RS232 Null Modem vs Straight-Through: Pinout, Wiring and How to Tell Them Apart

The One-Sentence Answer

If you are connecting a PC to an AV device — projector, display, matrix, processor — you almost always need a null modem (crossover) cable.

Straight-through serial cables exist for connecting a computer to a modem. Almost nothing in an AV rack is a modem. Yet straight-through cables are what ships in the box with most equipment and what you find loose in the toolbag, which is why this is the highest-frequency physical-layer failure in commissioning.

Why the Crossover Is Needed

RS-232 defines two roles:

• DTE (Data Terminal Equipment) — a PC. Transmits on pin 3, receives on pin 2.

• DCE (Data Communication Equipment) — a modem. Transmits on pin 2, receives on pin 3.

A straight-through cable connects pin 2 to pin 2 and pin 3 to pin 3. That works for DTE to DCE because their transmit and receive lines are already opposite.

The problem: most AV devices are wired as DTE, the same as your PC. Connect two DTEs with a straight-through cable and you have joined transmit to transmit and receive to receive. Both ends shout, neither listens, and nothing happens — with no error, no indicator, and no clue.

A null modem cable swaps them back:

     PC (DTE)              Device (DTE)
   pin 3  TXD  ──────────►  RXD  pin 2
   pin 2  RXD  ◄──────────  TXD  pin 3
   pin 5  GND  ───────────  GND  pin 5

DB9 Pinout Reference

For AV control, only pins 2, 3 and 5 carry data. The rest are handshaking signals from the modem era.

Three-wire is usually enough

A minimal null modem cable is exactly three conductors: 2↔3, 3↔2, 5↔5. This drives the overwhelming majority of projectors, displays and matrices.

The exception that costs people hours: some devices require the handshake lines to be satisfied before they will transmit. Panasonic projectors are the well-known case — several models need pins 7 and 8 (RTS/CTS) linked together at the device end. Without that jumper, the device never sees "clear to send" and silently discards everything.

A full null modem cable handles this by crossing the handshake pairs as well:

2 ↔ 3     RXD ↔ TXD
3 ↔ 2     TXD ↔ RXD
5 ↔ 5     GND
7 ↔ 8     RTS ↔ CTS
8 ↔ 7     CTS ↔ RTS
4 ↔ 6     DTR ↔ DSR
6 ↔ 4     DSR ↔ DTR

Rule of thumb: try three-wire first. If the device is silent and you have verified baud rate and framing, suspect handshaking — either use a full null modem cable, or loop 7-8 together at the device end.

Identifying an Unknown Cable

You will inherit cables with no markings. Two ways to identify one in under a minute:

With a multimeter (definitive). Set to continuity, probe pin 2 at one end against pins 2 and 3 at the other:

• pin 2 → pin 2 continuous = straight-through

• pin 2 → pin 3 continuous = null modem

By connector gender (a strong hint, not proof). Null modem cables are usually female on both ends, because they join two DTE devices which both present male ports. A cable with one male and one female end is most often a straight-through extension. Treat this as a first guess and confirm with the meter when it matters.

Buy a null modem adapter and keep it in the bag. A short DB9 female-to-male crossover adapter converts any straight-through cable on the spot. It costs very little and removes this entire class of problem from a site visit.

USB-to-Serial Adapters

Most laptops have no serial port, so a USB adapter is standard. Three things go wrong with them:

1. The COM number changes with the USB port. Windows assigns per port, so moving the adapter to a different socket gives you a different COM number and your control software points at nothing. Use the same physical port every time, or check Device Manager after plugging in.

2. Counterfeit chipsets. Cheap adapters using cloned FTDI or Prolific chips can be non-functional with current drivers. The symptom is a device that enumerates and appears in Device Manager but never passes data. Buy adapters with genuine FTDI chips for field work — the price difference is trivial against one wasted site visit.

3. Some adapters do not carry the handshake lines. Bare three-wire USB adapters exist. If you need RTS/CTS for a device like a Panasonic projector, an adapter that only wires 2/3/5 cannot help you no matter which cable you attach.

There is also a fourth, subtler one worth knowing: an adapter can affect wake behaviour on devices whose control port is only alive when the device is on. LG documents this for its ka power command — with a true RS-232C connection the command works in both power states, but through a USB-to-serial converter it may only work while the display is already on. If power-off works and power-on does not, the adapter is a suspect and not just the display's standby mode.

Other Connectors You Will Meet

RJ45 serial (Cisco-style). Common on matrices and processors to save panel space. There is no single standard for the pinout — check that device's manual, because RJ45 serial pinouts differ between manufacturers and a Cisco console cable will not necessarily work.

3.5 mm TRS serial. Used by Samsung Ex-Link and some other consumer-derived gear. Tip/ring assignment varies between model years, so a cable that works on one unit may do nothing on another. See Samsung Ex-Link commands.

Phoenix / terminal block. Usually RS-485 rather than RS-232, labelled A/B or D+/D−. Different electrical standard, different rules — see Modbus RTU vs TCP.

DB25. Legacy. On DB25, TXD is pin 2 and RXD is pin 3 — the opposite assignment to DB9, which is a pleasant trap when using a DB9-to-DB25 adapter.

Distance

RS-232 is specified to about 15 metres at 9600 baud, and less as baud rate rises. In practice a good cable often goes further, and a poor one fails sooner.

Some manufacturers specify much shorter limits — Samsung states 4 metres for MDC. Where the vendor gives a number, believe the vendor.

If you need distance, do not fight it with a longer cable. Use an RS-232-to-Ethernet gateway and put the serial link on the network. In a hall where the rack is far from the fixture, this is both more reliable and easier to service. Many devices also accept the same commands over TCP directly, which removes the serial layer entirely — see Epson ESC/VP21 and Sony ADCP, both of which are transport-independent.

Diagnostic Order

When a serial device does not respond, work in this order — it goes from cheapest to most expensive to check:

• Is the device's serial control enabled in its menu? Many devices ship with RS-232 disabled or with LAN control taking precedence.

• Cable type — meter it, or fit a null modem adapter and retry.

• Serial parameters — baud, and parity (Sony wants Even; Modbus RTU often wants Even; most AV wants None).

• Framing — terminator, checksum, ASCII vs HEX.

• Handshaking — full null modem, or 7-8 jumper.

• Test with a terminal, not the control system. PuTTY, Tera Term or Hercules removes an entire layer of possible causes.

Step 6 is the highest-value habit in this list. If a terminal on your laptop can drive the device, the device and cable are proven and the fault is in the control system's configuration. If the terminal cannot, stop looking at software.

With SoftControl

SoftControl exposes baud rate, parity, data bits and stop bits per device for serial and tcp_serial, and its command tester shows the exact bytes sent, the raw bytes returned, and the request in hex — so "the cable is wrong" looks different from "the command is wrong", which is the distinction that matters when you are on a ladder.

For fundamentals see the serial communication guide; for brand-specific quirks see Panasonic (the 7-8 jumper), Sony (even parity) and Samsung MDC (4 m limit).