National Instruments 3.22E+04 用户手册

The scratch register is located at offset 7 from the base address of the port. 
For example, if COM2 were located at base address 0x3F8, and you want 
to set the PCI-485 board to two-wire mode with DTR control, you would 
write a 0x02 to address 0x3FF. The PCI-485 board would immediately 
switch to the two-wire mode with DTR control.

The NI-Serial software extends the 

DeviceIoControl

 Windows function 

for programming the transceiver control mode. To program the transceiver 
control mode using 

DeviceIoControl

, complete the following steps:

1.

Add the following lines to your source code:

#include <winioctl.h>

#define IOCTL_SERIAL_SET_TRANSCEIVER_MODE

CTL_CODE(FILE_DEVICE_SERIAL_PORT,37,

METHOD_BUFFERED,FILE_ANY_ACCESS)

2.

Use the Win32 function 

DeviceIoControl

, as follows:

a.

Use the defined control code value listed in step 1 to set the 
transceiver mode.

b.

Use the input buffer values (unsigned long) listed in Table 4-3 for 
programming different transceiver modes.

Table 4-2.  Transceiver Mode Control Bytes

Four-wire mode

0x00

Two-wire mode: DTR with echo

0x01

Two-wire mode: DTR controlled

0x02

Two-wire mode: TXRDY auto control

0x03