User manual

106
Communication Protocols
Q (Query) is a signal sent from the peripheral devices to the PLC after
setting the network ID number and the function code for the PLC to
communicate with.
QA (Query Acknowledge) is a signal sent from the PLC to the
peripheral devices, indicating that the Q signal from the peripheral
device was received.
RR (Response Request) is a signal sent from the peripheral device to
the PLC, indicating that the QA signal from the PLC was received. This
signal is sent when Q
→
QA is normal.
When the PLC receives the RR from the peripheral device, it
determines that the communication with peripherals is successful and
sends R (Response) signal to the peripherals. This R signal contains
how the original Q signal from the peripheral device handled its
function code. The communication cycle for one function code ends
when the PLC sends the R.
Communications delay
The 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.
CPU ID
All devices connected to the network need a network ID number for
communication. There is an available range of 0 to 191 network ID
numbers for the NX series.
Redundancy is not permitted. When a single PLC and a peripheral
device are connected, usually 0, 1, or 255 is assigned as the network ID
number to the PLC. When the peripheral device wants to communicate
to a connected PLC regardless of its programmed network ID number,
it can use global network ID number 255, to which any PLC will
respond. However, the NX series can not be used to communicate with
more than two CPU modules at one time, so if you assign ID 225 as an
ID of more than two CPU modules at once, it will cause
communication errors.
Step 2-QA
Step 1-Q
Step 3-RR
Step 4-R
Query
Query
Query
Query Acknowledge
Query
Response
Query
Response Request