Data Transmission And Reception Timing - Mitsubishi Electric MELSEC iQ-R Series User Manual

12.2

Data Transmission and Reception Timing

Half-duplex communications uses C24 RS-232 interface CD (DCD) and RS (RTS) signals to control communications.
If the target device can transmit and receive data according to ON/OFF of C24 RS (RTS) and CD (DCD) signals as shown
below, half-duplex communications is possible.
• RS (RTS) signal : C24 turns ON/OFF this signal as shown below.
When data is transmitted from C24, this signal is turned ON. When transmission is completed, this signal is turned OFF.
• CD (DCD) signal: Turn ON/OFF this signal on the target device side as shown below.
To transmit data from the target device, turn ON this signal. When transmission is complete, turn OFF this signal.
The data transmission/reception timings for data communications with half-duplex communications are shown by the CD
(DCD) signal and RS (RTS) signal of C24.
Timing when transmitting data from target device
Transmit data by controlling C24 CD (DCD) signal according to the setting value for "simultaneous transmission priority/non-
priority designation" of the parameter.
■When C24 is designated to "Priority"
Target device
CPU module
SD(TXD) (Send Data)
RS(RTS) (Send Request)
CS(CTS) (Send Possible)
CD(DCD)
(Data Carrier Detect)
RX(RXD) (Receive Data)
 When transmitting data, check the RS (RTS) signal. If the RS (RTS) signal is OFF, turn ON the CD (DCD) signal. If the RS
(RTS) signal is ON, wait until it is turned OFF, and then turn ON the CD (DCD) signal.
 Transmit data after turning ON the CD (DCD) signal.
 After data transmission is completed, turn OFF the CD (DCD) signal.
 If the RS (RTS) signal is turned ON during data transmission, terminate data transmission and turn OFF the CD (DCD)
signal, and perform data reception processing. (Simultaneous transmission occurred.)
 After transmission from C24 is completed, retransmit all the data terminated at the step .
*1 Take the following measures between the devices communicating data as a countermeasure against ignoring received data by C24.
 Transmission and reception of a response message in reply to data transmission
 Time-out check of a response message and retransmission of data due to time-out error (target device side)
12 DATA COMMUNICATIONS USING HALF-DUPLEX COMMUNICATIONS
244
12.2 Data Transmission and Reception Timing
Data A
Ò
Ó
Ô
Data A
Terminate data
transmission.
Data B
Data C
Since the C24 has priority, it
continues to transmit even if the
CD(DCD) signal is turned ON.
Store this data at the
target device.
Data C
Õ
The C24 ignores
the receive data.
(*1)
Data B
Data B
Ö
Data B
Retransmission