I'm working on Linux system that using modbus rtu RS485 to send and receive data. My device is master and just send "request latest data"(8Bytes include 2Byte CRC) to slaver (from now is only 1 slaver) every 1second. when Slaver receive request, they will prepare data (71Bytes include CRC) and send back to Master. I can't see the source of Slaver because this is commercial product. Both of Master and Slaver using same baud rate 38400.

Result:

1. When check communication between Master and Slaver, sometimes (average is 1-2hours) data from Slaver is lost some first Bytes, and first Byte that received has been modified with other value that sent from Slaver (sometime lost some first Bytes only)
2. Sometime data from Slaver not come (timeout but do not receive any data). I tried to increase timeout by 500ms or 1sec but still occurs without any change
3. I tested Slaver by communication with Teraterm and there is no error like above. Data that sent and received are OK. With Master, I also tested with Teraterm and there is no error.
4. When I try to catching data while Master and Slaver send & receive data, when problem are occurred(not receive any data or lost some first bytes) on both of Master side and on my PC side (try to catching data byte Teraterm on PC).

I believe that problem is on Master side, and maybe on serial port setting, but I don't know where were wrong. Please help.

Sorry for my poor English!

CodePudding user response：

I used to work with RS485 bus a lot. And one problem that sometimes appeared was very similar to your. Because RS485 is half duplex bus, there is mechanism that switching receive mode and transmit mode on RS485 bus driver. And this was exactly cause of my problems.

When master device sent some data, slave was ready to reply (and replied) before bus driver (on master side) was switched to receive mode. This behavior ended with data loss.

May I suggest you to check, using oscilloscope, that slave sent data correctly? If so, you probably don't have too much options to do as possible solutions are:

• Slave have to wait some time before sending reply to master.
• Change HW, some RS485 driver that will be faster in switching modes or use different BUS.