EIA RS-232 serial line interface

The EIA RS-232 serial line interface standard (now often known simply as RS-232, or EIA) is an interface which was commonly used to connect both terminals and modems to computers (the other option for such communications being 20mA current loop).

It comes in two 'sexes', the 'Data Communication Equipment' (DCE), usually properly found on modems, and the 'Data Terminal Equipment' (DTE), usually properly found on terminals and computers. The concept was that at each end of the link, a DTE would plug into a DCE; the two modems with the DCEs would communicate over a telephone line.

Connecting a terminal to a computer (both wired as DTEs) thus usually requires a so-called null modem, a small device wired on both sides as a DCE. Internally, it connects appropriate signals together, so that e.g. one side's 'Transmitted Data' signal is connected to the other side's 'Received Data'. (In theory, there could be a DCE-DCE connecting device, but in practice this combination never occurs.)

Connectors

The RS-232 standard initially specified use of DB-25 connectors: male (DB-25P) for all DTE's, and female (DB-25S) for all DCE's. That way, if one RS-232 device or cable could plug into another at all, it was guaranteed to be a correct connection.

Alas, some equipment does not correctly follow this standard, requiring use of a so-called 'gender mender' - similar to a null modem, but without the signal cross-connects found in a null modem - to convert the offending connector to the standard. Unlike null modems (which are always female-female), gender menders come in both male-male and female-female variants.

With the rise of personal computers, the DB-25 connectors were too large, and in any event many of the signals in the full RS-232 specification were not needed. PC's therefore standardized on DE-9 connectors, following the lead of the IBM PC-AT, which first used them. DE-9 connectors are now officially part of the RS-232 specification.

Signals

The following table provides an (incomplete) list of RS-232 signals and pin assignments, for the most used signals, along with a few others. Direction is given with respect to the DTE - 'In' means the signal goes to the DTE, 'Out' means it comes from the DTE; the signal names also derive from this convention.

Name	Abbreviation	Direction	DB-25 pin	DE-9 pin
Transmitted Data	TxD	Out	2	3
Received Data	RxD	In	3	2
Request To Send	RTS	Out	4	7
Clear To Send	CTS	In	5	8
Data Terminal Ready	DTR	Out	20	4
Data Set Ready	DSR	In	6	6
Data Carrier Detect	DCD	In	8	1
Ring Indicator	RI	In	22	9
Force Busy		Out	25	N/A
Signal Quality		In	21	N/A
Signal Rate		Both	23	N/A
Serial Clock Transmit		In	15	N/A
Serial Clock Receive		In	17	N/A
External Clock		Out	24	N/A
Signal Ground	G	Common	7	5
Protective Ground	PG	Common	1	N/A

The signal ground provides a voltage reference for all the other signals, and also serves as a common return for them.

Null modem wiring

To create a null modem, Transmitted Data is connected to Received Data, and vice versa; Request To Send to Clear To Send; and Data Terminal Ready to both Data Set Ready and Data Carrier Detect.