Sunday, December 1, 2019

Studying Control Logic to allow me to debug loading and repair second PCB

STATE OF THE CONTROL LOGIC PCBS

I have two boards, one from each of the two tape drives. One of them is clearly defective, because the supply reel will slowly rotate at power-up. The other is seemingly working, but I do have the problem that my drive won't autothread successfully nor dump and go ready successfully.

I would like to repair the bad board which is justification enough to study this board and debug it. But, it is possible that the other board is not working properly causing the loading faults, thus it would be great to debug that one as well.

APPROACH FOR DEBUGGING

I could attempt to record all the signals as the boards work while installed in the drive card cage. The other method I can use is to hook it up to a connector on the test bench and provide suitable input signals, thus verifying its behavior with full access to all component leads as well as the board connector signals.

This needs to be powered with +5V, +12V, -12V and 12VAC, thus I need my triple power supply and a separate AC source. The PCB connector is dual sided, 36 fingers per side. 53 of the fingers are used for inputs and outputs, another 13 for the power connections. Six are unused, but I need a convenient way to hook up to the 66 that are active.

Some will be permanently wired (power and steady input signals), some will be probed only to watch outputs, and the others will be variously toggled on and off to drive the logic board through its paces.
Pushbuttons command actions such as Load/Rewind, tape drive sensors report on the presence of vacuum, tape at various points, light at the BOT sensor and so forth. These need to be switched.

ANALYSIS OF CONTROL LOGIC

This board monitors the operator pushbuttons (e.g. Load/Rewind, Reset, and Unload) to perform the requested actions. It monitors the photosensors looking for the beginning of tape and end of tape markers and the sensor detecting that the take-up reel is almost empty of tape. It sees the vacuum and pressure switches that detect a auto-open cartridge, tape wrapped around the take-up hub, vacuum in the vacuum columns, and the end of the tape present near the entry throat to the tape path chamber.

A large potion of the logic is involved with the autoloading capability of the drive. This is centered on a state machine built with integrated circuits, not a controller or microprocessor chip. A binary to decimal decoder chip outputs one of eight valid states of the machine or has all turned off for the remaining inputs.

The state machine assumes each new value by a trigger signal. It is a line that is raised by combinatorial logic when one of the conditions occurs which should cause a state transition. As such, the circuitry is mainly asynchronous rather than clocked.

At power-on or when the Reset button is pressed, the flip flops are set to zero and the decoded state is 0 - the REST state. The first event that occurs to cause a clock to change the state is when the Load/Rewind button is pressed. This causes the decoder to output a 1 for the WAIT state.

The trigger signal to advance the state can be generated to kick off autoloading by the Load/Rewind button, but it also is generated by two classes of events. First, a physical sensor is activated, such as when the tape is detected wrapped around the take-up reel hub. Second, a timer goes off if the desired activity has not completed in the alloted time. .

In each of the states where some physical action has to occur, such as the two above, a 555 timer is set. The timer can be set to various durations, all multi-second. If the physical sensor is triggered before the timer pops the timer is reset and the state machine successfully advances.

The single 555 timer is used to provide several different time intervals by switching in resistors. Three transistors are used, switching in a 330K, 200K and 100K resistor respectively. The highest selects an 8.5s interval. In addition to three times using a single resistor, more than one can be switched in to produce additional durations.

Some logic on the board will drive the tape forward or in reverse, either momentarily while a test switch is pressed or in multisecond cycles driven by a pair of interlocked timers. This is for manual testing.

Commands either from the tape control unit or the test switches request the capstan to move tape forward or reverse. The control unit can simply move the tape or it can watch the data flowing from the read board or send data to the write board if it is performing a read or write. Rewind and Unload are two other commands that can come from the control unit or locally by button presses.

Each state of the autoloading sequencer commands different behaviors from the two reel motors and the capstan. For example, the BACKWRAP command rotates the supply reel counterclockwise until air pressure flips the end of the tape over the vacuum sensor hole near the entry to the tape path chamber. This is during the WAIT state.

After detecting the end of tape, the state machine advances to THRD-1 which asserts the WRAP command to cause the supply and takeup reels to both rotate clockwise. This feeds the tape into the tape path chamber, down through the head and out to feed onto the take-up reel.

Other logic on the board will control the rewind and unload processes, much simpler than the autoloading process thus only needing a few flipflops and not a state machine.

HINTS FOR WHERE TO LOOK FIRST TO DEBUG THE BROKEN CONTROL BOARD

The bad behavior manifested by the broken control logic board is counterclockwise rotation of the supply reel immediately at power up. This would be caused by the BACKWRAP command being asserted even though the autoload state machine is still at the idle REST state.

I can probe the state of some pins on the backplane while the broken board is installed. After I verify that BACKWRAP is asserted, I can check that the state machine is in the REST state. If so, the problem is in a few gates that generate BACKWRAP. If not, the state machine logic is not working properly. Either way, I can dive in from there.

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