MODBUS RTU 485 or RS232 (Multiple or single slave)

Hi,

Can someone confirm me:

If there is case with RS232 protocol there can be only 1 slave per 1 master on communication link

On RS485 can be 256 slaves bouned on 1 master?

naturephoenix said:

Hi,

Can someone confirm me:

If there is case with RS232 protocol there can be only 1 slave per 1 master on communication link

On RS485 can be 256 slaves bound on 1 master?

Click to expand...

Correct on RS232, its 1:1.

RS485 is 1:n but the limit without repeaters is 32 nodes, you can have addresses 1-256

tragically1969 said:

Correct on RS232, its 1:1.

RS485 is 1:n but the limit without repeaters is 32 nodes, you can have addresses 1-256

Click to expand...

Actually RS232 and RS485 aren’t protocols (Modbus RTU is) they are standards for serial communications. Because RS232 has a dedicated transmit and receive line it is sometimes perceived that it’s point to point only but it’s not. There are a lot of communication protocols that use RS232 in a multipoint format (including Modbus RTU). Having said that if it’s hardwired then you can’t do multipoint it has to go through something that separates the signals (usually radio modems). If the communication lines are hardwired and the application is multipoint then you do have to use RS485.

you can also use a splitter if it is hardwired. The one I have used the most is made by black box. You have the wire length limitation on rs232 but in my experience even at distances of a couple hundred feet rs232 will usually still work well. I wouldnt setup a new system in that way but if wire was already in place it is worth a shot.