Appendix A: D50 PLC Communication Protocol 157
Step 5
Repeated Response
Once the original Q has been sent to the D50 PLC, the R message containing the requested data for
that query can be repeatedly received by sending only the RR message again.
Communications Delay
The D50 PLC will return a signal after receiving a Q or an RR within a specific time. However, due to
errors in the communications network, CRC values, and communication speed flux, there are
occasions when the PLC will not receive the signal from the peripheral device. The peripheral device
should allow up to three seconds for a response from the PLC. If there are no responses to the Q or the
RR message, the communication is considered to have failed, and the Q or RR should be sent again.
Example
1. No communication error.
2. When QA is not received.
3. When R is not received.
4. Repeated Response communications.
Peripheral device
QQARRR
For the internal processing of the PLC CPU
send RR at least 5 msec after receiving QA.
3 sec
Peripheral device
QQQARRR
Peripheral device
QQARR RRR
3 sec
Peripheral device
QQARRRRRR
PLC
PLC
PLC
PLC
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