3-pin RS232 to RS485

[AlfredoQuintero]

Hello:
I have to connect a RS232 Modbus RTU slave that only has Tx, Rx and GND signals to a Modbus RTU master.

However, all RS232 to RS485 converters that I have managed to find only work if the complete set of RS232 signals are present.

Interestingly, while trying a number of things after hours of work, accidentally I wired the following setup and worked!
To double check, I used two different serial boards on the PC (a PCIe one and a USB one) with same results:

Windows PC __________________ | ___ Modbus RTU
MBPOLL ______________________ | ___ (RS232C)
PCIe or USB to RS485 ________ | ___ Slave
Converter _____________________ |
___________________ (Pin 1) A+ <-|-> GND
____________________ (Pin 2) B- <-|-> Rx
__________________ (Pin 5) GND<-|-> Tx

The above setup works, but I cannot figure out why and do not want to recommend the solution to the customer unless I can explain why this works.

Will be grateful for advice on this issue. Thanks.

 

[AlfredoQuintero]

Never mind, I cracked the problem. With this product.
DSD TECH- Connecting the future

 

[danw]

Interesting dilemma. It's been at least a decade since I connected a Modbus RTU slave on an RS-232 port to a Master, but I thought that the USB/RS-232 oonverter (probably an FTDI chipset) would have worked using only Tx, Rx and GND, but I honestly can't remember and I can't even put my hands on a USB/232 converter because I've only used RS-485 over the past decade.

Glad to hear you found a solution, though.

 

[AlfredoQuintero]

Dan, hi, yes thank you.
Since I am sure this will be of interest to you and perhaps others, I elaborate a bit.
This is one refinery which has dozens of Rosemount FF transmitters (pressure, temperature), a plain-vanilla sort of application.
The DCS is an old Azbil DCS that only supports RS232C, but it is far from the field devices so they use an RS232 to RS485 media converter.
The media converter is connected to a Rosemount 3420 which monitors the IO and maps the FF data to Modbus RTU server, which Azbil uses.
The Rosemount 3420 is discontinued and the customer wants me to provide FG-200 and then engineering which would achieve their goal of being able to swap Rosemount 3420 (to be kept as a backup) for the FG-200, without having to change the Azbil DCS.
But I had never used FG-200 with RS485. For this conversion you need an externally powered converter. Of course, I will offer a more industrial grade solution. There are some which use 16 VDC - 30 VDC and are DIN-rail mounted, but the setup I have now I will use to configure my FG-200 and do the mapping while I wait for the device to provide to the customer.
This is how they system looks like in my messy lab.
Have a good weekend.