Thursday, May 25, 2017

Building PCB design for IBM 1130 light enhancement, plus working on disk tool debugging


I think I have worked out some light modifications to allow me to trigger my disk tool for writing, without having an actual disk drive attached, such that I can capture the stream being written to debug any further bugs.

I was ready to make the changes, but had to find where I stored my FPGA boards in order to load the logic onto it. I spent a bit of time putting in the changes to help with testing tomorrow. We should have a good session tomorrow at Marc's home, collecting data to get my cartridge writing logic working properly.


I launched the PCB design software and began to tediously build the panel that supports the 16 lights each for the six registers (IAR, SAR, SDR, AFR, ACC and EXT). This board will have 96 thyristors, 96 resistors, 96 single pins for the signal inputs, 96 double pin headers for the lamp attachments, and 3 turret terminals to provide power, ground and lamp test voltages, capable of delivering 1A on AC and ground turrets when all lamps are lit.

The design begins with a schematic to define all the connections, which I built up first by drawing a single register of 16 circuits and then replicating that six times across the sheet. When I transfer this to the PCB for layout, I will have hundreds of parts to find and place in the correct location.

If I left the sequentially assigned part numbers, such as JP3 for an input pin or D34 for a thyratron, the layout would be extremely tedious and error prone. I took the time (considerable mind numbing time) to rename all the parts with a logical structure that instantly identifies which light position they implement. Thus, S103 is the input signal for register 1, bit 3. T311 is the thyristor for register 3, bit 11. R200 is the resistor for register 2, bit 0. and O415 is the 2 pin header for the lamp for register 4, bit 15.

With this, I can find and drag the parts to their desired locale on the PCB. The only parts with strict locations are the output (2 pin) headers for the lamp mounting. The power headers and the various signal pin headers just need approximate siting - signals near their thyristors and the power turret terminal in the right edge zone.

This first PCB, for the six registers, will be 8 1/2" wide and 4 1/4" high. The right hand 1/2" is the zone for the six double-pin power connections. The lamps are separated 1/2" center to center running left to right within a register, and the register rows are separated 3/4" center to center from top to bottom. The board has a 1/4" margin on three sides and 3/4" on the power connector edge.

The remaining lights in the pedestal will be covered by two identical but smaller boards, each hosting an array of six rows of eight lamps each. Some lamp positions are not implemented, depending upon the signals being displayed and the arrangement of the faceplate.

Image result for ibm 1130 pedestal
Faceplate - registers on left half, status at mid right and other data on right
One board handles the mid-right section that has the T and X clock states and various status lamps, while the other board on the far right covers the op code, registers, interrupt levels, and so forth. One design will produce both of the smaller boards, with the board sparsely populated to fit its role.

The schematic relabeling of the register board took many hours to complete, before I could lay out the physical board and place components. I wasn't done with the relabeling until the evening. Therefore, I am just beginning on the layout.

However, I found that spacing will be much tighter than I expected, because the spacing between lights is just 1/2" horizontally and 3/4" vertically. The SCR is more than 1/4" by 7/16" which fills a lot of the space between adjacent lights in the grid. The resistors and signal pins are easy to fit, but the SCR is the dominant factor.

I tried out a different organization to see if I can get the parts to fit in the tight grid. I rotated the 2 pin header from horizontal to vertical orientation. This allowed the thyristors to fit between the columns of lights and my 0805 sized resistor and signal pin will fit as well. I had to lay out an entire row across the top and the entire left column to ensure that everything fits.

The only unknown is routing. The path from the resistor and input pin is very direct to the thyristor, as is the path from thyristor anode to output pin. Further, the ground and AC voltage lines are on the inner planes of the four layer board, saving the need to route those lines. The only line that will be significant for routing is the lamp test signal.

It will take me another day to finish laying out all the components across the board before I can attempt the routing. 

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