MODBUS slaves

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Thread Starter

Prem Sanil

I have two questions:

Why does Modbus allow only 247 slaves to be connected to the same network?

How it this limitation overcome by Modbus TCP/IP?
 
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Besides, on Modbus RTU or ASCII, you probably don't want to talk to more than maybe 50 slaves with any kind of frequency anyway. If you're conversing with each slave once an hour or something or only on a sporadic event-driven mode per node, then that might be a different matter.
 
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Lynn August Linse

> Why does Modbus allow only 247 slaves to be connected to the same network?

Keep in mind this is a fairly "new" limitation, so not all slave comply. A Master needs to be able to poll slaves with address 1 to 255 (You have been warned). For example I buy a cool little $40 thermostat device with 8 thermistor inputs, but it comes factory default as address 254, and you need to write a modbus register to change to another.

Modbus/TCP bypasses this because the 'address' is the IP, so you have billions of potential nodes with IPv4, and I'm not even going to mention the limit with IPv6!

- LynnL @ Etherios
 
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