RS485 vs RS232: why one is actually faster, and where the real limits are

The question I get is almost never "what is the difference." It is narrower. Somebody has a serial device on the bench, two interface modules next to it, and wants to know why the 485 module is rated for so much more speed and distance when both are three wires and a shield. RS485 vs RS232 gets answered with "one is differential, the other is not." Everybody nods. Nothing about the speed has actually been explained.

Differential signaling is the reason, sure. But the mechanism sits a step further down, in how much voltage each standard has to physically move before a receiver will call it a bit.

A twisted pair landed on one terminal block and a D-sub hanging off the next: RS485 vs RS232 shows up as a wiring decision long before it shows up as a spec sheet.

What RS485 vs RS232 actually comes down to

RS-232 is single ended. One conductor per signal, and the receiver measures that conductor against signal ground. Whatever noise couples onto the wire, and whatever difference exists between the ground at each end, lands directly on top of the data.

A compliant RS-232 driver into a 3 kilohm to 7 kilohm load puts out at least 5V. The receiver decides at plus or minus 3V. That leaves 2V of room between "the driver did its job" and "the receiver read the wrong bit."

RS-485 splits the signal across a twisted pair, and the receiver only looks at the difference between A and B. Noise coupling into the pair hits both conductors nearly equally, so subtracting one from the other cancels it. Ground shift gets rejected the same way. TI's RS-485 Basics Series white paper, publication SLLA545, is explicit about why the common-mode window runs -7V to +12V: those bounds exist to allow up to plus or minus 7V of ground potential difference between driver and receiver.

That rejection is what lets the receiver decide on 200 mV instead of 3V. TSB-89-A, TIA's own application bulletin for the standard, works the budget out: 1.5V of guaranteed driver output against a 200 mV threshold leaves about 1.3V to lose to attenuation and noise.

Compare those honestly and the raw numbers point the wrong way. RS-232 has 2V of margin, RS-485 has 1.3V. RS-232 still loses. What matters is not the size of the margin but what eats it. Induced noise and ground offset come straight off the RS-232 budget, since both land on the signal itself. Neither touches the RS-485 budget, because the receiver already subtracted them. A 3V offset between two panel grounds is a non-event on one and fatal on the other.

Why RS485 vs RS232 is a speed difference at all

Here is the part that gets skipped. Data rate is capped by edge rate.

SLLA545 states the rule plainly: the driver's output rise and fall time limits the maximum data rate the transceiver can operate at, and the working guideline is that rise and fall time stay under about a third of the total bit time. So "how fast can this run" is really "how quickly can the driver get the line from one valid state to the other." Which depends on how far it has to travel.

To be compliant, RS-485 has to produce 1.5V differential across a 54 ohm test load, per the test circuit in SLLA545. Call the full excursion between the two logic states about 3V at the pair.

RS-232 has to drag the line from at least -5V to at least +5V under load. That is a 10V trip minimum, often more, since the standard permits up to plus or minus 15V. The same standard also caps driver slew rate at 30 V per microsecond, and limits total load capacitance to 2,500 pF.

Run the arithmetic on the edge by itself. A 10V transition at the fastest slew rate allowed takes 333 ns, with no cable attached yet. Fit that inside a third of a bit and the bit cannot get much shorter than a microsecond. Cable capacitance only pushes it further out. TI's comparison chart in SLLA545 puts real RS-232 near 100 kbps over a few meters, and RS-485 at 50 Mbps over a short run.

Less voltage to move, faster edges, shorter bits. Differential signaling is what makes the small swing possible in the first place. The speed just falls out of it.

The 1734-232ASC covers the RS-232 side on a POINT I/O rack. Swap it for a 1734-485ASC and the same terminal base takes it.

The spec numbers, side by side

Parameter	RS-232	RS-485
Signaling	Single ended	Differential, balanced pair
Driver output, loaded	plus or minus 5V to plus or minus 15V	1.5V to 5V differential
Receiver decision threshold	plus or minus 3V	plus or minus 200 mV
Receiver input resistance	3 kilohm to 7 kilohm	12 kilohm minimum, equals one unit load
Common-mode range	Not specified, referenced to ground	-7V to +12V
Driver slew rate	4 to 30 V per microsecond	Not capped by the standard
Maximum load on the driver	2,500 pF total	32 unit loads plus two 120 ohm terminations
Devices per link	One driver, one receiver	32 unit loads, up to 256 nodes on 1/8 UL parts
Maximum cable length	Not specified as a distance	1,200 m, 4,000 ft
Data rate over a short run	About 100 kbps	Over 10 Mbps in the standard, 50 Mbps on current silicon
Topology	Point to point	Multipoint bus

 

Both columns of the RS485 vs RS232 table above trace to published documents. The RS-485 side comes from TI's standards overview SLLA070D Table 4, the design guide SLLA272D, SLLA545, and TSB-89-A. The RS-232 column is the standards summary table published by Analog Devices in its guide to selecting and using the RS-232, RS-422 and RS-485 data standards. And that 50 Mbps is not one vendor talking up its own silicon. SLLA070D footnotes TI devices running to 50 Mbps, and TSB-89-A states independently that current semiconductor technology allows signaling rates up to 50 Mbit/s.

The 50 foot rule is not in the RS-232 standard

Distance is where RS485 vs RS232 picks up the most folklore. Fifty feet is the number everybody repeats. It is not in the document. What the standard limits is load capacitance: 2,500 pF total, everything the driver has to charge.

Which explains something that otherwise looks random. The same RS-232 link runs fine at 80 feet on one job and drops characters at 25 feet on the next. It is not the distance, it is the cable. Capacitance per foot varies a lot between constructions, so 2,500 pF arrives sooner on some than on others. Fifty feet is roughly where ordinary multiconductor lands, which is how it became a rule.

RS-485 does put a real distance in its standard, 1,200 m or 4,000 ft. TSB-89-A gives the trade against speed as a rule of thumb: keep cable length in meters times data rate in bits per second under 10 to the eighth. A thousand meters at 100 kbps. Want 1 Mbps, keep the run inside a hundred meters. You do not get both.

How many devices you can actually hang on the bus

Device count is the other half of an RS485 vs RS232 decision, and the number everybody quotes is 32. That 32 is a load budget, not a device count.

TIA defined a unit load so drivers had something measurable to be held to. One unit load is the standard's 12 kilohm minimum receiver input resistance. What a compliant driver has to do, per SLLA545, is hold 1.5V differential across 32 of those in parallel with two 120 ohm terminations. That is the budget. Everything on the bus draws from it.

Transceivers get built at fractions of a unit load and the node count scales straight off that. SLLA545 Table 3-1: 1/2 UL parts give 64 nodes, 1/4 UL gives 128, 1/8 UL gives 256. Same 32 unit load budget every time.

Biasing spends from that same budget, and this is the part that catches people. Add external failsafe bias so an idle bus reads as a known state instead of floating, and those resistors load the driver like anything else does. SLLA272D puts 523 ohm bias resistors in series with the 120 ohm termination and notes the ceiling drops to 96 nodes once that is in place. SLLA545 works a 645 ohm external bias pair out at roughly 18.6 unit loads on its own. Over half the budget, on two resistors. Transceivers with internal biasing avoid it, so read the data sheet before designing around a node count.

One more, since it comes up on retrofits. TSB-89-A illustrates seven wiring configurations and flags the star outright as not recommended. Daisy chain it, terminate the two physical ends and nowhere else, keep stubs short.

RS-485 is not a protocol, and DH-485 is not RS-485

Worth stating plainly, because on legacy Allen-Bradley gear this one causes real confusion.

RS-485 specifies the electrical characteristics of drivers and receivers. That is all it does. SLLA545 says so on its first page: the standard does not specify a protocol. Modbus, Profibus and DMX512 all reference RS-485 for their physical layer, then define their own framing, addressing and bus access on top.

So "it is RS-485" tells you what the voltages do. It tells you nothing about whether two devices will talk.

DH-485 is Rockwell's protocol running on RS-485 hardware, which is where the name came from. A MicroLogix with a DH-485 port and a Modbus RTU meter can sit on electrically identical physical layers and never exchange a byte. Master count is the same story. Whether a bus tolerates more than one master is a protocol question: Modbus RTU allows one, DH-485 passes a token and allows several.

1734-485ASC, the RS-485 and RS-422 module in the same family. On an RS485 vs RS232 decision this is the side that also brings termination and bias with it.

When RS-232 is still the right call

RS485 vs RS232 is not a question I answer the same way every time, and I would rather not pretend otherwise.

One device, short run, inside the same enclosure, and it already has a 232 port. Use the 232 port. Nothing to terminate, nothing to bias, no A and B polarity to get backwards. And when you do get it wrong, one device goes quiet instead of the whole bus. Console and configuration ports on drives and meters are RS-232 for a reason.

Second device on the same run. Or the run leaves the cabinet. Or the cable shares a tray with anything switching real current. Then it is RS-485, and it is not close.

RS485 vs RS232 questions that come up on the job

RS485 vs RS232: can I use the same cable for both?

Not on a real run. The connector may fit; the electrical requirements do not match. RS-485 needs a twisted pair with a controlled characteristic impedance so the 120 ohm terminations match the line. TSB-89-A tabulates five cable constructions in the 60 to 120 ohm range, and 120 ohms is what the transceiver design guides assume. Ordinary RS-232 cable is not built to a characteristic impedance at all. Short and slow, you might get away with it. On anything long enough to matter you end up chasing reflections instead of commissioning a machine.

Can you convert RS-232 to RS-485?

Electrically yes, and that is the whole of what a converter does. It re-levels the signal. If the two ends speak different protocols, a converter does nothing for you and you need a gateway that translates. That is the most expensive mistake in an RS485 vs RS232 job: buying a level converter for what turns out to be a protocol problem.

RS485 vs RS232: which is faster in a real panel?

RS-485, by a wide margin, and it widens with the length of the run. Roughly 100 kbps against tens of Mbps once the run is short enough. The catch is that RS-485 only holds a high rate over a short run, per the length times rate rule above, so the honest answer on a 300 meter line is that both of them are slow and only one of them still works.

Do I need termination resistors on RS-485?

On any run of length, yes. Two of them, 120 ohms, one at each physical end of the bus and nowhere else. Not one per device. That mistake looks harmless and is not: every extra termination loads the driver further and drags the differential voltage down toward the 200 mV threshold.

Is RS-485 still worth designing in?

It has been in service since 1983 and it is still what most field instrumentation ships with. Modbus RTU, Profibus DP and DMX512 all sit on it. For a handful of devices at moderate rates, nothing has replaced it on cost.

Which module, once you have picked a side

If you have already worked out which side of RS485 vs RS232 your device needs and just want the part, the POINT I/O pair is the easy case: 1734-232ASC for RS-232, 1734-485ASC for RS-485 or RS-422, same form factor and same rack. We keep both in stock as surplus, along with the ILX34-MBS485 when the job is Modbus RTU over RS-485 rather than raw ASCII, and the 1763-NC01 cable for MicroLogix DH-485.