RS-232 interface

RS-232 is simple, universal, well understood and supported. It was introduced in 1962, and despite rumors for its early demise, has remained widely used through the industry.

The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). The "idle" state (MARK) has the signal level negative with respect to common, and the "active" state (SPACE) has the signal level positive with respect to common. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol.

The RS-232 interface presupposes a common ground between the DTE and DCE. This is a reasonable assumption when a short cable connects the DTE to the DCE, but with longer lines and connections between devices that may be on different electrical busses with different grounds, this may not be true. RS232 data is bi-polar. +3 to +12 volts indicates an "ON or 0-state (SPACE) condition" while A -3 to -12 volts indicates an "OFF" 1-state (MARK) condition. Modern computer equipment ignores the negative level and accepts a zero voltage level as the "OFF" state. In fact, the "ON" state may be achieved with lesser positive potential. This means circuits powered by 5 VDC are capable of driving RS232 circuits directly, however, the overall range that the RS232 signal may be transmitted/received may be dramatically reduced. The output signal level usually swings between +12V and -12V. The "dead area" between +3v and -3v is designed to absorb line noise. In the various RS-232-like definitions this dead area may vary. For instance, the definition for V.10 has a dead area from +0.3v to -0.3v. Many receivers designed for RS-232 are sensitive to differentials of 1v or less.

Pin	Name	ITU-T	Dir	Description
1	GND	101		Shield Ground
2	TXD	103		Transmit Data
3	RXD	104		Receive Data
4	RTS	105		Request to Send
5	CTS	106		Clear to Send
6	DSR	107		Data Set Ready
7	GND	102		System Ground
8	CD	109		Carrier Detect
9	-		-	RESERVED
10	-		-	RESERVED
11	STF	126		Select Transmit Channel
12	S.CD	?		Secondary Carrier Detect
13	S.CTS	?		Secondary Clear to Send
14	S.TXD	?		Secondary Transmit Data
15	TCK	114		Transmission Signal Element Timing
16	S.RXD	?		Secondary Receive Data
17	RCK	115		Receiver Signal Element Timing
18	LL	141		Local Loop Control
19	S.RTS	?		Secondary Request to Send
20	DTR	108		Data Terminal Ready
21	RL	140		Remote Loop Control
22	RI	125		Ring Indicator
23	DSR	111		Data Signal Rate Selector
24	XCK	113		Transmit Signal Element Timing
25	TI	142		Test Indicator

Data is transmitted and received on pins 2 and 3 respectively. Data Set Ready (DSR) is an indication from the Data Set (i.e., the modem or DSU/CSU) that it is on. Similarly, DTR indicates to the Data Set that the DTE is on. Data Carrier Detect (DCD) indicates that a good carrier is being received from the remote modem.

Pins 4 RTS (Request To Send - from the transmitting computer) and 5 CTS (Clear To Send - from the Data set) are used to control. In most Asynchronous situations, RTS and CTS are constantly on throughout the communication session. However where the DTE is connected to a multipoint line, RTS is used to turn carrier on the modem on and off. On a multipoint line, it"s imperative that only one station is transmitting at a time (because they share the return phone pair). When a station wants to transmit, it raises RTS. The modem turns on carrier, typically waits a few milliseconds for carrier to stabilize, and then raises CTS. The DTE transmits when it sees CTS up. When the station has finished its transmission, it drops RTS and the modem drops CTS and carrier together.

Clock signals (pins 15, 17, & 24) are only used for synchronous communications. The modem or DSU extracts the clock from the data stream and provides a steady clock signal to the DTE. Note that the transmit and receive clock signals do not have to be the same, or even at the same baud rate.

Some specs

Note: Direction is DTE (Computer) relative DCE (Modem).
Note: Do not connect SHIELD(1) to GND(7).