Hardware Reference
In-Depth Information
Figure 2.28
Simplified block schematic of the Dual-422
the PC/AT bus and permits serial communications at speeds up to 57.6 kilobaud
at distances of up to 1.2 km (the 9.6 kilobaud limitation imposed by most PC
communications routines can be overridden in most cases).
The simplified block schematic of the Dual-422 is shown in Figure 2.29.
Both ports operate independently and each has its own case address and inter-
rupt selection controls. A VLSI Universal Asynchronous Receiver/Transmitter
(UART) device is used to form the basis of each channel and this device is
augmented by external line drivers and receivers.
The UART employed is the National Semiconductor INS 16450 (an improved
device which is compatible with the original 6250 device employed in the legacy
PC). The INS 16450 is fully programmable and offers a choice of serial data
word length (5, 6, 7, or 8 data bits) with selectable even, odd, or no parity
checking. Baud rates are also selectable in the range 120 baud to 57.6 kilobaud.
Base address selection (for each port) is obtained via a dual-in-line switch
(see Figure 2.30). Links are used to select the desired interrupt level (either
channel can be configured as MS-DOS serial port COM1: or COM2: or any
other interrupt level may be selected) whilst a further link is provided in order
to enable or disable CTS/RTS data transfer control.
Programming the Dual-422 interface is extremely straightforward. Assuming
that the ports have been configured as COM1: and COM2: (and that no other
communication device has been configured to the same interrupt level), the
following BASIC code transmits a test string (T$) output via COM1: for input
via COM2: to the received string, R$:
Figure 2.29
I/O connector
pin assignment for the
Dual-422
REM Test string
T$="The quick brown fox jumps over the lazy dog"
REM Open the serial ports using 4800 baud
REM COM1 will be associated with channel 1
