Information Technology Reference
In-Depth Information
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Returned data - this contains the data returned by the slave, either register values or a
status.
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Error-checking field - this allows the master to validate the message integrity.
A standard Modbus network only contains masters and slaves. On a Modbus Plus network
controllers can operate as a master or a slave. The Modbus protocol is still applied to the
transaction. This typically occurs over a network.
Master
Slave
Slave
Slave
Slave
Figure 18.1
Master-slave
18.1.2 Transmission modes
Modbus transmits values from the master to the slave either using ASCII or RTU (remote
terminal unit). All the devices on the network must be set to the same setting. These are:
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ASCII - Modbus transmits the bit values as ASCII characters which represent the hexa-
decimal of the transmitted bit values. The transmitted characters will range from '0' to
'9' and 'A' to 'F'. For example, if the transmitted bit stream is to be:
0110 1111 0001 0011 1100 1100 1011 0000
this would be transmitted as the ASCII characters:
'6' 'F' '1' '3' 'C' 'C' 'B' '0'
In this mode, a start bit is transmitted, followed by a 7-bit ASCII character, an optional
parity bit and then two stop bits. The least-significant bit of the ASCII character is sent
first.
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RTU - an 8-bit value is sent as two hexadecimal values. For example:
0110 1111 0001 0011 1100 1100 1011 0000
this would be transmitted as the following:
