Chapter 4 Using Your Serial Hardware
Serial for Windows NT
4-2
©
National Instruments Corporation
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 must control 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 asserted. To transmit, your
application must first clear the
DTR
bit to enable the transmitter. After
the data is fully transmitted, your application once again sets the
DTR
bit
to disable the transmitter. Because the receiver is always enabled in this
mode, you not only receive packets from other devices, you also receive
the packets sent 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 must control 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 packets that you have transmitted.
Two-Wire Mode: TXRDY Auto Control
In this mode, the hardware transparently enables the transmitter and
receiver in a two-wire system. This mode removes the burden of flow
control from the user software. By connecting the transmitter to the
TXRDY (Transmit Ready) line, the hardware 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 packets that
you have transmitted.
Note:
It is recommended that you use the two-wire
TXR DY
auto control mode
when you are communicating with a two-wire device. 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.
