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Deployment PtP communication
Modbus - Function codes
Modbus
Telegram structure
Start sign
Slave address Function Code
Broadcast with slave
address = 0
ASCII, RTU mode
Supported Modbus proto-
cols
6.8 Modbus - Function codes
Naming convention
Range definitions
166
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:
Data
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.
In opposite to a "normal" send command, the broadcast does not require a telegram
n
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
Modbus has some naming conventions:
n
Modbus differentiates between bit and word access; bits = "Coils" and words = "Reg-
ister".
n
Bit inputs are referred to as "Input-Status" and bit outputs as "Coil-Status".
word inputs are referred to as "Input-Register" and word outputs as "Holding-Reg-
n
ister".
Normally the access at Modbus happens by means of the ranges 0x, 1x, 3x and 4x.
0x and 1x gives you access to digital bit areas and 3x and 4x to analog word areas.
VIPA System SLIO
Flow control
HB300 | CPU | 013-CCF0R00 | en | 16-40
End sign
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