[ct-user] Re: do you have a TS-940 with IF-10B installed?

[Dick Green]

First, signal ground and chassis ground do not have to be synonymous and
often are not. So I wouldn't worry about the .07V reading on the TTL signal
ground pin.

Second, I'm pretty sure that the level converter chip in the required
external interface (IF-232C, W1GEE, etc.) inverts the TTL signal. The chip
datasheets I was able to find on the web seem to imply this (e.g., "the
input is held HIGH by an internal pullup resistor; output will be LOW".)
Also, the chip internal schematic shows an inverting buffer between the
input and the output. If I'm right, a +5V signal will be translated to a
mark condition (-5V to -15V) at the output of the converter, while a 0V
signal will be converted to space (+5V to +15V.). I assume that you do have
an external level converter and are not trying to plug the 940's output
directly into the computer.

Assuming an inversion, the voltage measurements you are getting seem normal
to me, with the possible exception of CTS. Unfortunately, there's a long
history of incorrect use of handshaking signals like CTS. It may be that the
IF-10B patches CTS high (or to follow RTS) just in case the other device
happens to need it. That's generally not a good idea, because the other end
might be holding the pin low. One approach would be to not connect the CTS
lead between the IF-10B and the level converter. I know that some of the
level converters out there patch the handshaking signals so that both ends
will always be satisfied.

I'd troubleshoot it by putting a breakout panel (available for $15 at Radio
Shack) between the level converter and the computer. You'll be able to
evaluate the bottom line on the data and handshaking signals that way. If
it's not working, there's some liklihood that the computer wants to see a
handshaking signal that's not be being held high. I recall encountering this
issue once with a 940 or 950 and an IC-232C level converter.

73, Dick WC1M

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