Volume VII started off interesting right off the bat with page 15.49.01.1. First of all, I ran into more cases where “B” level signals fed into DOT functions, and recognized these situations are ANDs because the gates transistors go to 0V when on (logic 0, rather than one) and go to 6V when off (logic 1, rather than 0). Any single transistor turning on is enough to bring the signal to logic 0 – a logical AND. This triggered a move to have logic generation recognize when a DOT function is feeds a gate with a “B” level input or feeds a sheet edge with a B level signal, and makes it an AND – which I then later changed to snagging the logic function to use out of the signal levels table.
Also, on this same page, I discovered that I had mis-interpreted the logic function for two logic gates on card type DND as NOR, when they are actually NAND. As a result of investigating what to do about page 15.50.05.1, I also discovered I had goofed on card type YLJ as well, confirmed by comparing the logic on page 16.20.13.1 with the ILD figure 30. (Later I also found the same miss-interpretation for card type YLH).
Trouble in River City, page 15.50.*.1
Page 15.50.05.1 (and, really, all of the pages 15.50.01.1 – 15.50.09.1) are problematic, because they are not for the IBM 1410 Accelerator feature. This feature uses a fair amount of DTL (IBM DDTL) logic to speed things up. The card location diagram for panel 11C3 shows the DTL cards, but the ALD’s I have don’t match, and generally use SDRTL. For the most part, it doesn’t matter to the actual logic that I will produce for the FPGA. Rather than figure out exactly what the DDTL would be, I have been deferring to the ALD’s. However, on page 15.50.05.1, I ran into a problem. that page takes two inputs not shown on the ALD to produce an output which is also not shown on the ALD (or ILD, for that matter) and that signal is used on page 15.50.10.1 – which is for the accelerator feature.
The input signals are: “-B B CH MIN+INV PLUS SIGN GATED” (which I will refer to as signal “A”) and “-B B CH PLU+IN MIN SIGN GATED” (which I will refer to as signal “B”). The page also produces an output signal, “+B B CH PL+INV MIN SIGN GATED”, which I will refer to as signal “C”). Note that the wording of the output signal is slightly different than the input signal “B”, and is also +B vs -B.
Page 15.50.10.1 also uses signal “A” “-B B CH MIN+INV PLUS SIGN GATED” which originates on page 16.16.06.1, and it is , logically speaking (B CH Minus and Use B Ch Sign) or (B Ch Plus and Use Inv B Ch Sign) as one of many signals that can set the NOT Zone C Bit.
The ILD for page 15.50.10.1 does not match the ALD – it shows a pair of signals used among others in a large “OR” arrangement to set the ASSM Ch Zone C Bit: B CH Plus Sign Gated and B CH Invert Minus Sign Gated – which clearly matches signal “C”), and does not show “A” at all.
The upshot is, the equation for signal “C” might be as simple as C <= not B, but it is quite possible that it should be C <= not B and A. That latter equation would be equivalent in most circumstances, should not be harmful, and would automatically prevent this signal from setting the Zone C bit when its partner (signal not A) is setting the NOT Zone C bit – so that is what I chose to do.
To make that happen, I “fudged” some gates. I had on sheet 15.50.05.1 a DND gate (B level inverter) and a YLJ (NAND, B level input S level output) from the card location chart (which is for the accelerator feature) that I was not actually using as they don’t appear on the ALD. That combination produces NOT(NOT “B” and “A”) – the desired logic, but at a “-S” level. But 15.50.10.1 expects a “+B” level input. So I “tacked” on a gate “borrowed” from card type YLK 11C3F05. YLK is a an inverter accepting “-S” level input and producing “+B” level output. I doubt that this is how it was actually done, but since my ALD is using mostly SDRTL (“S” level) logic, and there aren’t any accelerator feature ILD’s, I went with this approach. Time will tell if it works.
Back to the more mundane stuff
On page 15.62.01.1 I discovered a signal name error. Usually I catch these the first time I come across them. This one was a little different, because the origin page, being for the 1311 disk, is not present in my print set. On page 15.62.01.1 I had miss-read the signal as “-S RECEIVER LATCH*1311” when the correct signal name is “-S RECOVER LATCH*1311” – the latter also appeared on an earlier page as well. Easy fix.
Page 15.62.03.1, which controls the strobe of I/O data was a tiny adventure. First, although the VDHL I wrote to handle SMS gate type TAM has the capability of being forced reset by an attached DOT function, I hadn’t told the database to assign that function to the faux pin “T”. In addition, both of the triggers on this page have their gates tied to ground (since these are SDRTL, that means LOGIC ONE), but the database did not reflect that – an easy fix. Finally, the ILD figure 44 has an error where it shows the various buffer strobes going into an OR which feeds the forced reset on the E Ch Strobe trigger, when in fact they feed into the clock (gated set) input on the trigger. It was pretty obvious when the test I wrote based on the ILD failed the first time that forced reset came into effect.
Page 15.70.33.1, the last page in this volume, also had a signal name error: there was an extra blank on the signal name “-C CPU TO CH TAU C BIT” which prevented group generation from generating a bit vector for it. Easily fixed. (FYI, TAU on the IBM 1410 stands for Tape Adapter Unit).
Triggers are a Pain
In working on page 15.62.04.1 I was reminded that the SMS trigger circuits are a pain. I had already dealt with the fact that another single gate “DOT-ed” with an on or off output can force that output on (either the “on” output or the “off” output can be so affected.) A second issue, though, is that some inputs are typically AC coupled – edge triggered, if you will. That means tracking state, because once an AC coupled input has reached logic 1, it cannot trigger again until it goes back to logic 0. I had dealt with the edge triggering before, but on this page my test tripped up because the variables that were tracking the edge were undefined. That bit me for the first time on this page, because I had neglected to force the trigger counters (4 of them, as there are 4 AC inputs, each requiring tracking 3 FPGA clock pulses to look for a logic-one-going pulse) to reset when the trigger is forced to a 1 or 0 – just initializing them to logic 0 didn’t help. This issue cropped up with card type TAM, which I fixed, but could also happen with card type DEY or DEZ (and maybe others I haven’t run into yet) as well. Did I mention that triggers are a pain? 😉
And, indeed, page 15.62.05.1 had the same issue, with card type SMS_DEY. Did I mention that triggers are a pain? 😉
Another thing about triggers: sometimes they are daisy-chained together, which means one has to be very careful about the relationship between test assertions and clock pulses. In a couple of tests I had to save what the state of the first trigger in a chain WAS before clocking in the 2nd trigger, which sets the 2nd trigger based on the state of the first one, but also clears the first one. Did I mention that triggers are a pain? 😉
Work to do merging switches
Not surprisingly, several switches on the IBM 1410 are multi-deck affairs. Right now the HDL group generation code does not recognize them, such that if a group contains multiple decks of the same switch, it gets it wrong. What it ought to do is recognize when the name of the switch and its number of positions match, and use a common bit vector for them. Until this gets fixed, I have to either test such pages separately, or tweak the generated group and test bench by hand.