SN65HVD78: Serial port to RS485

Hi Jerry,

SN65HVD78 is a half-duplex transceiver, which means that it uses the same pair of signaling wires both to send and to receive data.  The DE pin is used to enable the driver when transmitting and the /RE pin is used to enable the receiver when receiving.  When you connect multiple nodes together on a half-duplex network, you need to make sure to that only one node is transmitting at once while all other nodes receive.  (Otherwise, the two transmitters would interfere with one another and corrupt the data.)

Some serial port or UART implementations provide signals to control the DE and /RE pins of a half-duplex transceiver.  If so, these could simply be connected to pins 2 and 3.  Other implementations will provide a "direction" control.  If this is the case, you can typically tie this single control to both DE and /RE.  If your serial port does not support either of these controls, then there are a couple of options you could choose.

One option is to operate the network as a "full duplex" network, meaning that there are separate differential pairs used for signals being transmitted versus being received.  This would require additional cabling (four wires rather than two) and a different transceiver device that has separate transmit and receive pairs.  An example of such a device is THVD1451.

Another option is to keep the network "half duplex" but control the DE and /RE signals some other way.  You can refer to this post for a couple of recommended circuits for implementing this:

https://e2e.ti.com/support/interface/f/138/t/830860

By the way, for a half-duplex implementation SN65HVD78 is a good device.  You might consider the newer THVD1450 transceiver, though, since it is a pin-for-pin functional equivalent with improved performance.

Please let us know if you have any further questions on this.

Regards,
Max