National Instruments 3.22E+04 用户手册

Signal names with an overscore, such as DTR, indicate that the signal is active low.

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.

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.

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 the packets 
sent from your transmitter.

Table 4-1.  Transceiver Control Modes

Four-wire mode

Always enabled

Always enabled

Two-wire mode: 
DTR with echo

Enabled with 
DTR unasserted

Always enabled

Two-wire mode: 
DTR controlled

Enabled with 
DTR unasserted

Enabled with 
DTR asserted

Two-wire mode: 
TXRDY auto control

Enabled with 
TXRDY asserted

Enabled with 
TXRDY unasserted