Modbus - Access To Multiple Slaves - YASKAWA CP 040 RS422/485 Manual

VIPA System SLIO

5.6.2 Modbus - Access to multiple slaves

HB300 | CP | 040-1CA00 | en | 18-28
4. Configure the slave section. The configuration of the CP 040 as Modbus master
happens via the hardware configuration. In addition you need a PLC user applica-
tion for the communication with the following structure:
OB 100: One-time call of the handling blocks FB 60 - SEND and FB 61 - RECEIVE
(or FB 65 SEND_RECV) with all parameters and set R for initialization.
OB 1: Call of FB 60 - SEND (or FB 65 SEND_RECV) with error. For this an area
starting at 0 is stored in the CP 040 where the master may gain access via Modbus.
The FB 61 - RECEIVE with error evaluation allows you to transfer a data area into
the CPU. At a data change by the master, only those data are transferred to the
CPU where changes occurred.
At deployment of multiple slaves with RS485, there cannot occur bus conflict errors
because the master may only communicate with one slave at a time. The master sends a
command telegram to the slave specified via the address and waits for a certain time
where within the slave may send its respond telegram. During the latency the communi-
cation with another slave is not possible. For the communication with multiple slaves
every slave needs a SEND data block for the command telegram and a RECEIVE data
block for the respond telegram. An application with several slaves would consist of an
according amount of data blocks with commands.
These are executed in sequence:
1. slave:
1. Send command telegram to slave address 1. slave
2. Receive respond telegram from slave address 1. slave
3. Evaluate respond telegram
2. slave:
1. Send command telegram to slave address 2. slave
2. Receive respond telegram from slave address 2. slave
Serial communication protocols
Deployment - Modbus > Modbus - Access to multiple slaves
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