Testing my new seek logic for the Virtual 2315 Cartridge Facility - part 12

TRYING OUT A TERMINATOR AT THE 1130 BACKPLANE TO IMPROVE SIGNAL

The +Access Ready signal that is generated by the disk drive in real mode of the Virtual 2315 Cartridge Facility (V2315CF) or by the V2315CF in virtual mode. It is feedback to the disk controller logic in the 1130 as it moves the disk arm to seek to different cylinder locations. There was some ringing and glitches on the signal which did cause issues for the disk controller and for the V2315CF which needs to monitor the disk movement to keep track of the current cylinder. 

I have tried multiple things to improve the signal as received at the pin on the 1130 in gate A, compartment C1 where the disk driver logic resides. I changed the impedance of the terminator installed in the V2315CF, which is on the sending end of the signal to the 1130 and receiving end for the signal from the disk drive. I removed the terminating resistors entirely. I shorted the +Access Ready signals from the 1130 and the disk drive together on the 2310 Interface Board so that the V2315CF is electrically uninvolved other than additional cable lengths. 

All of these changes made some improvement. The last one gave me excellent agreement between the disk drive arm and the intent of the XIO Seek commands I issued. However, the V2315CF failed to completely shadow the movements, missing a couple of cylinders on longer seeks. 

I therefore decided to add termination right at the destination pin on the 1130 backplane. The IBM design did not have terminators there, other than whatever pullup resistor they used on the backplane after the signal arrived. I couldn't find any resistor or source for a pullup. The signal is generated by a single shot timer in the disk drive itself. 

I have no component level circuit schematic for that gate so I can't tell if it has pullup or not. In any case, having the pullup on the source side which is typical for the 1130 logic would not protect the input pin from ringing caused by inductance and capacitance in the cabling over to the SLT backplane. 

I therefore decided to mount termination resistors right at slot L4 of the backplane, where the +Access Ready signal is connected at pin D04. I used a 330 ohm pullup to +3V which is available on pin D02 of the slot and a 470 ohm pulldown to ground which is available on pin D08 of the slot. 

I had to find a good way to install this in that area. I chose to solder the two resistors together with very short leads at the connection. I then installed wirewrap wires to pins D02, D04 and D08, with their other ends loose. Once I find a way to mechanically secure the resistors in place between the rows of pins on the backplane, I will cut the wires and tack solder them to the common point and other ends of the resistors. For the time being, I did a quick and dirty installation.

OBSERVATION OF SIGNALS AND BEHAVIOR WITH TERMINATOR RESISTORS IN PLACE

The signal looked almost exactly like the signal leaving the disk drive at the backplane pin of its electronics, thus I was not getting any distortions based on the run of the signal through various cables, boards and traces on its way to the 1130 backplane pin. 

The drive would still fail to seek properly on some longer seeks, stopping a few cylinders short of its intended destination. It would also lock up and buzz at a point short of cylinder 202 but well along in its travel. Something is still wrong with the drive that I have to resolve. I need to go through the entire sequence of settings per the manual.

Below is the image I took of a seek of 193 cylinders.

Yellow is the +Access Ready signal as it is delivered to the 1130 disk controller backplane pin. Green is the -Access Go command from the 1130 disk controller that triggers each step. Purple is the -10/+20 Mil signal which makes the first step 10 mil and the remainder be 2 cylinders. Blue is the source +Access Ready signal at the disk drive electronics backplane pin. 

The expected waveforms and timing are shown in one of the maintenance manuals, which I will be checking while the drive performs some steps. 

There are four adjustments I will check and change if necessary that control the timings shown above.

1. Tachometer amplifier static balance - no output when carriage is not moving
2. Dead band of servo - adjust to eliminate oscillation but achieve proper movement
3. 10 mil and 20 mil stop voltages - duration of acceleration for each step size
4. 10 millisecond timer duration - time that +Access Ready stays low

I will also verify the 2.6 ms single shot is the proper duration to allow the detents to retract before arm movement starts. Watching voltages and timing of other signals above help identify if any components are out of spec and causing issues.

The FE maintenance manual has multiple methods of adjusting the drive values above, including a very promising method that observes the continuous movement of the arm for erratic detent operation and a means to tweak the adjustments for the most reliable and consistent stepping.