Chapter 4 Using Your Serial Hardware
PCMCIA Serial for Windows Me/9x 4-2 ni.com
Note Signal names with an overscore, such as DTR, indicate that the signal is activelow.
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 the packets
sent from your transmitter.
Table 4-1. Transceiver Control Modes
Mode Transmitter Receiver
Four-wire mode Always enabled Always enabled
Two-wire mode:
DTR with echo
Enabled with
DTRunasserted
Always enabled
Two-wire mode:
DTR controlled
Enabled with
DTRunasserted
Enabled with
DTRasserted
Two-wire mode:
TXRDY auto control
Enabled with
TXRDY asserted
Enabled with
TXRDY unasserted