The standard file-transfer (null-modem) cable with two MMJ (or RJ11/14) connectors will connect: 1-6, 2-5, and 3-4. Note that such a cable supports DTR/DSR flow control which is not supported (yet) by Linux. Making up your own standard 6-conductor file-transfer cable is very simple if you understand that the ordinary 4-conductor telephone cable from the wall to your telephone, used in hundreds of millions of homes, is also a file-transfer cable. Find one and wire your cable the same way.

If you lay such a cable flat on the floor (with no twists) you will note that both plugs on the ends have their gold contacts facing up (or both facing down). Although it's symmetrical, it is also file-transfer if you think about it a bit. One may put a few such cables together with inline couplers and everything works OK because each inline coupler is also a file-transfer (null-modem) adapter. Two file-transfer cables in series result in a straight-thru connection.

Here's a custom cable diagram (by Mark Gleaves) for connecting MMJ to a 9-pin serial port using RTS/CTS flow control:

[blockquote shade=yes]
DEC MMJ Linux PC DB9
Pin Signal Signal Pin
=== ====== ====== ===
1 DTR -----------------------|---> DSR 6
|---> CTS 8
2 TxD ---------------------------> RxD 2
3 SG (TxD)--------------------|--- SG 5
4 SG (RxD)--------------------|
5 RxD <--------------------------- TxD 3
6 DSR <-----------------------|--- RTS 7
|--> DTR 4
|--> CD 1
(no connection) RI 9

8-conductors and 10-conductors

RJ45 and RJ48 are 8-conductor modular telephone plugs. There exists some 10-conductor connectors which are allegedly wider and will not mate with the 8-conductor ones. People have called the 10-conductor ones RJ45 and/or RJ48 but this may be incorrect. These connectors are used for both flat telephone cable and round twisted pair cable. The cable end of the connector may be different for round and flat cable. RJ48 has an extra tab so that a RJ48 plug will not push into a RJ45 jack (but a RJ45 plug will mate with a RJ48 jack). They're used on some multiport serial cards and networks. Heres the pin numbers for an 8-conductor:

[blockquote shade=yes]
Plug Jack (or socket)
(Looking at the end (Looking at the cavity
end of a cable) in a wall)
.__________. .__________.
| 87654321 | | 12345678 |
|__. .__| |__. .__|
|____| |____|

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