Interface box and the 1130, where I didn't see any problem. I will have to keep watching this, because of the perfect alignment to the driver chip.

It is definitely not the SLT card in the 1131, since bits 6 and 7 are on one card and the remaining bits are on another, plus other bits on those cards work fine. No common point in the 1131 that would produce the symptoms seen.

I continued my careful study of the VHDL and all Xilinx warnings, tightening up code where it makes sense. Nothing stood out as a problem. Until I can get time to go into the workshop and test with more diagnostic information, I won't lick this issue.


I resumed my investigation into interfacing tape drives to the 1130, through my SAC box, as I now have enough to meet my two requirements - software which uses the tape drives and an adequately detailed description of the programming interface (XIO commands, interrupts, status words and so forth).

Peter Vaughan of the National Museum of Computing in the UK gave me the successful lead - a document on Bitsavers of a field developed program for accessing the mag tape drives from both assembler and Fortran - 00.3.003 - which includes listings of the programs including the interrupt handler.

It is clear that the attachment of the 2400 tape drives (usually 2415 models which had an integrated control unit that attach to a S/360 selector channel) was through the Selector Channel RPQ that put such a channel inside the 1133 Multiplexor box. This is adequately described in the Attachment Channel RPQ document on Bitsavers and matches what I see in the software.

The Selector Channel is accessed as area code 27 and interrupts on level 4 with ILSW bit position 12 indicating it is the source of the interrupt. This however adds a bit of convolution to the interface since the XIO Write function in the 1130 are used to pass a string of Channel Command Words to the selector channel, each three words long. Further, the XIO Sense Device uses its modifier bits to select which of four Channel Status Words rather than returning a single word of status.

The tape drives are programmed by CCWs just as they are on a S/360 system, which does allow me to reference the 360 library manuals for the 2400 drives to properly model the drive's behavior. The software supports a string of up to eight tape drives on a control unit.

Implementing a full selector channel involves a lot of capabilities - Transfer in Channel which is a branch to a new CCW location in core memory, Data chaining and command chaining, program controlled interrupts and other nuances. I am tempted to dig through the software and implement only those parts of the selector channel function that are used by the code.

I will set up the code from the manual as machine readable card images and test with the 1130 simulator to be sure I have a clean assembly and the code looks good. That can occur as I work out the Selector Channel RPQ function in the SAC Interface Box.

I will still have to figure out how I attach the actual tape drives - I have two that are SCSI based which would either require me building SCSI hardware into my SAC box or they could be attached to a PC with a SCSI card and have the data pass over the USB link. My current PC for the USB link and the Python program is a laptop running Linux so even the PC attachment alternative would be a bit more disruptive than it seems. 

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