RS-232 Pinouts: DB9 and DB25 Explained

RS-232 serial ports still run point-of-sale terminals, industrial controllers, network switches, lab instruments and older computers. The connector on the back is usually a DE-9 (commonly called DB9) or a DB25, and the pinout depends on whether the device is wired as DTE or DCE. Get that wrong and the link simply will not work; connect RS-232 to a low-voltage TTL header and you can destroy the board.

The DB9 pinout

The DE-9 connector has nine pins in two rows. The standard PC (DTE) pinout is:

Pin 5 is the signal ground and is always required. Pins 2 and 3 carry the data: TXD is the output from the computer, RXD is the input to it. The remaining pins are handshaking and modem control lines. A minimal link needs only pins 2, 3 and 5, but many devices wait for DSR or CTS before they will transmit.

The DB25 pinout

The DB25 uses 25 pins and was the original RS-232 connector on IBM PCs and many modems. The DTE pinout is:

Pins 1 and 9 through 19 are mostly unused or reserved. The important difference from the DB9 is that TXD and RXD sit on pins 2 and 3 in both connectors, but the handshake lines move: DTR is pin 20 on the DB25 and pin 4 on the DB9, while DSR is pin 6 on both. A DB9-to-DB25 adapter must map these correctly, and not every cheap adapter does.

DTE and DCE: which end is which

DTE means data terminal equipment: a computer, terminal or controller. DCE means data communications equipment: a modem, or a device that behaves like one. The pinout above is the DTE view. On a DCE device, TXD and RXD swap, and so do the handshake pairs.

A straight-through cable joins a DTE to a DCE. To join two DTEs, such as two computers, you need a null modem cable or adapter that crosses TXD and RXD, crosses RTS and CTS, and crosses DTR to DSR and DCD. Signal ground runs straight through. A gender changer only swaps male and female shells; it crosses nothing.

RS-232 voltage levels

RS-232 is single-ended and uses inverted logic at higher voltages than modern digital circuits. A voltage between +3 V and +15 V is a logic 0 (space). A voltage between -3 V and -15 V is a logic 1 (mark). The region between -3 V and +3 V is undefined and is treated as noise.

Those levels are what make RS-232 robust over cable but incompatible with microcontroller pins. A TTL or CMOS serial port runs at 3.3 V or 5 V and is not RS-232. Joining the two directly can damage the low-voltage side. A level shifter or a USB-to-TTL adapter is needed, and adapters with TTL output are not RS-232. See USB to serial adapters: drivers, levels and pitfalls for the converter chips and driver problems that come with them.

How far RS-232 runs

The standard allows up to 15 m (50 ft) at 19,200 baud. Lower baud rates run further. The limit is set by cable capacitance, not by the driver, so a low-capacitance cable can stretch the distance and a long coiled cord will not. For runs beyond that, or for multiple drops on one line, RS-422 and RS-485 are the differential alternatives; the differences are covered in RS-422 vs RS-485.

What goes wrong

TTL on an RS-232 port. The most expensive mistake. A 3.3 V or 5 V microcontroller header connected to a real RS-232 port sees voltages far outside its supply range. The low-voltage side can be damaged. Always check whether a port is RS-232 or TTL before connecting.

Wrong cable type. A straight-through cable between two DTEs will not pass data. A null modem cable between a DTE and a DCE will not either. If a link does nothing, the cable is the first thing to check.

Missing ground. Pins 2 and 3 alone are not enough if the two devices have no common ground reference. Pin 5 (DB9) or pin 7 (DB25) must be connected.

Handshake lines ignored. Some devices assert DTR or RTS and wait for DSR or CTS before sending. A three-wire cable will appear dead. Loopback jumpers at the connector can satisfy the device, but they must be fitted at the correct end.

Counterfeit adapter chips. USB-to-serial adapters contain a converter chip that needs the right driver. Counterfeit chips can be blocked by the genuine maker’s drivers, leaving the adapter unusable after a driver update.

DB9 and DB25 mixed up. The two connectors use different pin numbers for the same signals. A passive adapter that only changes the shell shape will not work unless it also remaps the pins.

Choosing and checking a serial cable

Match the connector at each end, then match the wiring. If both ends are DTE, use a null modem cable. If one end is DCE, use a straight-through cable. If the device is unknown, a breakout box or a continuity tester will show which pins are connected and which cross.

For new installations, a USB-to-serial adapter with the correct driver is usually simpler than finding a PC with a real port. For equipment that must keep its original connector, a correctly wired DB9 or DB25 cable is still the right answer. Label both ends, and keep a note of the pinout used, because a cable that works on one device may be wired differently from the next.