Compatibility between RS232 and RS485

[vangde]

Hello

I need to connect an electronic scale (designed to weight vehicles) to a PCB with microcontroller that im making.
The scale unit has a DB9 output (serial port) that is labeled RS485.

Thing is that i tried a usb to RS232 cable-converter and it worked fine plus i ordered a module for RS232 to TLL to integrate to the PCB.
I red that RS485 works with differential voltages and can tolerate lower voltages than RS232.

My question is: Is the label RS485 wrong(since it worked as RS232) or the two standards are compatible?
Is it safe to connect my usb cable-converter or the RS232 to TLL module for long periods of time and not endanger the scale unit?

 

[FvM]

It sounds unlikely that the RS485 scale interface works with a RS232-to-USB cable, unless the scale supports both interface standards in parallel on different pins or has a feature to switch the interface type. You should absolutely check the operators manual.

RS485 is preferred for longer cables due to higher noise immunity. RS232 is O.K. for local interfaces. You'll use a RS232-to-TTL level converter, e.g. MAX232. No problem to connect it permanently.

As main difference besides using different signal levels, RS485 uses differential signaling and half duplex while RS232 is single ended and full duplex (separate Rx and Tx lines).

 

[vangde]

It sounds unlikely that the RS485 scale interface works with a RS232-to-USB cable, unless the scale supports both interface standards in parallel on different pins or has a feature to switch the interface type. You should absolutely check the operators manual.

RS485 is preferred for longer cables due to higher noise immunity. RS232 is O.K. for local interfaces. You'll use a RS232-to-TTL level converter, e.g. MAX232. No problem to connect it permanently.

Thank you for your answer.

No, there is no switch to select between RS232 and RS485.
The unit is very old and made by a small company therefore its impossible that i find schematics.

Even if there were dedicated pins for RS232 in the DB9 output of the module, how the RS232 to usb converter would know that and comunicate through those pins.
This is my Converter: https://www.ebay.com/itm/USB-to-RS232-Serial-9-Pin-DB9-PIN-PL2303-Cable-Adapter-Convertor-AD-/112173355766?epid=1349416111&hash=item1a1e0d92f6:g:8tgAAOSwCEdYT2eW

 

[KlausST]

Hi,

I doubt a RS485 can directely interface to RS232. The standards definitely are not compatible.
Therefore your assumtion that this indeed is no RS485 device may be true.

RS485 uses two wires per channel with difference voltage of +5V/-5V (at least +/-2V at the driver and +/-0.2V at the reciever)
RS232 uses one wire per channel with absolute voltage of -9V/+9V (at least -5V/+5V at the driver and -3V/+3V at the reciver)

So you have a wiring problem: 1 wire --> two wires
And you have a voltage problem: RS485 generates no negative voltages reference to GND, but RS232 needs this.

How are they connected?

There are all converters you need available:
* RS485 to RS232
* RS232 to USB
* RS485 to USB
* RS485 to TTL
* RS232 to TTL
* USB to TTL
so you are free to connect them in a specified and reliable way.

Klaus

 

[FvM]

If the device is working with RS232-to-USB cable, it can be expected to expose at least the minimal RS232 pins 2,3 and 5. If you don't have any documentation, be happy that it works and try to operate it with your RS232 interface.

Problems might arise if additional handshake lines are required. The USB cables also drives pin 4 (DTR) and 7 (RTS) high when connecting with an application.

 

[vangde]

So, if the DB9 output of the scale was truly RS485 my usb converted should had not worked, right?

I have ordered https://www.ebay.com/itm/MAX232CSE-...e=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649 to interface my pcb and i dont have enough time to order different chip now if the one i ordered is not suitable.

 

[FvM]

The interface module is probably fine. Needs Tx and Rx crossed when connecting to the scale.