6-wire 6-position RJ11 (sometimes called RJ12) modular phone connectors are used for all RS-232 communications at Apache Point Observatory. This has several advantages, including:
RJ11 Wire
Signal
DTE
DCE
1
hardware flow control transmit (optional)
RTS or DTR
CTS and/or DSR
2
transmit
3
transmit ground
4
receive ground
5
receive
6
hardware flow control receive (optional)
CTS or DSR
RTS and/or DTR
Where "transmit" means the signal is output from the jack (into the cable, if one is plugged in), and "receive" means the signal is input into the jack (or the device "behind" the jack).
Unfortunately, the RS-232 name of the hardware flow control lines depends on whether the port is a DTE or a DCE. This is shown in the table. Note that you have a choice of two different pairs of signals; the port will be configured to use one or the other. If you your device supports both, you probably should use the RTS/CTS pairs, as they are more commonly used for flow control and the device may do better flow control using them (for instance the Xyplex terminal servers).
There are two different names for the connectors we use: "6-wire 6-position RJ11" appears to be most common, but "RJ12" is, or was, also used. I believe the former term is less likely to cause confusion. There are other styles of RJ11 (including 4-wire 6-position, which are common for telephones, and 4-wire 4-position, which are common for telephone handsets). if I don't specify, I always mean 6-wire 6-position, since that is what we use.
All RJ11 jacks have the same polarity. Hence connecting cables must have a twist such that pin 1 at one end is connected to pin 6 at the other end. Jack-to-jack couplers must also have this twist, and extension cords (plug at one end and a jack at the other) must not. Fortunately most or all standard telephone parts are made using these conventions.
Many standard phone cables use only the middle 4 wires of the 6 available (4-wire 6-position). These are fine for most serial connections, but beware of special cases in which the outer pair (hardware flow control) is in use.
A few cables, especially those supplied with consumer telephones and modems, have only the middle two wires connected. These are useless for serial communications, and should be thrown out to avoid confusion. Accidentally using these cables has cost us many hours of work.
The standard described may be modified for RS-422 signals. "Receive" and "receive ground" become "receive-" and "receive+", and similarly for "transmit". Unfortunately, you cannot support hardware handshaking in this mode, because there is no ground. Without hardware handshaking, most or all of the existing RJ11 wiring (distribution patch panels and such) could be used unchanged with RS-422, once the appropriate DB/RJ11 adapters were made.
