REVIEWING TERMINATOR CARD FOR SIGNALS ENTERING THE V2315CF MAIN BOX
The Virtual 2315 Cartridge Facility (V2315CF) sits in between the IBM 1130 disk controller logic and the internal 2310 disk drive inside the 1130, routing signals into the main box of the V2315CF. Some signals are directly connected between disk and 1130, but a tap sends the same data into the V2315CF, while others are either generated in the main box or received there and then rebroadcast on another line.
The main box has a plug-in board with terminator resistors. One pulls the signal line up to +5V while the other pulls it down to ground. This sets the signal, when no other element drives it, at about 3V and yields an impedance of 104 ohms which is close to the ribbon cables that carry the signals between the main box and the 2310 Interface Board that routes signals to the 1130 and 2310 cables.
The 1130 or 2310, depending on the direction of the signal, also provide a pullup resistor to +3V for most signals. This can interfere with the impedance and voltage levels of signals. I wanted to observe each signal to see what when they need the termination resistors installed and when they are better off without them.
WORKING ON SEEK SIGNALS FIRST
The signals involved in a disk arm movement (seek) are -Access Go, emitted by the 1130 disk controller logic to request a movement, along with -10/+20 Mil Step and -Rev/Fwd Direction signals that the 1130 sets prior to the falling edge of the first signal. The 2310 disk controls the +Access Ready feedback signal, which provides a handshake to the 1130 indicating when the arm has finished moving.
+Access Ready will drop low about 5 milliseconds after the 2310 has seen a falling edge on -Access Go. The 1130 will respond to the drop of +Access Ready by returning -Access Go to logic high. After a further 10 milliseconds, +Access Ready returns to logic high indicating that the arm movement has completed. All four signals involved in the seek are directly connected by the electronic switch chips while the V2315CF is in real mode, so the output of the 1130 is directly connected to the 2310 and the output of the feedback from the 2310 is directly conneccted to the 1130.
The FPGA in the V2315CF snoops on those signals so that it can shadow the position of the disk arm, knowing exactly where the drive arm is sitting at any time. I had hammered out some logic that I believed would be unaffected by the kinds of glitches I saw on the signal lines when I observed them using my logic analyzer.
TRYING TO MOVE THE ARM RESULTED IN HARD SWING TO 202 OR 0
Initially, even when I tried to move just one cylinder, the arm shot all the way to the extreme and kept trying to move. The oscilloscope showed me that the +Access Ready signal was not dropping close enough to ground to be reliably detected by the 1130 controller logic, so it kept asserting -Access Go causing incorrect movement.
I removed the termination resistors for that signal line and the system cooperated. The arm moved correctly and the signal didn't look too bad. No significant ringing on the timescale that can cause problems for the Solid Logic Technology (SLT) circuitry in the 1130 and 2310.
CHECKING THE SHADOWING LOGIC OF V2315CF
I ran a program that repeatedly moved the arm from cylinder 0 (home cylinder) to cylinder 200 and back to 0. The disk arm faithfully moved between the two positions, however after a minute of operation, when I stopped the program, I found that the V2315CF shadowing was off by a few cylinders, showing the arm at a low non-zero cylinder when the actual arm was at 0.
Drat - I am going to need to do even more to ensure that the glitches don't get my logic out of sync. I will work on that over the next day or two. I really want to see the shadowing fully working before I move on to checking the other signals and their need for termination.
ADDED ERROR MESSAGE ON V2315CF FOR WHAT I EXPECT IS A COMMON ERROR
The physical 2310 disk locks the handle so that a cartridge cannot be inserted or removed until the UNLOCK lamp is lit on the 1130 main console. As soon as the motor begins spinning a cartridge in the 2310, the UNLOCK lamp is turned off and will not turn back on until the platter is stationary again.
When the user tries to load a virtual 2315 cartridge, they insert it into the V2315CF and flip the switch on that box to LOAD. However, if the UNLOCK lamp is not on, then it silently ignored the attempt to load the virtual cartridge. This reflects real life behavior of the 2310, but the user may not realize what is happening if they start spinning up the 2310 then try to load the virtual cartridge.
To make this clear, I now display an error message on the LED screen of the V2315CF "Must be unlocked". This is a reminder of the correct operational sequence - load virtual cartridge first, then start up the 2310 drive.
ISSUE WITH THE CE SWITCHES ON THE BACK OF THE 2310 DRIVE
The 2310 disk drive has four switches on the back that are used by the Customer Engineer (CE) to test the drive. The let the CE move the arm forward or backward in steps of 10 or 20 mils, either one time per press or continuously while the switch is held. These are supposed to be inactive when the 2310 is cabled to the 1130 system, but work if the cable is disconnected from the disk drive.
The ciruitry for these is hidden across five pages of logic diagrams in two different binders (1130 and 2310) but not completely. The rest of the diagram is shown in the manual Field Engineering Theory Of Operations for the 2310 drive.
The 1130 side of the cable is connected to the +3V power rail. Inside the 2310, a pulldown resistor to the -3V rail keeps the switches negative unless the cable is connected so that the +3V is present instead on the line +CE Interlock. I previously identified a defect in my design, where I was grounding +CE Interlock which still permitted the CE swtiches to operate. I modified the 2310 Interface Board to connect the incoming +CE Interlock line from the 1130 to the 2310, which should deliver +3V and inactivate the switches.
I measured the voltage at the 2310 and +CE Interlock was negative. I pulled the cable from the 1130 and checked the signal coming in, which was about 2.4V not 3V. I moved back into the 1130 where the wire originates. The voltage on pin D07 of A-C1 slot N7, the connector end inside the 1130 logic gate, was 2.4V. That pin should be directly connected to the adjacent slot (A-C1 M7) pin D03 which is the pin where the +3V rail enters SLT cards. The pin on M7 D03 was 3V.
The issue was a failure in a trace that should connect A-C1 M7 D03 to A-C1 N7 D07. I added a wire wrap connection and did have +3V delivered on the 1130 cable where +CE Interlock enters the 2310 Interface Board. That restored the intended lockout of the CE switches.
REWORKED THE POWER WIRING OF V2315CF
The power to the V2315CF is connected through a power PCB but with wiring to external parts - a timer module, a smart trickle charger, a 12V motorcycle battery, the incoming +12V rail of the 1130 and the power supply board that comes with the RK-05 Emulator kit upon which the V2315CF is built.
It had included diodes to isolate the battery and +12V inputs, a circuit to detect when the 1130 12V supply dropped, and connections among the other external parts. The diodes caused an unacceptable voltage drop in the supply to the emulator power supply, so I redesigned to eliminate them.
The timer is always powered by the 12V battery now. Its sense line to turn it on for 40 seconds is connected to the incoming 1130 +12V rail. Its SPDT switch is connected with the emulator power supply + input hooked to the armature, the 1130's 12V rail connected to the N/C terminal and the 12V battery + terminal connected to the N/O terminal. This switches the emulator power supply from the 1130 system over to the battery for 40 seconds when the 1130's power drops.
A smart trickle charger maintains the voltage of the 12V motorcycle battery as long as the 1130 system is plugged into the building power, even when the 1130 system is otherwise powered down. Thus the battery is topped off and always ready to support the 40 seconds of power necessary to rewrite the current cartridge contents back to the microSD card in the mini 2315 cartridge.
I can pull the diodes from the existing PCB and make a few minor changes to utilize it under the new design. I rewired everything and gave it a test on the bench to be sure that it works as intended.
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