Appendix C - Construction Notes for an External Tone Generator
Operation of the Morse Academy designed external tone generator is controlled
with parameters on the first line in the file PORT. It contains five
numbers. The first value is the port used to control the Morse Academy tone
generator. The default setting 888 (378 Hex) is for LPT1, the standard
printer port. If your PC has additional printers, it can be set to other
ports by editing the ASCII file PORT.
Printer |
Hex |
Decimal |
LPT1 |
378 |
888 |
LPT2 |
278 |
632 |
LPT3 |
3BC |
932 |
No testing has been done with communications ports, but logically keying
those ports would seem to be possible with the coding provided in Morse
Academy. Those hardware ports are:
Serial |
Hex |
Decimal |
COM1 |
3F8 |
1016 |
COM2 |
2F8 |
0760 |
The program is written to support sending four different tones needed for
the full support of Morse Academy sessions:
Frequency |
Purpose |
Hex |
Decimal |
0 Hz |
Keyer off |
FF |
255 |
300 Hz |
Low pitch long tone for error |
FE |
254 |
730 Hz |
Keyer on |
FD |
253 |
1800 Hz |
High pitch short tone for success |
FC |
252 |
The choice of frequencies and function codes is arbitrary and can be changed
to accommodate other hardware implementations. These four values are stored
immediately after the port address in the element PORT in the Morse Academy
directory. They can be modified to support other hardware designs.
The author would prefer that any hardware built for Morse Academy support all
four frequencies so the full functionality of the program is preserved.
There are however many approaches to providing the necessary keying, and many
different possible uses. Although the author has not confirmed it, the
simplest circuit may be to drive a Keyer or transceiver thru a diode directly
from the parallel port, e.g. connect the anode of the diode to the positive
side of the Keyer, the cathode to pin 2 of the parallel port, and pin 25 to
the Keyer ground. This works only with Keyers that operate by closing a +5
volts to ground at less than 10 mA. To prevent excessive current flow a 100
ohm resistor in series with the diode is recommended.
SAMPLE CIRCUIT
To debug the program, a simple circuit was designed which fits onto a circuit
card about one inch square. Power was taken from the parallel port. This is
the reason that all function codes indicated above had the high bits set.
The top six bits were set to have their power output in parallel to drive the
circuit. Care must be taken in the design to not overload the PC's outputs.
Drawing excessive current will damage the port driver chip. The pin
assignments for the 25 pin parallel port are:
Pin |
Bit |
Output |
25 |
- |
Ground |
2 |
0 |
Function 0 |
3 |
1 |
Function 1 |
4 |
2 |
+5 volt power |
5 |
3 |
+5 volt power |
6 |
4 |
+5 volt power |
7 |
5 |
+5 volt power |
8 |
6 |
+5 volt power |
9 |
7 |
+5 volt power |
An alternative design could have used three master bits for tone selection
using only five power supply pin outputs. A note of caution on use of the
output pins as a power source. While the circuit can be designed to
represent "Keyer off" as the absence of function bits, depending on the
circuit you may still want to select all power bits to insure a constant
voltage. Keying all bits may result in chirping.
The sample circuit uses a single sound manufactured by Radio Shack called a
LED flasher that almost meets the three-tone requirement of Morse Academy.
The part is an eight pin LM3909 - Radio Shack part number 276-1705. Only
fourteen parts are required:
Part |
Qty |
Description |
LM3909 |
1 |
LED Flasher Chip, Radio Shack P/N 276-1705 |
Socket |
1 |
Eight pin IC socket |
1N270 |
6 |
Germanium diode (or similar) |
Resistor |
1 |
1/4 watt 100 Ohm |
Resistor |
1 |
1/4 watt 12,000 Ohm |
Capacitor |
1 |
10 microfarad (electrolytic) |
Capacitor |
1 |
0.1 microfarad (mylar) |
Connector |
1 |
25 pin parallel port connector (male) |
Connector |
1 |
audio phone jack |
Circuit board |
1 |
about one inch square (length of 25 pin connector) |
In testing this circuit on various computers if was found that the tone
frequencies are very sensitive to the parallel port output voltage. This is
especially true for battery powered laptops and notebooks, where in some
cases the tone off function actually generated a low frequency tone.
Adjustment of the circuit values R1 and or C1 may be necessary to generate
the proper frequencies. As a suggestion you might consider use of a variable
resistor to be able to adjust the circuit for your particular computer's
parallel port design. A different circuit design may be more appropriate to
the wide range of computers on the market.
By Joe Speroni
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