rs485 mixing voltage

[slurf]

Hi,

I am having problem to communicate with a PLC (slave) from a pic32 (master) on a rs485 busline.
I cant really find any documentaion on the PLC rs485 voltage levels and the driver that im using is a 3.3V.

The protocol i am using is modbus and I have emulated it to the same specifiaction of the PLC but on a rs232 to a computer and it works flawlessly. And if I put the transceiver on and the receiver on at the same time sending im receiving the correct data.

Could the problem be that the plc rs485 driver doesnt handle the 3.3V, that the threshold is to low to recognise ? If so whats the best solution to solve it ? Except changing the current rs485 driver on the pic32 card.




Best regards

the_most_hunted

 

[Analog Ground]

If the RS485 driver and receiver both meet the RS-485 specification, then the transmitting and receiving voltage levels are OK. It does not matter if there is a mix of 3.3 or 5V parts. This is the point of using RS-485. Now, the termination on the signal lines also has to be compatible with the specification. Modbus adds another communication layer on top of the RS-485 spec but I don't think it changes the basic RS-485 physical layer (voltage levels).