Q: What is the difference between full duplex, half duplex and simplex operation with radio modems.

A. It is useful to split this answer into two - an analysis of the actual serial data messages as if they were being sent on a cable and a separate look at the radio link and how it handles data messaging.

Serial data messaging, be it RS232, TTL-level, RS422 or RS485 can be one-way or bi-directional. An example of a one-way message is sending ‘On/Off’ commands from one place to another without any serial data response. Perhaps the messages are Lights-On and Lights-Off commands to a remote floodlight platform. No serial data response is needed as you will be able to see that the transmitted commands have been received. One-way serial data like this is called Simplex data.

Bi-directional serial data messaging is used where serial messages are sent and received from each end of the link e.g. requesting a remote telemetry unit for measured values. The request is in one direction and the response is in the opposite direction. This bi-directional data is Duplex data.

In a cabled system, one-way serial data is called Simplex data. Bi-directional data is Duplex data.

Next let’s look at the radio modem...

Radio channels are a scarce resource and need to be used sparingly. For this reason, the same radio  frequency is generally used for transmitting and receiving, however not at the same time but sequentially – switching between transmitting and listening (receiving) as in walkie-talkie operation. While transmitting, no receiving is possible until the transmit button/line is released. This TX/RX switching is done automatically with radio data modems. Modern designs as used in Wood & Douglas’ radio modems have extremely fast TX/RX switching to give as close as possible a wireless –equivalent of a serial data cable.       


If a radio link is used for one-way simplex serial data the radio link itself is said to be operating Simplex.


If a radio link is used for bi-directional duplex serial data the radio link itself is said to be operating half-duplex. Half-duplex indicates that the radios are switching between TX and RX even though  they are handling full-duplex serial data.


Full-duplex radio links are available from Wood & Douglas to some licenced radio users who have particularly demanding data throughput requirements. Separate TX and RX frequencies (separated by several MHz) are used and the radio has full time TX and full time RX circuitry. Because of the scarcity of radio channels, thesefull-duplex radio channels are licenced by governments to users like utilities, military, police, broadcasters etc.


Full-duplex radios have TX and RX circuitry operating full time, not just switching between the two. It allows messages to be transmitted to another outstation B while simultaneously receiving from outstation A.


If in doubt what you need for your particular application, please call or email the sales team at Wood & Douglas for advice.

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