Saturday, April 20, 2019

more refinements of the replacement console panel lighting for the IBM 1130 - bitstream, PS test, fidelity test

Loading the FPGA bitstream

I had completed the logic design (VHDL code) while on holiday, checking its operation with the simulator, so I was ready to configure the Spartan 7 based board that will drive the console lights. The toolchain provides everything needed to load the board, so it was a relatively quick and easy process.

Power supply testing

My power supply takes the 7.5VAC from the 1130 lighting circuit, rectifies it and, using two buck converters, generates both 5V and 3.3V to power the LEDs, I/O multiplexer chips and the FPGA board. The cumulative draw of 154 LEDs comes to 7.4 A with them all at full brightness during a lamp test; if they were turned on to white instead of yellowish the draw would be more than 9.24 A.

The enclosure on the 1130 that houses the control panel has a swing down door on the back for access. I will mount the power supply elements on that swing down door, making use of several raised posts with screw hole taps that are spread across the rear of the door.

I first set up a test of the power supply components I will be using. I found a wall-wart transformer that outputs 7.5VAC, allowing me to power the big bridge rectifier and the two buck converters that generate the 3.3 and 5.0VDC for my PCBs. The test confirmed that everything was good up to the current limit of the wall-wart, which is far too low to power the fully lit display panel but does permit a limited number of LEDs to be active.

Testing rectifier, buck converters and such driven by a 7.5VAC wall-wart
Next up, I hauled out the scope and monitored the ripple and noise on the DC power, both from the bridge rectifier and from the buck converter boards. While the output of the converters was good, as I expected, I do need to add a filter capacitor on the outputs of the main 5V converter. It will also provide some buffering of power closer to the LEDs that running through the 1130 all the way back down to the 7.5VAC supply.

Mechanical assembly

I had to add an extension that will raise daughter board higher off the rear of the main PCB, This gives me room to fit the signal wires from the 1130 logic onto the pins on the main board underneath. I simply used a set of Arduino shield socket strips to form the extension.

Daughterboard raised with extensions to allow access to signal input pins below it
Establishing the realistic filament characteristics

My first check of the reasonableness of the incandescent behavior simulation was to switch various LEDs on and off using an Arduino program. The effect looked good, in real time, to my eye and memory. The next check was to display all 32 steps from full off to full on, letting me judge the color shift and apparent brightness changes.

Displaying the 32 steps from full off to full on

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