Half-duplex communication is a way of communicating while switching between transmission and reception using one transmission line.
Therefore, it is not possible to transmit and receive at the same time as full-duplex.
However, the way of communicating with multiple devices using both multidrop connection and half-duplex communication is widely used.
Especially in the case of RS-485, there are many cases where the communication distance is long.
For this reason, this way which allows only one transmission line, can be expected to reduce the amount of wire installation work and reduce the cost of electrical materials.
On the other hand, the communication LSI (Large Scale Integration) chip installed in the device on the PC side is designed on the premise of full-duplex communication.
For this reason, our company adopts a technology that realizes half-duplex communication by changing the RTS output signal
and not receiving the message sent by the own station in the receive buffer of the own station.
The CPU load can be reduced, the risk of missing telegrams can be reduced, and stable communication can be expected.
Both hardware and software settings are required for half-duplex communication with our devices.
In the hardware settings, the RTS (Request To Send) output signal on the LSI chip is input to CTS (Clear To Send), and RxD (Receive Data) is gate controlled.
If necessary, enable the Terminator (terminal resistor) to prevent unwanted signal reflections.
Hardware specifications and how to set the hardware differ depending on the device used, so please refer to each manual.
Reference circuit : For COM-4PDHN-USB
After hardware setting to half-duplex communication, it is necessary to change the RTS output signal with appropriate timing by software.
The setting method differs depending on the software to be used.
For details, refer to Tutorial/RS-485 communication.
Reference software settings: COM property advanced settings (When the half-duplex setting is enabled, the RTS output signal is automatically controlled)
