Hello all,

I have a lot of Modbus device - about 160 or 170 devices. So is it possible to read data from them if they are connected on a same bus by this way: there will have about 6-7 RS-485 repeaters placed on that bus?

Thanks.

 

Nothing in the Modbus specification prohibits such a configuration. It's really more a question of whether the repeaters introduce any significant delays, and if so that you must ensure your master waits long enough for responses.

But the delays should be insignificant with most repeaters (compared to the overall slave's response latency, etc.)

Darrin

 

From the Modbus spec: "The address field of a message frame contains two characters (ASCII) or eight bits (RTU). Valid slave device addresses are in the range of 0 – 247 decimal. The individual slave devices are assigned addresses in the range of 1 – 247. A master addresses a slave by placing the slave address in the address field of the message. When the slave sends its response, it places its own address in this address field of the response to let the master know which slave is responding.

Address 0 is used for the broadcast address, which all slave devices recognize. When Modbus protocol is used on higher level networks, broadcasts may not be allowed or may be replaced by other methods. For example, Modbus Plus uses a shared global database that can be updated with each token rotation."

 

I guess you have them in a Daisy Chain bus right?
And according with the definition you can connect 247 devices.

Regards

 

Dear friend:

From the logical point of view modbus protocol can address 247 different nodes. If you are using a RS-485 or RS-422 electrical interface the limit is 32 nodes or 1.000 meters per electrical segment, the first what happens. To overcome this limit you must use repeaters. And depending on the speed, the delays must be taken in account.

Good luck!