All receive and send circuits of 1130 Expander tested, LEDs fixed, finished assembling IOB6120

FIXED THE THREE RECEIVE CIRCUITS THAT DIDN'T WORK PROPERLY

Yesterday I tested each circuit to verify that when the input wire from the 1130 is pulled low, the circuit outputs +5 on the pin going into the FPGA for that signal. Three of the 36 circuits did not respond correctly, in two the signal didn't get past the 500 ohm output resistor and in one case I didn't see the output on the transistor.

All of these were resolved by a quick touchup with the soldering iron. Solder didn't flow between the PCB pad and the component, in all three cases, instead blobbing up atop the component and appearing superficially correct. 

DEVELOPED TEST PLAN AND CHECKED OUT ALL SEND CIRCUITS

The send circuits take a logic low from the FPGA, which has those outputs operating with a weak pullup, inverts it and pulls down the output line to the 1130, which is also open collector. Thus, I needed to add a pullup resistor on the output lead I was using to check each circuit, then connect the FPGA pin associated with that circuit to ground. 

I found all send lines pulled to ground in the absence of the ground input, but they all popped up to +5V when I grounded the inputs. In practice, the 1130 supplies a pullup to +3V for those output lines, so the actual behavior will be 0 when the FPGA sends high and +3V when the FPGA is at logic low. All forty send lines were working perfectly. 

SWITCHED LEDS OVER TO 12V SUPPLY AS THEY WON'T LIGHT WITH 5V

The LEDs would not light with proper voltage applied, if you remember. I hooked them up directly to +5V through a load resistor and got no glow at all. I then tapped into the +12V that is delivered by the PC power supply and we had illumination! The load resistor that worked well is a 200 ohm part. I swapped the SMD resistors on the PCB from 90 ohm to 200 ohm.

The LED that should light simply because the power supply is active was hooked to the Power Good pin of the supply, but that is not a high enough voltage. Instead, I hooked up the three LEDs (Box Power On, CPU Run and CPU Parity Check) to the +12V supply, adding a connector to allow disassembly during future maintenance. The Box Power On LED is always grounded through a 200 ohm resistor, thus it turns on as soon as my box is plugged in. 

The two signal reporting LEDs - CPU Run and CPU Parity Check - are triggered by the input signals received on the A5 and B8 circuits. I manually tested and when I ground the input terminals, the LEDs glowed properly. The last LED is hooked directly across the 1130 power sequencing lines, which is at 24VAC when the machine is turned on - I previously verified that this one will light in that situation. 

SOLDERED REMAINDER OF IOB6120 BOARD TOGETHER

I soldered the remaining fourteen surface mount integrated circuits and then all the through hole parts which were mostly connectors. After I soldered each chip in place, I used a tiny screwdriver tip to push on each chip lead. Any that moved were resoldered until all connections were known to be good. 

SOME ADDITIONAL WORK REMAINS FOR THE IOB6120

Since I substituted a larger flash ROM for the one in the design, I have an address line or two that may be floating as the board was designed. Fortunately all the pads match between the two versions of the chip other than some address lines which are on pads that were unassigned in the design. After I check the pad to see whether it is connected to anything, I will make sure the line is routed to ground to ensure good operation of the flash chip. 

There is a fix that has to be applied to the version of the board that I built, because the original design will drain the two 3V coin cells too quickly. This involves hooking up four diodes and four resistors, cutting traces and perhaps adding a short wire or two. 

OPEN QUESTIONS

Since I had to source components, particularly the flash ROM and the four SRAM chips, from eBay sellers there is the possibility that I was sold counterfeit chips. If these were not SMD versions I could have easily breadboarded up a test of the chips, but that is quite inconvenient for the surface mount versions. We shall see what happens when I attempt load the flash, program the FPGA and then have them access the SRAM.