While the RS-232 standard originally specified a 25-pin D-type connector, many designers of personal computers chose to implement only a subset of the full standard: they traded off compatibility with the standard against the use of less costly and more compact connectors (in particular the DE-9 RS-232-C version used by the original IBM PC-AT). The desire to supply serial interface cards with two ports required that IBM reduce the size of the connector to fit onto a single card back panel. A DE-9 connector also fits onto a card with a second DB-25 connector. Starting around the time of the introduction of the IBM PC-AT, serial ports were commonly built with a 9-pin connector to save cost and space. However, presence of a 9-pin D-subminiature connector is not sufficient to indicate the connection is in fact a serial port, since this connector is also used for video, joysticks, and other purposes.
Some miniaturized electronics, particularly graphing calculators and hand-held amateur and two-way radio equipment, have serial ports using a phone connector, usually the smaller 2.5 or 3.5 mm connectors and use the most basic 3-wire interface.
Since most devices do not use all of the 20 signals that are defined by the standard, smaller connectors are often used. For example, the 9-pin DE-9 connector is used by most IBM-compatible PCs since the IBM PC AT, and has been standardized as TIA-574. More recently, modular connectors have been used. Most common are 8P8C connectors, for which the EIA/TIA-561 standard defines a pinout, while the Yost Serial Device Wiring Standard is common on Unix computers and newer devices from Cisco Systems. 10P10C connectors can be found on some devices as well. Digital Equipment Corporationdefined their own DECconnect connection system which is based on the Modified Modular Jack (MMJ) connector. This is a 6-pin modular jack where the key is offset from the center position. As with the Yost standard, DECconnect uses a symmetrical pin layout which enables the direct connection between two DTEs. Another common connector is the DH10 header connector common on motherboards and add-in cards which is usually converted via a cable to the more standard 9-pin DE-9 connector (and frequently mounted on a free slot plate or other part of the housing).
The following table lists commonly used RS-232 signals and pin assignments.
| Signal | Direction | Connector pin | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Name | V.24 circuit | Abbreviation | DTE | DCE | DB-25 | DE-9 |
MMJ | 8P8C (RJ45) | 10P10C (RJ50) | ||||||
| EIA/TIA-561 | Yost (DTE) | Yost (DCE) | Cyclades | Digi (ALTPIN option) |
National Instruments | Cyclades | Digi | ||||||||
| Transmitted Data | 103 | TxD | Out | In | 2 | 3 | 2 | 6 | 6 | 3 | 3 | 4 | 8 | 4 | 5 |
| Received Data | 104 | RxD | In | Out | 3 | 2 | 5 | 5 | 3 | 6 | 6 | 5 | 9 | 7 | 6 |
| Data Terminal Ready | 108/2 | DTR | Out | In | 20 | 4 | 1 | 3 | 7 | 2 | 2 | 8 | 7 | 3 | 9 |
| Data Carrier Detect | 109 | DCD | In | Out | 8 | 1 | N/A | 2 | 2 | 7 | 7 | 1 | 10 | 8 | 10 |
| Data Set Ready | 107 | DSR | In | Out | 6 | 6 | 6 | 1 | 8 | N/A | 5 | 9 | 2 | ||
| Ring Indicator | 125 | RI | In | Out | 22 | 9 | N/A | N/A | N/A | N/A | N/A | 2 | 10 | 1 | |
| Request To Send | 105 | RTS | Out | In | 4 | 7 | N/A | 8 | 8 | 1 | 1 | 2 | 4 | 2 | 3 |
| Clear To Send | 106 | CTS | In | Out | 5 | 8 | N/A | 7 | 1 | 8 | 5 | 7 | 3 | 6 | 8 |
| Signal Ground | 102 | G | Common | 7 | 5 | 3, 4 | 4 | 4, 5 | 4, 5 | 4 | 6 | 6 | 5 | 7 | |
| Protective Ground | 101 | PG | Common | 1 | N/A | N/A | N/A | N/A | N/A | N/A | 3 | N/A | 1 | 4 | |
The signal ground is a common return for the other connections; it appears on two pins in the Yost standard but is the same signal. The DB-25 connector includes a second protective ground on pin 1, which is intended to be connected by each device to its own frame ground or similar. Connecting this to pin 7 (signal reference ground) is a common practice but not recommended.
