dgc@LOCUS.UCLA.EDU.UUCP (02/14/87)
We (the UCLA Mathematics Department) have been using them for over 5
years, with no problems. We use the 6-conductor series. We need only
5 wires: Signal Ground, Transmit, Receive and a "ready" signal in each
direction.
We short pins 3 and 4. This protects us completely if we inadvertently
plug a device into an ordinary single-line telephone jack (shorting
these together, that is, shorting tip and ring does no harm to the
telephone circuitry). We use the shorted pair 3 and 4 as signal ground.
We use 1 and 6 for transmit and receive and 2 and 5 for "ready" (one for
each direction).
The standard "twist" in the modular phone cable has the effect that all
cables are null-modems. We wire all of our devices (DTE and DCE) to
look the same to this system. As a result we can connect any device to
any device with these cables without worrying whether they are DCE or
DTE. That is, for example, we may plug two terminals together and they
will talk to each other (useful for fixing broken terminals). We can
even plug a modem into a modem and have them both work.
I consider "protective ground" a potential hazard (for we run between
different wings of a building with different ground potentials) and I
wonder if the "engineers" who designed the RS232 specs had ever read
the national electric code. Ground wires are not required in this
situation, however Section 250-95 requires that protective ground wires
be as large as the power conductors serving the device. The conductors
serving our vax are number 12 and those going to terminals and modems
are typically number 18.
dgc
David G. Cantor
Internet: dgc@cs.ucla.edu
UUCP: ...!{ihnp4, randvax, sdcrdcf, ucbvax}!ucla-cs!dgc