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21: D
ATA
L
INK
C
OMMUNICATION
21-10
O
PEN
N
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C
ONTROLLER
U
SER
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S
M
ANUAL
Simultaneous Refresh Mode Communication Sequence
Unlike the separate refresh mode, the master station performs data link communication using an interrupt processing dur-
ing normal scanning. When communication with all slave stations is complete, the master station refreshes all received
data simultaneously.
As with the separate refresh, when a slave station receives a communication from the master station, the slave station
returns data stored in data registers assigned for data link communication to the master station.
Data refresh at the master and slave stations is done in the END processing at the respective station.
When the master station completes data refresh, special internal relay M8177 (all slave station communication completion
relay) goes on at the master station. Special internal relays M8140 through M8176 (slave station communication comple-
tion relay) do not go on at the master and slave stations in the simultaneous refresh mode.
The communication sequence in the simultaneous refresh mode is shown below:
Simultaneous Refresh Time at Master Station for Communication with One Slave Station (Trf)
When no transmit/receive data has been changed during communication at 19200 bps, the master station requires the fol-
lowing time to refresh data for communication with one slave station.
Tr f = 3.125 msec
When N words of transmit/receive data have been changed during communication at 19200 bps:
Tr f = 4.167 msec
×
(2 + N)
Total Simultaneous Refresh Time at Master Station for Communication with All Slave Stations (Trfn)
When the baud rate is set at 19200 bps, the master station requires the following time to refresh the transmit and receive
data for communication with all slave stations, that is the total of refresh times.
Tr fn =
∑
Tr f =
∑
4.167 msec
×
(2 + N)
Example: Refresh Time in Simultaneous Refresh Mode
When data link communication is performed with such parameters as transmit words 10, receive words 10, slave stations
8, average scan time 20 msec, and baud rate 19200 bps, then the total refresh time Trf8 for communication with all eight
slave stations in the simultaneous refresh mode will be as follows:
When no transmit/receive data has been changed,
Tr f8 = 3.125 msec
×
8 = 25 msec
When one word of transmit data has been changed at all eight slave stations,
Tr f8 = {4.167 msec
×
(2 + 1)}
×
8 = 100.0 msec
When 10 words of all transmit data have been changed at all eight slave stations,
Tr f8 = {4.167 msec
×
(2 + 10)}
×
8 = 400.0 msec
When the baud rate is 38400 bps, Trf8 for all slave stations is 400.0
÷
2 = 200.0 msec
Master Station
1 scan
Slave Station 1
Slave Station 2
Slave Station 31
Master Station
Completion M8177
END Processed
1 scan
Simultaneous Refresh for
Slave Stations 1 through 31
Phone: 800.894.0412 - Fax: 888.723.4773 - Web: www.clrwtr.com - Email: [email protected]