ABNORMAL TRACE RESULTS SUGGESTED ISSUE IN SYNC CONTROL LOGIC
The expected sequence of events is that when the card first moves into the photocell region, triggering OneDark, a set of timing happens to sync the remainder of the card with the positions in the middle of each of the eighty data columns. The pick is declared successful with Good Pick Reset then a counter is loaded with a preset count of the time it should take for the card edge to move past the cells and have them over the middle of column 0. The clock counts down until it reaches zero, turning on signal ZERO.
In the interim from when the OneDark first occurs until ZERO is turned on, the timing wheel will be passing its teeth in front of a magnetic pickup. The first such pulse from a tooth passage will start a counter OSCLK that runs until the ZERO activates. This is a calculated offset from a tooth.
Every other tooth passing afterwards causes OSUCLK to count up until its value matches the offset. This will tell the machine that we are directly over the middle of a column, activating CSDS signal.
In the machine currently, we see the present counter counting down and reaching zero, but OSCLK never counts nor do we get OSUCLK or any CSDS signals to tell us we are at column middles. Thus, the sync control logic is not working properly.
ITERATING THROUGH THE FLIPFLOPS AND LOGIC GATES
Sync control makes use of twelve flipflops to sequence through the states from the initial OneDark until we generate each CSDS signal for column positions 0 through 84. There are combinatorial gates to control when these flipflops are set, cleared or switched. All of these are sitting on the Clock card, inside the card cage and thus very hard to access.
I tacked wires onto three or four pins at a time, looking to see whether the gates are behaving as expected or not. For example, the gate that emits the OSCLK counting clock has three inputs, one being the 120KHz clock C1 and the other two gating its passage to the output to become OSCLK. One of the other inputs is the same signal that successfully gated the 120KHz clock to count down the preset amount, but was verified. The final input is from the logic that senses occurences of the teeth passing the magnetic pickup. That one was never activated.
I began to back up through the flipflops looking for which was blocking the deliver of the tooth signal (called TST2 at this point). It took some time to sample 3-4 pins and move along the chain. Eventually I decided to check the output of the op amp that produces the pulses from the magnetic pickup connections.
I didn't see pulses coming from the op amp. I checked the inputs and didn't see any pulses arriving! That would explain the symptoms that have developed in the last couple of days. Prior to that, I was seeing the pulses and the OSCLK and OSUCLK clock signals.
I pulled the card and measured the resistance of magnetic pickup across the two wires on the connector. Infinite ohms, an open circuit! By comparison, my good M600 reader measures a bit over 100 ohms at the same connector points.
THREE SCENARIOS TO CONSIDER HERE
The first scenario is that the wire itself has broken inside the cable, much as the 5V power wire to the card cage broken off on its own. This would be resolvable by replacement of the cable with a compatible one. It consists of two conductors with a ground shield around them.
The second scenario is that the magnetic pickup itself has suffered a broken wire inside the component but it can be accessed and repaired. This would require removal and reinstallation of the pickup, which forces some adjustments and tests before it can be assumed to work properly.
The third scenario is that the pickup is broken inside but can't be repaired. I would need to source a compatible replacement, but with no information about the part, its maker and its specs, that is very unlikely. A possible alternative is to install a new technology alternative to detect the position of the wheel and make that new circuitry produce the pulses in lieu of the old op amp and pickup.
