RS-232 is the old-reliable serial standard still wired into countless field installations, connecting an RTU to its radio, a flow computer to a controller, or a laptop to a device's configuration port. It is single-ended and point-to-point - one device to one device, over a short cable - which is both its limitation and, for those simple direct connections, its virtue. This guide explains how RS-232 signals data, its DB9 pinout and DTE/DCE roles, and how it differs from the multidrop RS-485 that often sits alongside it.
RS-232 Serial in one line: RS-232, also called EIA-232, is a single-ended serial communication standard that sends data between exactly two devices as voltage levels referenced to a common ground, over a short cable typically no more than a few tens of feet. It is point-to-point rather than networked, uses positive and negative voltages to represent the two logic states, and defines connector pinouts and the DTE and DCE device roles - making it the classic interface for wiring an RTU to a radio, a flow computer, or a serial console.
RS-232 is a single-ended, or unbalanced, interface: each data line carries its signal as a voltage measured against a shared signal ground. A negative voltage represents one logic state and a positive voltage the other, with a fairly wide voltage swing that gives the receiver a clear distinction between the two. Because the signal is referenced to ground rather than to a second wire, it needs only one conductor per signal plus the common ground.
That simplicity is also the source of its limits. Single-ended signaling is vulnerable to noise and to differences in ground potential between the two devices, both of which corrupt a voltage that is measured against ground. Combined with the interface's voltage levels and capacitance, this restricts RS-232 to short cable runs - a few tens of feet in typical field conditions - and to modest data rates. Push the cable longer or the environment noisier and errors creep in.
It is also strictly point-to-point. RS-232 connects one device to exactly one other device; you cannot hang several devices off a single RS-232 line as you can with a multidrop network. For a direct link between two boxes sitting close together - an RTU and the radio next to it - that is perfectly adequate and pleasantly simple, which is why the standard persists.
RS-232 is commonly wired on a DB9 connector, and its pins carry not just transmit and receive data but a set of control and handshake signals - request to send, clear to send, data set ready, data terminal ready, and others - that devices use to coordinate flow. Simple connections use only transmit, receive, and ground, while devices that need hardware flow control wire the handshake lines as well. Getting the pinout right is half the battle in the field.
The standard divides devices into two roles: data terminal equipment, or DTE, and data communication equipment, or DCE. Historically the DTE was a terminal or computer and the DCE was a modem, and the roles determine which pins transmit and which receive. A DTE transmits on the pin that a DCE receives on, so a straight-through cable connects a DTE to a DCE correctly. Connecting two devices of the same role - two DTEs, such as two controllers - requires a null-modem cable that crosses the transmit and receive lines so each device's output reaches the other's input.
This DTE/DCE distinction is the single most common source of confusion when wiring RS-232 in the field. A link that will not communicate often just has transmit and receive reversed because two like-role devices were joined with a straight cable instead of a crossover. Knowing each device's role and choosing straight-through or null-modem accordingly resolves most first-time connection problems.
RS-232 and RS-485 are often confused because both are serial, but they solve different problems. RS-485 is a differential, balanced interface that carries each signal as the difference between two wires, which cancels noise and tolerates ground differences, letting it run far longer cables and, crucially, support a multidrop network of many devices on one pair. RS-232 is single-ended and strictly two devices over a short run. Where RS-485 is the choice for wiring several instruments along a bus, RS-232 is the choice for a single direct link.
In practice the two coexist across a field installation. An RTU might use RS-485 to poll a string of meters or transmitters along a multidrop bus, and RS-232 to connect to the radio or flow computer sitting right beside it, and again to a laptop plugged into its console port for configuration. Both often carry the same protocol - Modbus RTU is common over either - so the difference is in the physical wiring and topology, not the message format.
For a cloud SCADA such as Merobix, RS-232 is one of the short first links in the chain that ultimately delivers field data to the cloud. A sensor's reading may cross an RS-485 bus to the RTU, hop over RS-232 into the radio, travel a wireless link to the master, and only then reach the SCADA host. Each of these physical layers is transparent to the data once it works, but a miswired RS-232 cable - transmit and receive reversed, or a missing ground - is a classic reason a newly commissioned site never reports, making an understanding of the standard practically useful for anyone bringing sites online.
RS-232 is single-ended and point-to-point, connecting exactly two devices over a short cable, while RS-485 is differential and multidrop, connecting many devices along one twisted-pair bus over much longer distances. RS-485's differential signaling cancels noise and tolerates ground differences, which RS-232 cannot. Use RS-232 for a single direct link and RS-485 for a networked string of devices.
They are the two device roles the standard defines: data terminal equipment, historically a computer or terminal, and data communication equipment, historically a modem. The roles determine which pins transmit and which receive. A DTE and a DCE connect with a straight-through cable, but two devices of the same role need a null-modem cable that crosses transmit and receive so each one's output reaches the other's input.
The most common cause is transmit and receive being reversed - typically because two same-role devices were joined with a straight-through cable when they needed a null-modem crossover. Other frequent culprits are a missing signal ground, mismatched baud rate or framing settings, or unwired hardware handshake lines that a device is waiting on. Checking device roles, cable type, and port settings resolves most first-time failures.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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