PC to emulate paper tape


Thread Starter

Trevor Easton

I'm looking for an inexpensive way for a PC to emulate a paper tape reader on an old NC Milling Machine. I'm aware of some black boxes that will do this but want to do it on the cheap. Any advice?

If you're willing to fool around a bit, you can do it with some optoisolators running off a parallel port on the PC. On the one I interfaced to (a long time ago, maybe 1983), there were 7 bits (+ 1 parity bit?) and a strobe. The strobe was inverted. The easiest way to find out would be to look at the signals from a working one on a scope. If you can't do that, you should still be able to get it to work as there are a limited number of possibilities. There could be an ACK line, but I don't think there was on the one I worked on.

The parallel port drives some inexpensive optos through a 240 ohm resistor. If you put an LED in series, you can see the data but may have to lower the resistor value. You'll have to determine the logic levels on the other side
of the opto; you should be able to measure this with a voltmeter (even if the tape reader doesn't work anymore).

Your main concerns will be strobe polarity and timing, although the timing should not be too critical. As I recall, the data was positive true and the strobe was positive-going (latch on rising edge). Either of these could be inverted, however.

The bit rate on these things was 110 baud, or about 10 characters per second. You might be able to go somewhat faster, but there will be a
practical limit because these things had really limited handshaking, based on the fact that they were mechanical devices. Have fun!

Willy Smith
Numatico SA

Johan Bengtsson

It sounds like you need a null-modem cable and MSDOS.

I think it would be enough to just connect the NC to any of the COM-ports and copy the file you want to the selected comport.

/Johan Bengtsson

P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
Only on newer NC's. Lots of older NCs only had the paper tape reader input. Some had 20ma current loop for a serial output -- which was
expected to run a paper tape punch.

For those older machines you need to simulate the tape reader -- at least a couple of companies have products for this. I've used Decitek
products with maderate success.