User`s manual

213
CHAPTER 8 DATA COMMUNICATIONS USING HALF-DUPLEX COMMUNICATIONS
8
8.2 Data Transmission and Reception Timing
8.2 Data Transmission and Reception Timing
Half-duplex communications uses the Q series C24 RS-232 interface CD (DCD) and RS (RTS) signals to control
communications.
If the external device can transmit and receive data according to ON/OFF of the Q series C24 RS (RTS) and CD
(DCD) signals as shown below, half-duplex communications is possible.
• RS (RTS) signal: The Q series C24 turns ON/OFF this signal as shown below. When data is transmitted from
the Q series C24, this signal is turned ON. When transmission is complete, this signal is turned OFF.
• CD (DCD) signal: Turn ON/OFF this signal using the external device as shown below. To transmit data from
the external device, turn ON this signal. When transmission is complete, turn OFF this signal.
The following describes the half-duplex communications data transmission and reception timings by Q series C24 CD
(DCD) signal and RS (RTS) signal.
(1) Timing when transmitting data from external device
Transmit data by controlling the Q series C24 CD (DCD) signal according to the "simultaneous transmission
priority/non-priority designation" registered in "Transmission control and others system setting" screen of GX
Configurator-SC.
(a) When Q series C24 is designated "priority"
1) 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, then turn ON the CD (DCD) signal.
2) Transmit data after turning ON the CD (DCD) signal.
3) After data transmission is complete, turn off the CD (DCD) signal.
4) 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 generated)
5) After transmission from the Q series C24 is complete, retransmit all the data terminated at step 4.
*1 Take the following measures between the communicating devices as a countermeasure against ignoring of the receive
data by the Q series C24.
• Transmit and receive a response message in reply to data transmission
• Time-out check of the response message and data transmission due to time-out error (external device side).
1)
2)
3)
4)
5)
External device
Programmable
controller CPU
SD(TXD) (Send Data)
RS(RTS)
(Send Request)
CS(CTS)
(Send possible)
CD(DCD)
(Data Carrier Detect)
RD(RXD)
(Receive Data)
Data A
Data A Data B
Data B
Data C
Data B
Data C
Store this data at the
external device.
Terminate data
transmission.
Retransmission
Data B
Since the Q series C24
has priority, it continues
to transmit even if the
CD(DCD) signal is
turned ON.
The Q series C24
ignores the receive
data.
(*1)