5. Cabling serial ports together

Since you will be connecting two DTEs together, you will need to have a null modem run between the two devices. A null modem crosses transmit and receive, and ties a few status lines together so the application can open the port. This null modem can be a dongle that connects to the cable, or can be built into the cable. A dongle will get expensive if you have a large number of cables, so it is usually easier to get cables with the null modem built in.

Most PC hardware these days use DB-9 connectors, giving 9 pins for transmitting data and status, which is fine for us. Pre-built DB-9 cables can be had for a few dollars for a few feet of cable. More flexible is building a DB-9 to RJ-45 connector and building the null modem into that. The RJ-45 connector then accepts regular 10BaseT cables that can be custom-built, or with varying legths. This gives a lot of flexibiliy in arranging cables, since each cable can be the correct length to run between machines. Little extra cable is left lying around.

DB-9 to RJ-45 connectors can be purchased unassembled since there are no real standards for making this conversion. So long as Tx and Rx cross and CTS RTS cross, you have a null modem connection. The cabling I have here comes from my own design, and works just fine. Note that there have to be two different DB-9 to RJ-45 connectors because of the way pins are switched. I labeled them as "1" and "2". They can be placed on either end of the cable.

Table 1. DB9 to RJ-25 connector

Connector 1 Connector 2
DB-9 RJ-45 DB-9 RJ-45
1 5 1 5
2 6 2 4
3 4 3 6
4 7 4 7
5 3 5 3
6 2 6 2
7 1 7 8
8 8 8 1
9 n/c 9 n/c