Hello everyone!

We have an energy efficiency project in Brazil which uses the RS-485 serial to communicate with devices such as: electricity meters, temperature sensors and other devices. The protocol used is Modbus RTU @ 9600bps. The figure below shows a typical panel where the instruments are installed on the cover. In this case, there are ten three-phase electricity meters and four temperature meters.

Behind the cover, the voltage and current cables from the CT's are in the same bus together with the cables of the RS-485 communication circuit. The RS485 communication circuit was connected via a daisy-chain connection. The cable is shielded and has been grounded at a single point. A 120R terminating resistor was inserted into the last device on the network. The IOT-2040 gateway is located on the panel rail, where it receives the various connections, including the RS-485 bus. Observing the waveforms using an oscilloscope, it was possible to verify that there was no expressive noise on the RS-485 bus.

The problem is that despite the simplicity, bus size and number of connected devices, the RS-485 serial interface does not work, that is, it is not possible to communicate with the devices mentioned above. However, in one of several attempts to make the circuit work, one of the engineers came up with the idea of ​​using a USB/RS-485 converter, not to communicate with the devices on the bus, but only to check if there would be any changes in operation. In this case, all the devices on the bus started to communicate normally, and thus remain in operation after more than 3 months.

Opening the IOT-2040 gateway, we can see that the IC: EXAR SP339EER1 is used, which (according to the manufacturer) does not require the use of external resistors to adjust the bias. However, we are faced with the opposite situation in which, due to using a USB/RS-485 converter with bias voltage adjustment resistors, the devices started to communicate normally.

Would there be any possibility of inserting such resistors on the IOT-2040 board? If not, would there be any other alternative?

Attachment, pictures of the panel, pictures of the IC: EXAR SP339EER1 used in the gateway and also, pictures of the USB/RS-485 converter where it is possible to identify the bias adjustment resistors.

Thanks in advance,

Christian Vieira

Thanks for the answer!

The problem does not concern the configuration of the RS-485 serial interface, but the operation of it with the devices installed on the board!There is simply no communication with the devices without the bias resistors installed on the bus !!!

Iam facing same problem with Siemens CM PTP module(as Modbus Master.) communicating with Atlas Copco Mk5 Gateway Module as Modbus Slave

Communication was not working at the site So to test Mk5 ,we put USB to RS485 convertor and communication started. So now when we remove USB from laptop communication stops & when we connect USB  it start...

Chris Vieira have you got any solution to your problem

Hi swatipatwardhan,

are you using IOT2050 too? Or do you have a general issue with the communication? Please describe your isse more in detail.

swatipatwardhan

Iam facing same problem with Siemens CM PTP module(as Modbus Master.) communicating with Atlas Copco Mk5 Gateway Module as Modbus Slave

Communication was not working at the site So to test Mk5 ,we put USB to RS485 convertor and communication started. So now when we remove USB from laptop communication stops & when we connect USB  it start...

Chris Vieira have you got any solution to your problem

 I am facing same problem like you.

Have you found any solution