Friday, February 6, 2015

Documation fix to read right cut cards


I finished up testing of the new beta version of the PC software. It is being uploaded it GitHub.


I developed a small daughter card with a single 7433 Open Collector Quad NOR gate chip on it. I had to interrupt one connection, inserting this gate in between the formerly connected points, as well as routing the signal for column 81 (called 81CR on the Documation schematics) onto this card and to my daughter card.

The new gate will force row 12 to register as dark when we are at column 81, otherwise it will pass through the state of row 12's photocell detector. The error check at column 81 will not look at the top row, assuming it is dark. A read check will only occur if light passed through rows 11 or below.

The defect in the Documation reader design is that a check during column 81 (one past the last data column of the card), is close to the diagonal notch that is cut on some card stock - as a result the row 12 photocell detects the light from around the notch. The error checking during column 81 tests to see if ANY row is lit, generating a READ CHECK if any are on.

I thought I had located the trace on the board that needed to be cut in order to separate the two pins that are ordinarily connected, then I tack soldered wires to the two pins, leading up to the daughter board. One of the backplane positions carries the 81CR signal, but it was not picked up on the control card where the other logic resides. I tack soldered another wire to the gold finger that slides into the MB-13 motherboard socket slot.

When I first cut a trace, I mistakenly cut the wrong one - not realizing there was a trace hidden directly under the ship on the same side of the board, connecting the output of one inverter to the input of the next, It was that hidden link that I intended to cut.  While the modification with the wrong trace cut did stop the read checks for right-hand-notched cards, it emitted row 12 as a hole for every column regardless of the actual card state.

I discovered the flaw, bridged the improperly cut trace and tried to find a way to sever the link underneath the chip. I didn't see a way to do that unless I unsoldered the entire chip and removed it first. I didn't want to do that. Instead, I cut the chip lead for pin 13 before it touched the motherboard and bent the pin remnant up to break electrical contact. With the output of my daughterboard tacked onto the pin 13 remnant, the damaged trace corrected by a bridge wire, and the other wires still tacked onto pin 1 and CR81, the tests worked as expected.

The daughterboard sits parallel to the control board but slightly above it. I placed it on the other end of the board from the chip it was tied into, because that was where the CR81 signal arrived onto the board. I wanted to ensure that the signal delay of the wire for row 12 was longer than the delay for the CR81 that blocks row 12 detection. Otherwise I feared a race hazard where the notched card might light up the photocell just before we blocked it due to the column 81 signal.

The wires were stiff enough to keep the daughter-board above the components on the main board, although I still put some electrical tape on the bottom of the little card. The control card slides into the bottom slot of the Documation, the other three cards in a standard reader are the top three slots, leaving two empty slots and thus plenty of vertical room above the control board.

Fix with daughter card attached to Control Card
I read in about 2000 cards of line printer art all of which were punched on card stock with right hand notches, with only two read checks. Both were due to cards that slipped while moving through the reader, not advancing as far as it should. Thus, there was no light at column 84 time when the card should have cleared. No spurious read checks at all due to the notch. All data looked perfect otherwise.

Video showing Documation reading right notched cards

I read in many boxes of line printer art, pictures of Abraham Lincoln and Jack Kennedy, for example, There is other software that I can now read as well. After reading in quite a few, I uploaded them to share with other 1130 enthusiasts.


First heavy rain day since I completed the shed, which will give me a good chance to see how much or little it allows water in. It looked dry about four hours after the start of the rains, but it has been much heavier since then and of course sustained for quite a while.

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