RS232 and RS485 both turn up on the same vehicle-terminal spec sheet, and integrators routinely wire the wrong one. The choice is not about speed or age. It comes down to two questions: how many devices are on the line, and how far does the cable run?
The actual difference: single-ended vs differential
RS232 sends each signal as a voltage measured against a shared ground. That works over a short cable in a quiet electrical environment.
RS485 sends over a twisted pair and reads the difference between the two wires. Interference that hits the cable hits both wires roughly equally, and cancels out when the receiver subtracts one from the other.
Distance, device count and noise immunity all follow from that one design difference.
| RS232 | RS485 | |
|---|---|---|
| Signalling | Single-ended (vs ground) | Differential (twisted pair) |
| Topology | Point-to-point | Multidrop bus |
| Devices per port | 1 | Up to 32 (more with repeaters) |
| Practical cable run | ~15 m | ~1200 m |
| Noise immunity | Low | High |
| Termination | No | Yes, 120 ohm at both bus ends |
| Addressing | Not needed | Required (each node needs an ID) |
Which one for which peripheral
Anything within arm's reach of the operator and connecting to exactly one port goes on RS232. Anything that leaves the cab, down a boom, along a trailer or across a yard, goes on RS485, even if it is the only device on that line, because the distance alone rules RS232 out.
Where RS232 fails quietly
The failure that catches people out is not a dead link, it is an intermittent one. An RS232 run of 20 to 30 m will often work on a bench, and work on the machine while it is parked. Then it starts dropping characters once the hydraulics engage or a motor starts nearby.
Because the fault only appears under load, it usually gets blamed on software or the peripheral for weeks before anyone re-examines the cable. If a serial link is flaky on a working machine but fine in the workshop, check cable length and signalling type first.
RS485 wiring that people skip
- Termination: a 120 ohm resistor at each end of the bus, not at every node. Missing termination causes reflections that look identical to random data corruption.
- Daisy-chain, not star: RS485 is a linear bus. Branching it into a star causes reflections even with correct termination.
- Unique node addresses: two devices sharing an address will both answer and corrupt each other's replies.
- Twisted pair, not any two conductors: the noise cancellation depends on both wires following the same physical path.
Neither replaces CAN bus
CAN bus reads the vehicle's own network (engine data, fault codes, implement messages under J1939 or ISOBUS). Serial ports connect peripherals the vehicle maker never installed: scanners, printers, weigh scales, third-party sensors. A typical deployment uses all of them at once, which is why interface count, not just interface type, decides whether a terminal fits a job.
A real example
A contractor fitting terminals to self-propelled sprayers needed three things per machine: a cab-mounted barcode scanner for chemical batch logging, a legacy GNSS receiver, and four tank-level probes spread along a 28 m boom.
The first install put everything on RS232 with extended cables. The scanner and receiver worked. The boom probes dropped readings whenever the pump ran. Rewiring the four probes as a single RS485 bus with 120 ohm termination at both ends fixed it. The scanner and receiver stayed on RS232, where their short runs were never the problem.
FAQ
Can I convert RS232 to RS485 with an adapter? Yes, but each converter adds a powered component and a connection point that can fail in a vibrating vehicle. A native port avoids both.
Is RS485 faster than RS232? Not inherently at short distances. The real advantage is that RS485 holds its data rate over long runs where RS232 degrades.
Do I need termination on a short RS485 bus? Under a few metres it often works without, but termination costs nothing and removes an intermittent fault that is hard to diagnose later.
How many RS485 devices can share one bus? The standard allows 32 nodes. In practice, polling time becomes the limit before the electrical limit does, since the terminal queries each address in turn.
Which should I specify on a new build? Default to RS485 unless you have a specific reason not to. One device under 15 m can use either. More than one device, or any run past 15 m, and RS485 is the one that works reliably in a vehicle.
I work on rugged vehicle-mount terminals at CF-Device. The full write-up, including the spec-sheet checklist, is on our site: RS232 vs RS485 for Rugged Vehicle Tablets.
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