A request can be directed to a certain slave ore be send to all slaves as broadcast mes-
sage. For the identification of a broadcast message you have to set bit 5 to 1 in the ADR
byte. Here the slave addr. (bit 0 ... 4) is ignored. In opposite to a "normal" send command,
the broadcast does not require a telegram evaluation via FC/SFC 218 SER_RCV. Only
write commands may be sent as broadcast.
n
The Modbus protocol is a communication protocol that fixes a hierarchic structure
with one master and several slaves.
n
Physically, Modbus works with a serial half-duplex connection. There are no bus con-
flicts occurring, because the master can only communicate with one slave at a time.
n
After a request from the master, this waits for a preset delay time for an answer of the
slave. During the delay time, communication with other slaves is not possible.
n
After a send command, the acknowledgement telegram must be read by a call of the
FC/SFC 218 SER_RCV.
n
The request telegrams send by the master and the respond telegrams of a slave have
the following structure:
Telegram structure
Start sign
Slave address Function Code
Data
Flow control
End sign
n
A request can be directed to a special slave or at all slaves as broadcast message.
n
To mark a broadcast message, the slave address 0 is used.
n
In opposite to a "normal" send command, the broadcast does not require a telegram
evaluation via FC/SFC 218 SER_RCV.
n
Only write commands may be sent as broadcast.
Modbus offers 2 different transfer modes. The mode selection happens during runtime by
using the FC/SFC 216 SER_CFG.
n
ASCII mode: Every byte is transferred in the 2 sign ASCII code. The data are marked
with a start and an end sign. This causes a transparent but slow transfer.
n
RTU mode: Every byte is transferred as one character. This enables a higher data
pass through as the ASCII mode. Instead of start and end sign, a time control is used.
The following Modbus Protocols are supported by the RS485 interface:
n
Modbus RTU Master
n
Modbus ASCII Master
Broadcast with set bit 5 in
ADR byte
Modbus
Broadcast with slave
address = 0
ASCII, RTU mode
Supported Modbus proto-
cols
VIPA System MICRO
Option: PtP communication
Protocols and procedures
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