Text 128, 87 rader
Skriven 2006-05-31 18:56:38 av Peter Knapper (3:772/1.10)
    Kommentar till text 124 av Bob Ackley (1:2905/3)
Ärende: eCS 4 of 4.
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I managed to order some manuals from IBM, on the 360, DOS and COBOL.
They must have wondered why anyone was ordering them, but to their
credit they didn't take long to arrive and they were very cheap. I was a
little coy because I wasn't too sure about the provenance of the machine.

Sometime around December 1982 I had a stroke of luck when I saw
advertised an IBM 2203 RJE (remote job entry) station. This was being
disposed of by a government department, and they put it up for tender. I
tendered $151 for it, and got it. I think I was the only tenderer - I
could probably have saved $150. The boss of the office that had used it
was quite amused, maybe they expected to get $10000 for it or something.

The RJE station consisted of a card reader and a line printer. As I
didn't have a card punch, automatic or manual, the reader wasn't much
use, but the printer was. Unfortunately the whole system was designed to
run to a mainframe via a modem, so access to the whole thing was via a
serial link, and these were somewhat lacking on the /360. Still, once it
was moved into the office I was able to play with printing out the test
card deck, and again it came with a good set of manuals.

The printer was a reciprocating band type, in which the typeface moved
back and forth, and a hammer in each print position fired as the
appropriate character went past. With a full upper-case alphabet, and
numbers, I think about 44 characters, it would do 300lpm, which would
still be quite respectable. It made an awful racket, as the hammers were
all cocked back for each line, then fired with a screech.

Anyway, back to connecting this station to the system. I decided that if
I was going to do anything much I would need to have some way of getting
data in and out other than the console typewriter. There was nothing for
it but to design my own multiplexer channel peripheral. I can't believe
it now, but with the benefit of the operation manuals, circuit diagrams
and the microcode listings, I was able to deduce the operation of the
channel.

I decided to make a unit based on the 6802, as I was familiar with 6800
programming, and worked out the minimum amount of add- on circuitry
necessary to talk to the /360. Not having any spare channel cables (and
because a channel connector would be larger than the whole
microprocessor assembly), I wire-wrapped a new connector onto the
appropriate points within the CPU and ran a piece of flat cable out to
my little unit, including power feeds.

The channel interface circuitry was quite tricky, as the 6802 couldn't
respond as fast as the 360 required, necessitating the use of latches
and comparators to respond to the device addresses. Luckily, once a
transfer cycle was underway speed wasn't a problem. All transfers were
done a byte at a time, there was no buffering and no use of 'burst' mode.

The 6802, together with 4 6850 serial chips, looked like 8 independent
subchannels, numbered 8 through F to tie in with the default device
assignments. The 6802 could cope with all 8 running simultaneously. I
can't remember how I set the baud rate for each channel, I think there
were rotary switches. The RS-232 handshaking lines were used as the
status indicators when a program queried the device's readiness. One
full-duplex RS-232 link was run to the RJE station, and another to a
"System 80" (TRS-80 clone) which served as my data entry station.

I did have a few mysterious problems with my device causing the CPU to
stop with a channel check for no apparent reason. It happened so rarely
that I couldn't track it down, but it was a bit annoying (pun not intended).

That aside, I was now able to invoke the COBOL compiler, feed a program
in from the System 80 and produce a listing on the line printer. I wrote
a few simple programs in COBOL, number guessing games and the like, but
my heart wasn't really in it. The fun had been in getting the whole
system going - once it was going I had no use for it.

After that the system languished. I occasionally fired it up to
demonstrate it, but eventually I gave up paying the rent on the office
and had the power disconnected. The building has gone through several
owners since then. At some stage someone broke the office door down and
stole the bookshelf that held my manuals. I haven't been back there since.

My experiences with the system have proved useful. I have a great
respect for IBM hardware, and for the effort that goes into the design
of any system that complex. I am glad it wasn't a 360/91, that little
/30 was a lot cosier.

Lawrence Wilkinson
Auckland, New Zealand
May 18, 1993
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