PLC COMMUNICATION

M

Thread Starter

martin

hi sir..

why rs485 cables used in plc-pc communication ? why not rs232?

thanks for ur answer..

Martin
invensys automation
 
> why rs485 cables used in plc-pc communication? why not rs232?

rs232 is point to point only allowing 2 devices to communicate with each other.

rs485 allows 1 sender and multiple receivers at a time.

Typically you will have 1 master and several slaves.
The master will send a request and only the slave which was addressed will answer a response. Thus the bus arbitration is solved. Only 1 device (master or slave in question) will send to the wire. Modbus is such a master slave protocol.

There are also protocols that use tokens and multiple masters. The token is given from one master to the next. Each master is only allowed to hold the token a certain maximum time.
Once a master has got the token it can send requests to other masters and slaves. PROFIBUS is such a multi master protocol.

Another difference between rs232 and rs485 is the electrical signal on the wire. rs232 uses a +-12 Voltage. rs485 uses a differential signal and twisted pair cables thus making the communication more insensible against noise.
 
Top