WARNING:
In order to build the interface described below, you will at least require some basic electronics experience. If you don't understand, how the described circuitry works, it's best not to build it. Although the described procedure is relatively simple, you could cause serious problems to both your PC and your Motorola radio. So please build the interface and use it at your own risk.
Pay attention to the polarity of the capacitors these are marked with a + in the diagram next to each capacitor. You will find that in some cases the capacitors appear to be the wrong way around, however this is not the case, as they must be configured this way to generate the correct voltages within the MAX232.
Interface Parts List.
Item Qty Description
1 1 PCB (Single sided prototype board)
2 1 IC MAX232
3 1 IC 7407
4 1 IC 7805 (5v regulator)
5 5 Capacitors 1uf, 16v electrolytic
6 2 Resistors 680 ohm 1/4 watt
7 1 LED Green
8 1 LED Red
Component Side View
Track Side View
Click here to download a zip file that contains an AutoCAD 14 file of the images above.
PC Connection.
In the interface drawing above you will see the numbers 2,3,4,5 & 9. These numbers represent the 9pin serial port found on most PC's today. If you only have a 25pin serial port then see below for the pin-out translation.
9 pin to 25 pin conversion information.
9 pin 25 pin
2---------------------------3
3---------------------------2
4---------------------------20
5---------------------------7
9---------------------------22
Radio Connection (GM300 & GM350).
To connect to your GM300, all you have to do is connect your interface to the front MIC socket on the radio. See the information below for connection details.
 
The Interface design above has been tested on the following radio's:
GM300 (Connect to the Mic socket on the front panel of the GM300)
GM350 (Connect to the Mic socket on the front panel of the GM350)
GP300 (See my GP300 page for info etc.)
Maxtrac 840 - 800 MHz Truncking Radio (Try http://www.batlabs.com for connections).
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