Wednesday, March 4, 2015

1132 printer debugging and work on the new SAC interface hardware

Today is my usual day to visit with the 1401 restoration team at the Computer History Museum at midday, where we will tackle any remaining issues with the two machines as well as socialize over lunch. I worked on issues with the virtual tape drive and an intermittent error with a real tape drive. Others worked on the card reader of one machine and the punch of the other.


I hooked up the oscilloscope to the printer hammers and fired off a line worth of one character while monitoring the suspect column and another one. I could see both columns 60 (the bad one) and the other columns activating at the solenoid, but nothing prints. This means the first issue, missing column, is a mechanical problem not a logic/electrical one.

The second problem, failure to perform a carriage restore, involves a more circuitous path than one might naively presume. This button is not direct wired to the printer clutch that causes a skip. It is routed over the cable to the 1131 processor, where the printer control unit logic cards detect the signal, command the printer to skip (high speed move), watch the signals from the carriage tape channels, and issues a stop skip when channel 1 is detected.

The carriage space key is also handled indirectly, in this manner, but does work properly. Therefore to debug this second problem I have to move to the 1131 and probe signals there to see what is happening (or not happening).

The third problem on the punch list is incorrect character being printed. Since I experienced this with some single character line-wide prints in my earliest printing tests, this suggests that sluggish movement of levers is the cause. That should work its way out as I print many times, heating and moving the old lubricant around. I will take no action at this time while I work on the first two issues.


My FPGA board that left Brooklyn yesterday tagged in last night in Pennsylvania, then went back off the net. It seems to have gained some speed, perhaps, in its leisurely drift towards the west coast.

I found a better controller for my toaster oven hybrid I used to solder all the parts on a board simultaneously -- a tool called a reflow oven. The process involves a paste which is a mix of solder (tin and other metals) plus a rosin which boils away any oxidation to allow a good molecular bonding of the molten metal between the metal pad on a board, the metal pad of the component and the liquified solder itself.

A Kapton mask has holes cut out for the spots where this paste should be smeared in a thin layer onto the board - having openings where each component has a pad that attaches to the board. Using this and a spreader, the paste is applied to the board. The mask comes off and the components are placed into the board, held by the surface tension of the toothpaste-like consistency of the paste we spread previously.

The board with its loosely held components is put into the reflow oven and it begins a careful sequence of temperatures. First, the board and components are brought up to a baseline temperature below the melting point of the resin in the paste and soaked for a while to stabilize temperatures. Then the heat is increased to cause the resin to bubble, cleaning off any oxidation on the metal surfaces of board and components.

Continuing to rise, the heat causes the solder embedded in the past to become molten, a state called liquidous, where it is held for a defined period of time. The molten metal attaches to the metal pads on the board and the metal pads on the component. The miracle of surface tension then helps us - the liquid solder pulls itself into the most compact shape, which drags the component from its rough placement until it is as perfectly aligned over its mounting pads as possible. This minimizes the energy state of the liquid - surface tension at work - and ensures all the parts are exactly where they should be.

The oven then drops the temperature below the liquidous point, causing the solder to solidify. A careful cycle of cooling, to avoid thermal stress that might break the newly formed joints, allows the board to ease down to room temperature. All this heating and cooling happens in about a half hour or less, leaving me with a perfectly soldered PCB.

A few connectors and diodes are then hand-soldered to my board, because the diodes are through-hole parts with wire leads that thread through holes in the board and get soldered onto the back. The connectors, small headers with pins sticking up, also extend through holes and are soldered on the back. That completes a board; rinse and repeat three more times to build the set of boards needed for the SAC interface.

The final board can be built with a subset of the components since it has to implement only five circuits, not the twenty-four that a board usually provides, to complete all 77 signals on the 1130's SAC cable. On the other hand, since the components are relatively cheap, I can build a full fourth board which gives me spare circuits in case I ever need them.

The process of stepping heat up from the stabilizing 'pre-soak' level to liquidous and then the drop downward were accomplished by manually changing the digital setting of a power controller, one which controlled all four heating elements as a unit.

The new controller separates the upper and lower heating elements, adds an additional booster element to the bottom, and automates following the timed profile with a microcontroller that has a thermocouple monitoring the temperatures inside the oven. Since I had always achieved good results with my hand-control method, there is some tolerance to variations with the reflow process, but having a more complete control over the temperature profile has to further improve the quality of boards I produce.

The reflow oven controller was picked up today and I started to update my existing reflow oven. I was about halfway through the installation when light faded away outdoors. A couple of more hours should complete the task, then the oven needs about two more hours, training the controller software to learn how to properly control the heat inside.

The modification separates the upper and lower heating elements, controlling each independently. It adds a third element at the bottom as a booster to increase heat output. Each of the three heat elements is independently switched and its power modulates by use of three solid state relays. The inside of the oven is lined with gold tape to reflect back the radiant heat to maintain temperature.

The controller itself is a variation on an Arduino, designed to manage the heating elements based on the algorithms in the software, the temperature sensed by the thermocouple and by the training that allows it to understand how the oven heats and cools. 

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