User`s manual
336
14.2 External Devices Interlock Conditions
When using a multidrop line to communicate data between external devices and the programmable controller CPU in
an m: n configuration, interlocks must be provided between the external devices so that multiple external devices
cannot communicate data with the programmable controller CPU at the same time.
This section describes the conditions for providing the interlocks between the external devices so that any one of the
external devices can communicate data with the programmable controller CPU.
(Conditions for priority use (obtaining line ownership) of the line from the start to the end of data communications with
the programmable controller CPU.)
14.2.1 Maximum communications time per external device station
This condition determines the maximum time each external device can communicate with the programmable controller
CPU after obtaining line ownership. (Time in the illustration below.)
This prevents loss of data communications between other external devices and the programmable controller CPU by
shutdown of the external device that obtained line ownership.
● Make the maximum data communications time per external device station the maximum time of the external device that
requires the most time to communicate data with the programmable controller CPU.
● After system starting, complete data communications from the external device that obtained line ownership and the
programmable controller CPU within the maximum communications time.
(If data communications cannot be completed within this time, initialize the Q series C24 transmission sequence by
transmitting the EOT/CL code to the objective programmable controller CPU within the maximum communications time.
( MELSEC Communication Protocol Reference Manual)
● While an external device and the programmable controller CPU are communicating data, have the other external devices
check for timeout so that they do not transmit data during this time.
External device 1
Indicates the range within
which the target device
cannot obtain line ownership.
Time
Start of data communications
External device 2
External device 5
to
Indicates the range within
which the target device can
obtain line ownership and
can have data communication
with the programmable
controller CPU.