Thursday, November 1, 2018

Soldering more secure connections for the DSKY substitute


Nearing the end of the construction and testing of the DSKY substitute

The first two finished boards for input and output interface to the Apollo Guidance Computer passed their tests. I began to wire up the five addition input boards needed to connect my DSKY substitute to the AGC next week. This is assuming great progress from a standing start, but better to be prepared.

The output board is used in conjunction with relays. When activated, this connects the output line through a 2K resistor to ground. If not activated, the line floats up to +28V. This particular board can support 9 output circuits, although I am only using 8 for the DSKY.

First output board, used with relays to pull AGC input down towards ground
The input board is connected to a circuit in the AGC that will pull the line down towards ground (through a 2K resistor in the AGC and a 10K resistor in line the the transistor base, so the actual achieved voltage is quite a bit higher than 0V but sufficient to cause the transistor to conduct on my interface circuit.

That transistor delivers 28V through a voltage divider to ground, with the center tap sitting at just under 5V when active or at 0V when inactive. Thus this input circuit is also an inverter - a 28V input produces a 0V output, while when the input is pulled down towards ground, the output becomes 5V.

First input board, floats input at 28V, pulled down when AGC output activates it, gives inverted TTL output
I replaced some more breadboards with small solder boards like the ones used above, making all the connections to the I/O expander boards for the LEDs and for eight of the inputs from the first two input boards like the above that I completed.

I2C chain of I/O expander boards to drive LEDs and record some inputs
I soldered the alphanumeric (14-segment) driver onto another board and wired it to the I2C chain. I still have to do quite a bit:

  • wire up the two 8-channel relay modules
  • move the flasher logic to a circuit board
  • finish the input interface boards
  • wire in the 23 input channels
  • wire up the 8 output channels to the relay board
  • wire up the control lines from the Arduino to the output channel relay board
  • finalize the flashing control relay board wiring
  • ensure I have all the +5 and ground wires routed properly
  • check the Arduino prototype shield wiring for shorts or bad solder joints
One stretch goal is to label the keyboard with the proper text and to put a plastic overlay on the display lights with the correct text legends. I printed the text but the process of converting  them into rub-on transfers will take about an hour that I may not have tomorrow. 

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