smith@math.ucla.edu (Smitty) (08/14/89)
While Richard Tobier's RS232 scheme is clearly well-thought out, it
doesn't take advantage of the standard telephone modular wiring scheme
in which ALL modular cables have a built in "flip", so that in a
6-conductor flat cable, if the pins on the plugs are numbered 1-6, then
the connections between the two plugs is
1-6
2-5
3-4
4-3
6-5.
By taking advantage of this "flip", one can make ALL modular cables
effectively into "null modems" and at the same time prevent potential
terminal damage if a terminal is inadvertently plugged into a real
telephone jack. This means that ANY two devices can be connected by
a standard modular cable and "talk" to each other.
Here is the basic scheme which is used in many places:
A. From DTE (terminals, printers, and computers, etc.) to modular jack:
(the numbers in the first column are the positions in the modular
jack)
1 DTR (data terminal ready) (DB25: 20)
2 TD (transmitted data) (DB25: 2)
3 signal ground (3 and 4 are connected together wherever they appear)
4 signal ground (DB25: 7)
5 RD (received data) (DB25: 3)
6 DSR and DCD (connected together) (DB25: 6 and 8)
CTS and RTS are connected but don't appear at the jack (DB25 4 and 5)
B. From DCE (modems (and a few exceptional "brain-damaged" terminals and
printers which are wired like modems) to modular jack (the numbers in
the first column are the positions in the modular jack):
1 DSR (data set ready) (DB25: 6)
2 RD (received data) (DB25: 3)
3 signal ground (3 and 4 are connected together wherever they appear)
4 signal ground (DB25 7)
5 TD (transmitted data) (DB25 2)
6 DTR (DB25: 20)
CTS and RTS are connected but don't appear at the jack (DB25 4 and 5)
There are many variants, that work well, on the above. The key point
is that if ANY two devices are connected together with a standard 6
conductor modular cable (which, of course, has the built-in "flip") then
they can talk to each other.
Also, since 3 and 4 are connected together, if this assembly is plugged
into an ordinary single-line telephone jack, it simply shorts the
telephone line (3 and 4 are connected to red and green, which is where a
single-line appears) and puts no voltage on the RS232 connection.
To make this work, any two female jacks which are connected, must also
have the same "flip" described above (this means that the total number
of flips between any two connected devices is odd, and this gives the
desired "null modem" effect).
It's very, very convenient, for testing, etc) to be able to connect two
terminals together and have them talk (in full-duplex, of course), etc.
One can make convenient patch-panels using this scheme.
Of course, with the advent of terminal-servers using ethernets, which do
all of the above and much more, the necessity for this kind of thing is
going away rapidly (Thank God!).
smitty