Connection to Modbus Slave and PC over RS485 to RS232 Convertor


Thread Starter


I am finding difficulty to get connection to Modbus and PC where modbus slave got RS485 cable
for which I am using RS285/RS232 convertor to connect it to PC. So please help him what's may be the problem?

Daniel Chartier


We do it all the time using B&B port-powered converters. We use Modscan as testing software (free from the Web) and always manage to connect, if our cabling is correct, and if the communication port of the slave is powered and configured properly. Just make sure that comm parameters are set to the same values on the master and the slave (baudrate, number of charaters, parity [very important], number of stop characters)and that Modbus telegrams are enabled on your master.

Hope this helps,
Daniel Chartier
Possible problems:

1) RS-232 wiring: is it correct?
Signal ground wired on RS-232 side?
Signal ground wired on the RS-485 side?
Wiring correct for half duplex or full duplex, depending?

2) Handshaking: what are hand shake requirements? Are those connections wired, if

3) RS-485 side: A and B are not universally A and B, sometimes they are labeled the opposite of what one expects. Try swapping the RS-485 A and B lines.

4) If it's a port powered converter, use a powered converter or sometimes you can supply a
dc voltage from a power supply to a port powered converter, because port powered converters are notorious for not working on laptops and have a bad reputation for only sometimes working on desktops. How do you know the converter works at all?

5) Are there DIP switches on the 232/485 converter? How do you know they're set right?

6) How do you know your PC's serial port is available? Many Windows programs grab the
serial port and don't let it go. Palm's HotSynch manager, RSLinx, and any HMI that runs as a service that uses the serial port probably already has possession of COM1. Do you need
additional serial ports, or can you turn off what might be grabbling the serial port?

7) Have you configured the comm parameters correctly on both ends?
Baud rate
Word format: 8 bits, parity? stop bits? If you use no parity, can the slave handle 1 stop bit? Are you using Modbus RTU or Modbus ASCII?

8) Are you sure you have a Modbus master driver on the PC? Modbus slaves can't talk to Modbus slaves. Is your master a commercial package, or did you write it yourself?

9) If the register addressing is in hex, try to address plus or minus one from the expected
address to account for offset.

10) Are there blinkie lights (LEDs) on the converter? Does the Tx LED blink when the master on the PC polls the slave?

Yesterday I was on a start-up and the client pulled me aside because he couldn't connect to a Modbus device on the bench test setup.

He had a brand new B&B model 485SD9TB port powered converter and Win-Tech's Modscan 32 on a Dell Inspiron 4000 laptop with a native serial port. The error code in Modscan was "device not connected".

I went to Radio Shack, bought a 12Vdc plug-in-the-wall (wall wart) DC power supply, wired it to the + & - power terminals on the B&B converter and presto, Modscan connected to transmitter.

It's about 20th time I've witnessed that. I wrote down the model numbers because I wanted more proof than anecdotal "that's happened to me".

I'm really glad others have such great experience with port powered coverters, but I've found numerous instances where they wouldn't connect without external power.

I use the ADAM-4561 USB to RS-485 port with Modscan, as COM9 in my case. This works better than any converter I have ever used. Most converters do not work properly with laptops without some fuss.

Daniel Chartier

Hello David;

We have had good experience with the B&B 4WSD9TB model, propt-powered as I mn=mentionnded before, on a DELL Latatitude D510 also with native serial port.

To make sure we have enough power from the port, we use short runs, small network sections (we tend to work in groups of 10 slaves more or less for our tests) and remove any termination circuit from the bus; this is recomended by B&B support. We have had no major issues with the converter in this configuration.

I agree that a backup power supply would be useful, but I have not needed it yet, and my laptop is already too heavy to lug, with tools, cables, manuals and various media of backup... maybe I'm getting lazy.

Hope this helps,
Daniel Chartier