Siemens SENTRON PAC RS485 Manual page 86

Glossary
Function code
The function code defines the meaning of the message frame. The function code defines the
structure of the message frame.
Half-duplex mode
Bidirectional data traffic. Data is exchanged between the communication partners but only in
one direction at a time. In half-duplex mode, the driver switches the twisted-pair cable
R(A),R(B) of the interface between send mode and receive mode. Individual data flow
control characters (e.g. XON/XOFF) can be an exception to this. These can also be received
and sent during send/receive mode.
The following are ignored in this mode:
● All detected transmission errors before and after the receive message frames
● The BREAK level during the message frame breaks
Master
The master sends requests to a certain slave in unicast frames. In broadcast frames it sends
requests to all slaves in the network.
MODBUS RTU
MODBUS RTU is a master/slave protocol which runs on a serial channel. There is only one
master on a MODBUS RTU bus.
Parity bit
Depending on the setting, the parity bit supplements the number of data bits to be
transmitted to form an even or odd number. This is for the purpose of data security.
Report
All communication partners involved in data transmission must follow fixed rules for handling
and implementing the data traffic. Such rules are called protocols.
Response monitoring time
The response monitoring time is the time the master spends waiting for a response message
frame from the slave once a request message frame has been output.
Slave
A slave may only exchange data with a master after being requested to by the master.
Start bit and stop bit
One start bit and up to two stop bits are used with asynchronous serial data transmission in
order to enable synchronization to every transmitted character for the receiver.
86
PAC RS485
Manual, 02/2008, A5E02091800B-01