RS-485, protocol or physical link?

  • Thread starter a big man with a little doubt
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Thread Starter

a big man with a little doubt

I have the following doubt: when an equipment says that it has port RS-485, in fact to what talks about this?

for example protocol RS-232 you find it anywhere, has start bit, baudrate, stop bit etc. but in fact by I see, RS-485 only specifies the physical link ...and the communication protocol???
or I am mistaken?

Daniel Chartier


Have a look at the following link:

It lists the caracteristics of different RS-xxx drivers. Note that they distinguish them on the basis of line driver specs. The "Recommended-Standard" 232, 422, etc. only specify physical link layer elements.

Now, look at Modbus, on the other hand. When a vendor claims he will communicate data using Modbus (a protocol specifying the data link layer), he must specify the physical layer and parameters he will use. He will state Modbus RS-232 (8,9600,1,E), or Modbus RS-485 (4-wire) or even Modbus-TCP(a little different, as the Modbus data is encapsulated in a TCP/IP frame, but you could stretch the point).

Always remember the old analogy: being able to connect to another person over a telephone line (physical layer) will not ensure that you will understand wach other, if one speaks English and the other Pakistani (the protocol ensures data integrity and transferability).

Same thing applies on other physical layer definitions, like TCP/IP connections: Modbus-TCP and Ethernet-IP can coexist on the same Ethernet cables, but the processors will not exchange data.

Hope this helps,
Daniel Chartier

Nathan Boeger

Lots of good info about the protocol here:

I'm not sure what the typical port you would use and what devices support this. It's a serial protocol. Here's a pinout diagram to convert you serial port (DB9)->RS-485:

Hope this helps. I'd like to learn more about it if someone here is experienced with RS-485.

Nathan Boeger
Inductive Automation
"Design Simplicity Cures Engineered Complexity"
RS-485 is a physical layer electrical specification, not a communications protocol. RS-485 uses two wires and differential signaling (ie voltage differences across the two wires), and be multidropped to allow multiple devices on the network. Modbus-ASCII, Modbus-RTU, and Profibus-DP are three examples of protocols which use RS-485 as their physical layer.
> or I am mistaken? <

You are not.

RS-485 is about as meaningless as "ethernet". Both are hardware specs and tell nothing about the protocol used to transfer data.

OVer the years, there have been devices that have had comm ports with either RS-232, RS-485 or ethernet for which the manufacturer had no viable means of getting data into or out of the port.

On a daily basis I encounter people who think that plugging a cable into ethernet ports or RS-485 ports on two devices will magically transfer readable data to the other device.

You are right to be sceptical.


( Complete thread: )
485 is the physical link, you could indeed put a 485 adapter on each end and talk standard Serial across the link. The protocol is "typically" Modbus Serial, but it could also be anything else also like DF1 or a custom protocol.
RS-485 is a physical link and it comes in many forms: DB9, RJ45, or terminal blocks. Unlike RS232 standard, there aren't any RS-485 pin-out out there b/c there is no standard specifying which pin is TD+, TD-, RD+, RD-, and SG. Each manufacturer of serial devices have their our conventions of assigning pin-outs. Refer to the PLC vendor for RS-485 pinout info.

Tom Tran
[email protected]
im sorry if my question is stupid. i'm a bit new to automation. If profibus and Modbus both use rs485 as their physical layer, then why aren't their cables interchangeable (use same cable to connect to both protocols)? and then each protocol use its own standard in the upper layers?.

The specs that define the physical layer for RS-485 define the electrical characteristics, not the hardware connectors used.

The product manufacturer decides which hardware connector to use, so there's all sorts of variety.

I've seen RS-485 on various hardware connectors: screw terminals, RJ45, DB9. DB25, DB15, M12, Amphenol whatevers, RJ-11/12 and on-site adapters using wire nuts or electrical tape.

The nature of the market is that Profibus DP on RS-485 is not likely to be the same network link that one would drop a Modbus RTU slave device onto. The host port that deals with the protocol/data would each be a different port, dedicated to its protocol, so the connector the port uses then varies with whatever that turns out to be (different COM module in the rack with whatever connector it provides)