Some Cyclades cables
DB-25 to DB-25, straight-thru
DB-25 DB-25 M / F M / F -------- -------- 2 TxD ------------ TxD 2 3 RxD ------------ RxD 3 7 Gnd ------------ Gnd 7 20 DTR ------------ DTR 20 6 DSR ------------ DSR 6 8 DCD ------------ DCD 8 4 RTS ------------ RTS 4 5 CTS ------------ CTS 5
DB-25 to DB-25, crossover
DB-25 DB-25
M / F M / F
-------- --------
2 TxD ------------ RxD 3
3 RxD ------------ TxD 2
7 Gnd ------------ Gnd 7
20 DTR --------|--- DSR 6
|--- DCD 8
6 DSR ---|-------- DTR 20
8 DCD ---|
4 RTS ------------ CTS 5
5 CTS ------------ RTS 4
DB-25 to DE-9, straight-thru
DB-25 DE-9 M / F M / F -------- -------- 2 TxD ------------ TxD 3 3 RxD ------------ RxD 2 7 Gnd ------------ Gnd 5 20 DTR ------------ DTR 4 6 DSR ------------ DSR 6 8 DCD ------------ DCD 1 4 RTS ------------ RTS 7 5 CTS ------------ CTS 8
DB-25 to DE-9, crossover
DB-25 DE-9
M / F M / F
-------- --------
2 TxD ------------ RxD 2
3 RxD ------------ TxD 3
7 Gnd ------------ Gnd 5
20 DTR --------|--- DCD 1
|--- DSR 6
6 DSR ---|-------- DTR 4
8 DCD ---|
4 RTS ------------ CTS 8
5 CTS ------------ RTS 7
RJ-45 (Cyclades) to DB-25, straight-thru
RJ-45 DB-25 Cyclades M / F -------- -------- 3 TxD ------------ TxD 2 6 RxD ------------ RxD 3 4 Gnd ------------ Gnd 7 2 DTR ------------ DTR 20 8 DSR ------------ DSR 6 7 DCD ------------ DCD 8 1 RTS ------------ RTS 4 5 CTS ------------ CTS 5
RJ-45 (Cyclades) to DB-25, crossover
RJ-45 DB-25
Cyclades M / F
-------- --------
3 TxD ------------ RxD 3
6 RxD ------------ TxD 2
4 Gnd ------------ Gnd 7
2 DTR --------|--- DSR 6
|--- DCD 8
7 DCD ---|-------- DTR 20
8 DSR ---|
1 RTS ------------ CTS 5
5 CTS ------------ RTS 4
RJ-45 (Cyclades) to DE-9, straight-thru
RJ-45 DE-9 Cyclades M / F -------- -------- 3 TxD ------------ TxD 3 6 RxD ------------ RxD 2 4 Gnd ------------ Gnd 5 2 DTR ------------ DTR 4 8 DSR ------------ DSR 6 7 DCD ------------ DCD 1 1 RTS ------------ RTS 7 5 CTS ------------ CTS 8
RJ-45 (Cyclades) to DE-9, crossover
RJ-45 DE-9
Cyclades M / F
-------- --------
3 TxD ------------ RxD 2
6 RxD ------------ TxD 3
4 Gnd ------------ Gnd 5
2 DTR --------|--- DCD 1
|--- DSR 6
7 DCD ---|-------- DTR 4
8 DSR ---|
1 RTS ------------ CTS 8
5 CTS ------------ RTS 7
RJ-45 (Cyclades) to RJ-45 (Cyclades), straight-thru
RJ-45 RJ-45 Cyclades Cyclades -------- -------- 3 TxD ------------ TxD 3 6 RxD ------------ RxD 6 4 Gnd ------------ Gnd 4 2 DTR ------------ DTR 2 8 DSR ------------ DSR 8 7 DCD ------------ DCD 7 1 RTS ------------ RTS 1 5 CTS ------------ CTS 5
RJ-45 (Cyclades) to RJ-45 (Cyclades), crossover
RJ-45 RJ-45
Cyclades Cyclades
-------- --------
3 TxD ------------ RxD 6
6 RxD ------------ TxD 3
4 Gnd ------------ Gnd 4
2 DTR --------|--- DCD 7
|--- DSR 8
7 DCD ---|-------- DTR 2
8 DSR ---|
1 RTS ------------ CTS 5
5 CTS ------------ RTS 1
RJ-45 (Cyclades) to RJ-45 (Sun/Cisco), straight-thru
RJ-45 RJ-45
Cyclades Sun/Cisco
-------- ---------
3 TxD ------------ TxD 3
6 RxD ------------ RxD 6
4 Gnd --------|--- Gnd 4
|--- Gnd 5
2 DTR ------------ DTR 2
7 DCD ------------ DSR 7
1 RTS ------------ RTS 1
5 CTS ------------ CTS 8
RJ-45 (Cyclades) to RJ-45 (Sun/Cisco), crossover
RJ-45 RJ-45
Cyclades Sun/Cisco
-------- ---------
3 TxD ------------ RxD 6
6 RxD ------------ TxD 3
4 Gnd --------|--- Gnd 4
|--- Gnd 5
2 DTR ------------ DSR 7
7 DCD ------------ DTR 2
1 RTS ------------ CTS 8
5 CTS ------------ RTS 1
RJ-45 (Cyclades) to MMJ (DEC), crossover
RJ-45 MMJ
Cyclades DEC
-------- ---------
3 TxD ------------ RxD 5
6 RxD ------------ TxD 2
4 Gnd --------|--- Gnd 3
|--- Gnd 4
2 DTR ------------ DSR 6
7 DCD ------------ DTR 1
DE-9 to DE-9, crossover
DE-9 DE-9
M / F M / F
-------- --------
1 ----+--------- 4 (1 and 6 to 4)
6 ----+
2 -------------- 3 (2 connected to 3)
3 -------------- 2 (3 connected to 2)
4 --------+----- 1 (4 connected to 1 and 6)
+----- 6
5 -------------- 5 (5 connected to 5)
7 -------------- 8 (7 connected to 8)
8 -------------- 7 (8 connected to 7)
correct
incorrect