RS-232 Interface Board  (250-1006)  Features          Faxback Doc. # 5561

The addition of the RS-232 C option board to the Tandy 1000 opens up a whole
new world of compatibility. The interface is designed to plug into one of the
option card slots within the Tandy 1000. The external equipment can then be a
serial line printer, a video terminal or other communication peripheral.

This board incorporates a Western Digital WD8250 Asynchronous Communications
Element (ACE) which functions as a serial data I/O interface performing
serial-to-parallel conversion on data characters received from a peripheral
device or modem, as well as parallel-to-serial conversion on data received
from the CPU.

It should be mentioned at this point that although the RS-232 C Option Board
provides a means of communications between your Tandy 1000 and some external
device, via the necessary conventions regarding data identification,
transmission rates, send-receive sequences, error-checking techniques, etc, it
DOES NOT provide the programming necessary to use any particular external
device. Aside from the use of a recognized communications software, you would
have to write a "drive program" to permit custom operation of the appropriate
"non-standard" peripheral. the data necessary to assist in that task is
provided by the Western Digital Corporation and is included as part of this
guide.

In addition to the above, the ACE also has a programmable baud rate generator
which allows operation from 50 to 9600 baud. It may also be software tailored
to your requirements. For example, it will add and remove start bits, stop
bits, and parity bits. Five, six, seven, or eight bit characters with 1, 1
1/2, or 2 stop bits are supported. Additional features include:

* Full double buffering which eliminates the need for precise synchronization.
* Independent receiver clock input.
* False start bit detection.
* Line break generation and detection.
* Modem control functions: Clear To Send (CTS), Request To Send (RTS), Data
Set Ready (DSR), Data Terminal Ready (DTR), Rind Indicator (RI), and Carrier
Detect (CD).

THEORY OF OPERATION

The RS-232 C Asynchronous Communications Board has various modes of operation
that can be selected by programming the modes of operation that can be
selected by programming the WD8250 ACE. The WD8250 is programmed by selecting
the I/O address (HEX 3F8 to 3FE PRImary, and HEX 2F8 to 2FE SECondary), and
writing data out to the board. Address bits A0, A1 and A2 are used to define
the modes of operation by selecting the different registers. The selection of
certain registers is done by using the divisor latch access bit (Bit 7) of the
line control register.

One interrupt is provided to the system from IRQ4 for PRImary operation, and
IRQ3 for SECondary operation. This interrupt is active positive. Bit 3 of the
modem control register must be set high in order to send interrupts to the
system. When this bit is high, any interrupts allowed by the interrupt enable
register will cause an interrupt.

For additional data relative to the preceding, please turn to the Western
Digital pages in this guide.


(smm 08/03/93)