RS-232 - Computer Busses - page 229

the divisor MSB register. Finally, bit 7 is set back to a '0'. For example, for 9600 baud,
COM1 and 1.8432 MHz clock then 0Ch is loaded in 3F8h and 00h into 3F9h .
When bit 7 is set at a '0' then a read from the base address reads from the RD buffer and
a write operation writes to the TD buffer. An example of this is shown in Figure 13.9.

Write to TD/RD
buffer

TD

3F8h

TD buffer

RD

3F8h

Read from TD/RD
buffer

RD buffer

Figure 13.18 Read and write from TD/RD buffer

Table 13.5 Serial communications addresses

Primary Secondary Register Bit 7 of LCR

3F8h 2F8h TD buffer '0'

3F8h 2F8h RD buffer '0'

Divisor LSB '1'

3F8h 2F8h

3F9h 2F9h Divisor MSB '1'

3FBh 2FBh Line Control Register

Line Status Register

3FDh 2FDh

13.5 RS-232 programs

Figure 13.19 shows the main RS-232 connections for 9 and 25-pin connections without
hardware handshaking. The loopback connections are used to test the RS-232 hardware and
the software, while the null modem connections are used to transmit characters between two
computers. Program 13.2 uses a loop back on the TD/RD lines so that a character sent by the
computer will automatically be received into the receiver buffer. This set-up is useful in test-
ing the transmit and receive routines. The character to be sent is entered via the keyboard. A
CNTRL-D (^D) keystroke exits the program.
Program 13.3 can be used as a sender program (send.c) and Program 13.4 can be used as
a receiver program (receive.c). With these programs, the null modem connections shown in
Figure 13.19 are used.
Note that programs 13.2 to 13.4 are written for Microsoft Visual C++. For early versions
of Borland C/C++ program change _inp for inportb and _outp for outportb .