Friday, April 1, 2016

Work on paper tape plotter and other things

My daughter bought a used VW Passat wagon - perfect car for her to commute and bring her large dog along. However, it was leaking oil from the valve cover gaskets, producing smelly smoke that gets sucked into the ventilation system.

I decided to change the valve cover gaskets myself as the quote from the local repair shop was over $1,000 and as a retired person, I can definitely beat that cost. About $100 all together for parts and tools. I will move a bit slow to be sure I don't cause any problems. That plus the continued retirement tasklist pulls me away from the 1130.

After many hours I came to the conclusion that the special tool I bought to compress the timing chain tensioner was junk - I can compress the tension by hand but this tool just wedges into the place, too thick to push between chain and frame to reach the piston it must depress.

The repair cost just jumped up a bit for the replacement tool to compress the piston. New tool coming, but on Monday, so this will have to fester until then. At least I could replace the brittle and broken PCV hose assembly, as lack of PCV suction is one of the reasons that leaks develop.


Implementing mirror 1442 reader/punch

On the PC side, I added the logic to inject blanks for mirror mode and to issue the transactions that switch us between mirror and virtual operation. Again, I checked that the virtual function still works.

I can't test the mirror mode myself until my physical 1442 reader/punch is working, but the changes were quite minor thus it has a decent chance of working as written.

Paper tape reader and punch integration

I received all my components last night to hook the paper tape reader and punch units to the slave fpga board. A dual relay board to advance the tape motor of the two units, voltage translation boards for the incoming signals and a signal amplifier/driver board to fire the eight punch channels. Wiring and packaging commenced.

Further study of the units highlighted that I didn't need any voltage translation boards. The switches on the units that detect presence or absence of tape, and that detect holes for the reader, have a common connection that I can hook to signal ground and use pullup resistors on the fpga side to use these at the LVCMOS 3.3 voltage levels of the board.

I do need the signal amplifier/driver to drive the 48V solenoids that punch holes in the paper tape. My first plan was using a power driver board designed to light seven segment displays, able to drive inductive (solenoid) loads to 100ma per channel. However, the spec on the chip is a max of 33V thus I can't use this part with the 1055 punch.

It won't be hard to create my own driver board for the eight channels of punch output, but it will add a few days to the wiring task for the punch. This will be a very simple set of power transistors to conduct the solenoid down to ground when an input signal is present. If I choose a transistor with a beta of 100 and feed it a couple of ma from the fpga board, well within the drive capability of the board, I should be able to pull a couple hundred ma through the solenoid to punch the hole.


I removed the foam rubber that sat inside the center of the cylinder, in order to gain full access to the dented section of the drum. One possible  way to get most of the dent out of the drum would be a wooden inverse mold - allowing the outer surface of the drum to nestle into the reverse shape in the wood. I could then press and tap on the dent from the inside to get the surface more or less smooth. If I can restore it so that variations in height are just a few millimeters under the intended radius, I could fill those in.

I may speak with an auto body repair shop that has expertise with aluminum - they may be able to repair this much better than I could. Still undecided on the repair method to try first.

The plotter is missing an important piece. What I assumed was just the pen holder, which rides on two horizontal rails and is moved left and right, also has the solenoid that raises or lowers the pen. Part of many plotted outputs are commands to pull the pen up, reposition and then drop the pen to draw from the new starting point. 


  1. Re the "switches on the units that detect presence or absence of tape, and that detect holes for the reader": operating these at low voltage with (one assumes) very low current -- will they be reliable? It's faintly in my memory that contact switches that run "dry" (of current) don't make reliable contact.

  2. These were wired to IBM SLT logic, which would put them at about the ballpark voltage and current when they were conducting with the fpga as with SLT cards. I did remove oxidation on these and clean them up after years of storage - they worked well with an Arduino that I used as a testbed to investigate the two.