Designed power supply mounting, did some wiring, finished 160 pin connector wire extenders and now testing

POWER SUPPLY MOUNTING DESIGN

I have only enough room for a mini-ATX power supply in the case. There is about an inch to spare in width and height, more room front to back. I am using an extension that puts the power outlet on the rear of the case and then allows it to enter the PS on its side. 

The power cord is routed above the edge of the PCB, around the entire PS and then to the rear plug. The output cable from the supply also wraps around the entire PS before plugging into the PCB. The other output cables, typically connected to disk drives in a PC, are just placed in front of the PS in the empty area. 

The power supply has four 6-32 screws that fasten it to a PC frame, on the same side as the power socket. I worked out a design for a bracket, made by bending a 6" x 5" aluminum sheet to form two 3x5 planes at right angles to each other. On one plane I will cut out a center rectangle for the power socket and air entry, and drill the four holes for the PS mounting screws. The other plane gets two holes that match existing studs on the bottom of the case I am using. 

HOOKED UP LEDS, 24VAC CONNECTOR

I neatly wired up the four LEDs to the PCB screw terminals, then began to test out the lights. I first applied 24VAC to the circuit that shows the 1130 power is on - that worked fine. Next, I turned on the power supply expecting the second LED to light showing that box power is on. It didn't light. 

I measured 5V at the wire going into the LED but it didn't light. I have to investigate further - I was pretty sure that these LEDs would light with a lower voltage drop than the 5V I see delivered by the supply. Although, it might have a high internal resistance since I am tied to the power good output of the PS whose internal design is not known. I will do some more investigating on the next visit to see what is going on with the LEDs. 

When the 1130 powers on, it sends 24VAC out through the power connectors to each attached peripheral, intended to energize a relay to power up those attached boxes. I wired up the 1130 power connector socket on my box to the small connector on my PCB, so that when the 1130 turns on, it will light the leftmost green LED. 

COMPLETED THE WIRE EXTENSIONS FOR THE MAIN SIGNAL CONNECTOR

I finished adding all the 6" extensions for the output signals from my PCB to the 1130. I should be able to easily connect all the lines to the screw terminals on my PCB. After adding the extensions, I checked each and every wire to be sure that the labeled connection is hooked to the proper pin on the signal connector. There are 160 pins on the connector but only 77 active signals, the rest are either ground or unused pins. 

I have labeled the lines by the FPGA board connector pin they route to - there are two 2 x 32 pin headers that attached the board to my PCB. Each row is assigned a letter, thus the left header (viewed from the front of my board) has rows D and C, the right header has rows B and A. Vertically from the bottom of the board they are numbered 1 to 32. Some of the pins are used for power, ground and other purposes, but most are input-output lines to the FPGA.

For example, the outputs from my FPGA to the 1130 computer are assigned to rows C and D. Thus there are signals like C3, D3, C4, D4 etc. The inputs to the FPGA that monitor signals outputted by the 1130 are assigned to connections such as A4 and B20 on the other header. My wires have labels like A5, but they are hooked to 1130 signals, in this case CPU Meter Out which is also named CPU Run. That is wired to pin E1 of the 160 pin connector. 

I worked through all of the signal wires, verifying that each labeled FPGA connector pin went to the proper pin of the main signal connector. I also checked that the FPGA connector pin is tied in my VHDL code to the appropriate 1130 signal - so that A5 is indeed read as CPU Meter Out. 

I had spotted a swap between the assignments for two signals - CPU Parity Stop and X2 Clock - which were assigned to A8 and B8 but swapped in meaning. I corrected the labels and updated my documentation. All is now correct. 

TESTING TO ENSURE ALL IS WORKING PROPERLY

I checked that appropriate power is delivered everywhere it should be - 5V, 3.3V and 3V - and that it didn't appear on any signal pins. I tested for shorts. I validated that the terminals for received signals all had the expected resistance and voltage. The terminals for sent signals are open collector so they didn't have any output voltage. 

The received inputs are pulled low by the 1130 when they are asserted - logically true - and my receive circuit provides a pullup to 3V just to ensure there are not false positives. It is the same circuit that IBM implements for such signals sent over long cable runs. Because of this, I could temporarily ground each receive screw terminal which should flip the output on the FPGA header pin from 0 to +5V indicating the 1130 signal is true. 

I did this for all 36 received signal circuits and found three that weren't switching. Two of them switch at the output of the transistor on my receive circuit but are not making it through the 500 ohm coupling resistor. The third doesn't turn off the transistor regardless of the input signal. 

It should be easy to track down and fix those circuits. I then have to do the same thing to test my driver circuits which are simply high current open collector inverters. I can't see the outputs without adding a pullup resistor to the screw terminal, then delivering +5 and 0 to the FPGA pin associated with that circuit. The 0 input should let the output pull up to high, while a high input should drag the output down to ground. 

Once I know that all inputs and outputs work properly, I can finish the assembly and do a power on test with the FPGA board connected by USB link to the PC. 

PARTWAY THROUGH BUILD OF IOB6120

Since I received my PCB blanks from the fab yesterday and have all the parts in the shop, I started to assemble the board. I soldered down some very small parts first - a flash ROM and a Xilinx FPGA chip - then put on all the SMD resistors and capacitors. There are about a dozen ICs to add and then the through hole parts before the board is complete.