2 - Forth Interest Group

Forth in Control
Ken Merk
Langley, British Columbia, Canada
The following article is my attempt to contribute back
to the Forth community some of my experiences in solving
hardware interfacing problems using Forth. My formal
training is in hardware, so I consider myself a beginner
when it comes to Forth or any software-related projects.
I found Forth a powerful tool to interface the computer to
the outside world. Its speed and interactive qualities let
you build "modules" that can be tried out with immediate
feedback from your hardware. Once a solid foundation of
primitive words is established, building up from there
goes quickly.
The Forth package I use is F-PC by Tom Zimmer,
which I originally adopted to learn 8086 assembler language.
My first try at an application using F-PC was very
successful. Using SMENU.SEQ as a foundation for pulldown
menus (with mouse interface), and words like
BOXLFILL, SAVESCR, and RESTSCR for pop-up windows,
LINEEDITOR for text input with full edit capabili-
ties, FUNKEY. SEQ for function-key input, and~CoL0R. SEQ
to make it pretty, it turned out to be a professional-looking
program. I thought to myself-just think what I could do
if I knew what I was doing!
Using the PC, a Forth disk, and a few electronic
components, we can build a simple interface to control
devices in the real world. The first step is to get some I/O
lines out of the computer. There are four basic routes we
can go:
1. Use the expansion slots located on the computer's
motherboard. To do this, we need to build an address
decoder to select some specific I/O port space, and
some latches or PIA chips to get our I/O off the data bus
into the outside world. There is 1K of port addresses
available here, enough for any situation we can think of.
This is the most versatile and complex to implement.
2. We could use the RS-232 port to get control data in and
out of the computer. This again involves building some
electronics into our interface. We will need a serial-toparallel
converter to change the serial bitstream into
useful parallel control data. A UART and baud rate
generator would work for this application.
3. The PC game port is another pathway into your
computer. We can sense four digital inputs and four
analog inputs. There is no provision for data output, so
it limits us to just receiving data from the outside world.
4. The parallel printer port is the easiest and least com-
plex to implement. We can bring eight outputs to the
outside world for control purposes. The port is also bi-
directional, so we can implement some input lines to
receive data.
To keep this project as simple as possible, we will use
the parallel printer port and build an interface to control
eight devices. For now, the eight devices will be LEDs
(light-emitting diodes), which will represent the odoff
state of each bit on the port.
The first thing we need to do is determine what port
address is assigned to your parallel printer card. The three
possible parallel ports referred to as LPTI, LPT2, and LPT3
are supported by three base addresses. At boot up, the
BIOS searches for parallel ports at each of the three base
addresses. The search is always performed in a specific
order.
1. Location 03BC (hex) is polled first. A byte is written to
address 03BC and then read back to see if it matches
what was sent.
2. Location 0378 (hex) is polled second.
3. Location 0278 (hex) is polled last.
The first port that is found is assigned the name LPTI, the
next one is assigned LPT2, and the last one is LPT3.
When you first turn on your computer, the BIOS
displays an information screen which tells you the ad-
dresses of your parallel ports and LPT assignments. If your
BIOS does not support this feature, you can use the System
Information utility in PC TOOLS, under the I/O Port
heading, to obtain this information. When all else fails, we
can use Forth to get this information. On boot up, the
address of the first parallel port found is stored in address
locations 0040:0008 and 0040:0009. To view it, we can use
Forth's dump routine.
HEX 0040 0008 10 LDUMP
A chart will be displayed showing 16 consecutive address
1 locations, starting from location 0040:0008. Address loca-
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