How many slave devices cab be multidroped on RS-485 bus? I know we can have 32 devices on RS-485 bus. But I want to confirm which following combination is correct for MODBUS ASCII RS-485?

1) One Master and 31 slaves or
2) One Master and 32 slaves.

Which is right combination?

Thanks,
Vinod Gavande
vinod.gavande [at] ips.invensys.com

 

Hi,

For RS485, its 1 + 31.

Ramu

 

Hello,

One Master and 31 slaves = 32 devices. The master is the device as a slave.

Regards
Andrzej
www.modbus.pl

 

The Master is just another device, so 32 including the Master is your correct answer. Remember, the RS485 network is actually a 3 wire network, A, B and Common AND there are only 2 ends to the network eack of which require a termination resistor. Also, employ fail safe biasing for best results.

 

This is an electrical limitation of RS-485, not something to do with the protocol. It has to do with how much signal current standard RS-485 chips can supply and draw. A total of 32 nodes is the normal rule of thumb. You can have more though if you use special chips.

That means that for standard RS-485 the limit is a maximum of 32 nodes of any type, including masters, slaves, repeaters, gateways, and anything else you hook into it. That limit applies to any other protocol using RS-485 as well.

 

A couple years ago, I ran into a specific vendor's single loop controller that limited the number of devices to 15 on a Modbus ASCII link, because the slave addressing only went to 15 (maybe it was 16 counting from zero, I don't recall).

I guess that's what 'open' standard means: however the implementation comes out of the chute . . .

Bud