A Multi-mode PC to Radio Interface
For PSK, CW, PACKET, etc
Jose I. Calderon (DU1ANV)
Makiling Amateur Radio Society
In Recent years, the proliferations of soundcard based
multi mode communication softwares have invaded the Radio amateur Ham Shack. It
all started with keyboard CW. A computer based generation of the Morse
characters that allowed the ham operator to use his PC to key his CW transceiver
with ease. Speeds up to 60 words or more per minute was made possible by these
keyboard CW programs that made perfect Morse characters the way it should be.
The appearance of these PC-keyboard CW keyers placed the old reliable manual
keyer to be set aside to become only “the alternate - Just in case” status
inside the shack. Today, other computer-based digital modes of modulating the
radio transceiver has emerged to the point that Amateur Radio operating not only
to become even more enjoyable but extended the exciting world of Ham Radio into
a “Super Hobby”.
The Circuit design
The Interface circuit is divided into two general sections. They are:
Circuit A, the receive and modulation interface - During receive mode, speaker audio is fed to J1. The receive audio is sampled through the signal divider (1000 resistor), to prevent loading the speaker audio line into the attenuator (Pot 1). The signal is transferred via the isolation transformer, the output of which, is made available at J3 where the received audio is sent via a shielded cable to the soundcard “Line Input” for processing by the soundcard and the PSK program. The signal is finally displayed in the waterfall spectrogram (called trace or trail) while simultaneously displaying the decoded characters in the receive window. During transmit mode, the characters that are typed in the transmit window is processed by the program and the PSK modulation signal is sent via the soundcard “Line Out”. This signal is sent through a shielded cable to the input jack (J4) of the interface with the associated attenuator circuit. The modulation signal is transferred through the isolation transformer (T2) and finally fed to the audio mic input via a shielded audio cable to modulate the transmitter.
Fig 1. The Multi-mode PC Soundcard to Radio Interface circuit.
|B. The TTL and Mode Switch Interface:|
Circuit B, the “Transistor-Transistor Logic” (TTL) circuit. The two separate circuits are electronic switches and are identical in design. The upper diagram serves as the electronic switch for the PTT (labeled in many radio models as “standby or tx” terminal pin in the mic wiring diagram) of the transceiver. The second circuit (lower diagram) serves as the electronic keying circuit for keyboard CW work when you use a computer keying program such as “CWType”. The command signals for each function is taken from the serial COM port (such as COM 1 port) of the computer. This is normally available at the back of all PC’s in DB9 RS232 configuration. Only three (3) terminals are used to accomplish the switching function. The DTR (Data Terminal Ready) for CW-Key function and the RTS (Request To Send) for PSK function, or vice versa, and the GND terminal of the COM port.
How the circuit B works - When the program/computer is
idle, the DTR and the RTS terminals of the COM 1 port are -12/-5 VDC with
respect to GND, cutting the bias to the base terminals of Q1 and Q2 resulting to
non-conduction of current from each collector to GND, via the emitter junctions
(OFF state). When the transmit command is evoked by the computer, the DTR and
RTS terminals will swing to a positive voltage of +12/+5 vdc with respect to
GND. This positive bias triggers the two transistors to conduct current (ON
state) from each collector to GND via the emitters. This happens because the PTT
and Key terminal (when connected to the interface) of the radio have a normally
open terminal voltage of +5 to +7 VDC. During this conduction state, the
resistance between collectors and emitters appear to be very low (almost zero)
as seen from the PTT side and or the CW key side of the radio. Hence these
terminals become shorted to GND, actuating the PTT and or the CW key (as if they
were manually shorted to GND). When the base of the transistors are relieved of
the positive bias (computer is idle), the collectors will stop conducting
current (OFF state) and the radio will see an open connection of the PTT and CW
Key to GND, thereby stopping the transmission and the radio will go back to
receive mode ----- (QSK).
Assembly and hookup
The wires must agree to the wiring diagram shown in Fig 2 below. If not, you must open the case of the DB25 and rewire it yourself. Only three wires are needed (DTR, RTS and GND) and ignore the rest but leave them connected to their original terminals.
3. Once completed, plug the DB25 end to the input port of the Data switch and proceed to identify which switch terminal is associated to DTR, RTS and GND. In my case, the switch goes to A port and or B port (the Centronics terminals). Therefore, the switch must be a bank of multiple single pole double throw (spdt) type. Selecting only the three connection points is easy by using your ohmmeter as continuity tester. The final wiring hookup is shown below
4. Final check : As an additional insurance, you must check your final wiring to review the specific connection points of inputs and outputs of circuit A and B and the data switch connections. Make sure that the DTR, RTS and GND are coming from the correct terminals of the computer’s serial COM port and your RS232 cable is correctly wired. As a final guide, the common serial port of today’s computers is a DB9 (9-pins) and has the following standard pin configuration:
Those who are using outboard TNC’s (Terminal Node controllers) for Packet operation use the same RS232 cable configuration.
Final Hookup and Setup
The above cables complete the interface setup. And the Interface should be ready to fire once connected to the computer’s COM port and to the radio. Those radios without accessory jacks for PTT and audio input must use the mic/ PTT input in the front panel of the radio by disconnecting the standard microphone and replace it with an appropriate plug similar to its standard microphone connector and wire the output of the Interface to this. Plug your extension speaker to J2 of the interface and finally, if you want to maintain the computer’s sound card outboard amplified speakers, plug the input of this amplifier to J5 of the interface. That’s it folks! You are ready to enjoy PSK and other computer soundcard based digital modes.
Calibration and Use
Receiving PSK31 (RX or monitoring
Transmitting PSK31 (Transmit):
© Philippine Amateur Radio Association, Inc. 2009