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G4MKI-L Coverage

If G4MKI-L is active it should be listed in 101ENGLISH, 101GHILL or KENT...if not found here, try ECHOLINK




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Interface used to link transceiver and computer soundcard for G4MKI-L (when using IC-706MKIIG)

 Also used for data modes on the HF bands - primarily PSK. Click on the photo below

 for full details of the SignaLink SL-1+

Homebrew ptt Interface Circuits

A number of different homebrew approaches to ptt interfacing can be found on the eQSO website by

following this link:

ptt Interfacing

If you need a circuit suitable for use with a hand-held, try circuit 3 at the link below:

ptt interface for a hand-held

Interface with Fan Control (above)

The homebrew interface I constructed for RTTY on HF using my Icom IC-701 and a homebrew Terminal Unit.

Modified recently to key an old 70cms handheld for VOIP. Inside the die cast box is an RS232 to TTL

converter and a transistor switch for the ptt line. The simple mod for audio injection was based on circuits in the links above.

A Blast from the Past (on COM ports)

Or, to be more precise. RS232 serial communication ports.

There was a time, before the appearance of USB ports on the back panel of IBM compatible pc's, when COM ports, as in COM1, COM2 ,and if you were lucky, COM3 and COM4 were all the rage. In those days a mouse had, more often than not, a DB9 com port plug moulded on the end of the connecting lead, so that it could be plugged into COM1!! Nowadays, of course, they use a mini DIN.

 Most radio amateurs soon discovered that 2 COM ports were not enough and went to great lengths to install additional COM cards in their motherboards. They also discovered that modification was required before they would work because sadly COM1, COM3 both require access to  the same IRQ (pc bus interrupt) as do COM2 and COM4 (IRQ's 3 and 4 respectively). The IRQ's control the flow of data to and from the card, a different IRQ being required for each active COM port. Some printed circuit board land cutting and wire linking was required on the new COM cards to make use of unused interrupts. I ran 4 COM ports, using 2 modified cards on an old 386 machine for some years, primarily to support amateur radio software applications and a weather station.

The base addresses assigned to COM ports are as follows:

COM1 = 3F8h




It is to one of these addresses that I/O data is directed by the software controlling the card/port. If other software is being run simultaneously which also controls a COM port, it MUST use a different COM port to avoid conflict.

The introduction of PCl com port cards has made the addition of COM ports a much simpler process..

Another problem which used to arise frequently with older machines occurred during the installation of soundcards - they often required more than one IRQ for correct operation - suddenly the 'spare' IRQ you 'borrowed' to support an additional COM port wasn't spare any more!! - the sound card needed it!  Network cards were yet another possible source of conflict.

Fortunately, newer versions of Windows supporting 'Plug & Play' have, in most cases, eliminated these problems.

When using RTS signalling to switch a transceiver between receive and transmit another can of worms is opened. To be able to fully understand the problem reference needs to be made to the RS232 standard.

RS-232 signals

The RS232 pinout signals are represented by voltage levels with respect to a system common (power / logic ground). The "idle" state (MARK) has the signal level negative with respect to common, and the "active" state (SPACE) has the signal level positive with respect to common. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol.

The RS-232 interface presupposes a common ground between the computer and transceiver. This is a reasonable assumption when a short cable connects the computer to the transceiver, but with longer lines and connections between devices that may be on different electrical busses with different grounds, this may not be true. RS232 data is bi-polar. The standard specifies a maximum open-circuit voltage of 25 volts, but common signal levels are ?5 V, ?10 V, ?12 V, and ?15 V. Circuits driving an RS-232-compatible interface must be able to withstand indefinite short circuit to ground or to any voltage level up to 25 volts. From +3 to +12 volts indicates an "ON or 0-state (SPACE) condition" while A -3 to -12 volts indicates an "OFF" 1-state (MARK) condition. Some computer equipment ignores the negative level and accepts a zero voltage level as the "OFF" state. In fact, the "ON" state may be achieved with lesser positive potential. This means circuits powered by 5 VDC are capable of driving RS232 circuits directly, however, the overall range that the RS232 signal may be transmitted/received may be dramatically reduced. The output signal level usually swings between +12V and -12V. The "dead area" between +3v and -3v is designed to absorb line noise. In the various RS-232-like pinout definitions this dead area may vary. For instance, the definition for V.10 has a dead area from +0.3v to -0.3v. Many receivers designed for RS-232 are sensitive to differentials of 1v or less.

It is  possible that your transceiver will be grounded to an earthing spike. It is very likely that your pc will be grounded to the domestic electrical earth which may be 'floating' above earth potential unless it too is grounded to an earthing spike!

One obvious way of avoiding the polarity problems is to electrically isolate the computer from the transceiver using, for example an opto-coupler or a miniature dil relay switched by a suitably biased transistor. The very simple circuit using an opto-coupler is shown below has been found to work very well. Some of the other circuits I have tried didn't work!! (for me!)

The Signals

Circuit Class

Circuit Condition




Mark (Asynchronous stop bit, idle)

-25v to -3v



Space (Asynchronous start bit, break)

+25v to 3v



Mark (Asserted)

+25v to 3v



Space (Not Asserted)

-25v to -3v


 Note: The polarity is different for data and control

Recognising COM ports

They come in 2 flavours! 9 pin and/or 25 pin. They also come in 2 genders, male and female - the male with 9 or 25 pins, the female with 9 or 25 holes, pretty obvious really.

Pin assignments are as follows:

9 pin

25 pin


Carrier Detect

2 TX data


RX data

3 RX data


TX data

4 Request to Send


Data Terminal Ready

5 Clear to Send



6 Data Set Ready


Data Set Ready

7 Ground


Request to Send

8 Carrier Detect


Clear to Send

20 Data Terminal Ready


Ring Indicator


Ring Indicator


Copyright © 2009 Duncan Bray