The RS-232 standard defines the
interconnection of computers and modems, so there is little to go
wrong here by simply purchasing a pre-assembled cable. There are
two types of cable: cables with connectors for a standard 25-pin D
connector on the computer; and cables with connectors for a
proprietary 9-pin D connector used on the IBM
PC/AT and many other
computers. The cables have titles like
RS-232 25-pin computer
(DTE) to 25-pin modem
(DCE) or
RS-232 9-pin IBM
PC/AT computer
(DTE) to 25-pin modem
(DCE). Most modems are packaged
with a suitable cable.
If you need to manufacture your own cables, see the
Serial-HOWTO for the RS-232
pinout for your computer. Connect Transmit Data on the computer to
Transmit Data on the modem, Receive Data on the computer to Receive
Data on the modem, and so on for Signal Ground, Clear to Send,
Ready to Send, Data Set Ready, Data Terminal Ready, Data Carrier
Detect and Ring Indication.
For professional computer room installations consider routing
the serial cable through an RJ-45 patch panel.
There are two common pinouts on used on the
RJ-45 connector: Yost and
Cisco
2500-series console.
If you create your own pinout for unshielded twisted pair
cable then be sure that your pinout twists a Signal Ground wire
with the Transmit Data wire and another Signal Ground wire with the
Receive Data wire. Although the RS-232 signals
are not balanced, this twist will result in the least amount of
signal degradation and noise pickup.