Friday, April 15, 2016

M-32TL operation notes

I took a brief look at the M-32TL printer in my Canon P170-DH calculator. I haven't figured out all the details, but here's what I've learned so far.

 The printer has three electrical components:
  • An FN-130CW DC motor made by Standard Motor that drives the mechanism through a small gear train.
  • A tiny solenoid that activates both the printing of a single column and the stepping of the print head to the next column.
  • A 4-wire rotation position sensor.

In this image I found on the net, you can see the motor in the lower left. The solenoid is hidden behind the gears at the upper left (follow the white wire to it). The position sensor is the thin black box on the right end of the printer.

The calculator uses an LB1268 magnet driver to turn on the motor and activate the print solenoid. This driver is a curious device, intended to drive solenoids through channels 1 and 2, and a motor on channel 3. It's rated for a maximum of 8V and 1A on channels 1 and 2, and 2A on channel 3, though not continuously. This board uses only channels 2 and 3.

When the solenoid is inactive, the motor causes the three wheels of the print head to rotate through a reduction gearing system. Two of the wheels (one red, one black) contain the characters 0-9, a decimal point, a comma, a hash mark, and a hyphen. The third wheel contains various special operation symbols and prints only in the first column.  By activating the solenoid when the proper character is in position for printing, the character is printed and the print head advances to the next column. I have not yet determined how it selects which of the two number wheels is used to print. It also looks like the printer is capable of printing more than 12 columns (17 + an operation symbol?).

I haven't looked into the operation of the position sensor yet, but I found this photo of one someone disassembled:

If you look closely you'll see the circuit printed on the wheel contains a continuous band in the center, a "start of rotation" index in the middle, and 14 character position indexes around the outer edge. If you ground the wiper for the center ring, each of the other wipers will go to ground when it contacts the appropriate segment. This is very similar to the EP-102 printer in the Busicom PF-141, except the EP-102 had 18 print wheels and printed all the occurrences of a given character simultaneously, while the M-32TL can only print one column at a time.

Why two wipers on the outer edge instead of one? You'll note one is offset from the other, and will contact the character index a bit later. According to the website of Arne Rossius (in German), one indicates the start of a character and the other the end of the character. This would avoid having to be concerned with the rotation speed of the motor when driving the solenoid.

Unfortunately, everyone else seems to have a single-color M-31A printer, while I have a 2-color M-32TL. It looks like I'll have to figure out how to change colors on my own. I do wish I could dig up a datasheet for the Epson M-32TL printer!

Lastly, despite the markings on the calculator's internal transformer which clearly say "Output: 5.2VDC 600mA", the output of the transformer is AC. On the PCB I found a full-wave bridge rectifier made of discrete diodes and a large electrolytic capacitor filter capacitor. I measured almost 8VDC on the capacitor, but with the rest of the circuitry turned off that's probably its peak voltage. Not exactly a 5.2VDC output. Regardless, it means I'll have to include a circuit to generate the 36VDC the VFD will require.

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