Chapter 6
Using Your Serial Device Server
ENET-232 and ENET-485 Series User Manual
6-8
ni.com
Four-Wire Mode
Use the four-wire mode for most full-duplex systems. In this mode, the
transmitter and receiver are always enabled. This mode is the default.
Two-Wire Mode: DTR with Echo
Use this mode in half-duplex systems where the DTR (Data Terminal
Ready) line controls the transmitter. In the DTR-with-echo mode,
the transmitter is tri-stated when the DTR signal of the UART (Universal
Asynchronous Receiver/Transmitter) is unasserted. To transmit, your
application first must enable the transmitter by setting the DTR bit to assert
DTR. After the data is fully transmitted, your application once again clears
the DTR bit to disable the transmitter. Because the receiver always is
enabled in this mode, you receive packets not only from other devices, but
also from your transmitter.
Two-Wire Mode: DTR Controlled
This mode is similar to the two-wire, DTR-with-echo mode. Use this mode
in half-duplex systems where the DTR line controls the transmitter.
Although this mode uses the same method as the DTR-with-echo mode to
control the transmitter, the hardware automatically disables the receiver
whenever the transmitter is enabled. Thus, you do not receive the packets
sent from your transmitter.
Two-Wire Mode: Auto Control
In this mode, the serial hardware transparently enables the transmitter and
receiver in a two-wire system. Use this mode to remove the burden of flow
control from your application. The hardware automatically enables the
transmitter for each byte to be transmitted. Also, the hardware disables the
receiver whenever the transmitter is enabled, so you do not receive the
packets sent from your transmitter.
Note
When you are communicating with a two-wire device, National Instruments
recommends that you use the two-wire Auto Control mode. Because this mode handles the
transmitter/receiver enabling for a two-wire connection in your hardware, it reduces the
software overhead required to perform this operation in your application program.
For more information about serial communication in two- or four-wire
modes, refer to the
section in Appendix D,
.
