Modifications for the Motorola GP-300

Picture(s) of Motorola - GP-300

02-04-2000 Radio Programming, 8 to 16 channel expansion English language
02-04-2000 RIB-IVG (Programming Interface MkII) English language
16-04-2001 How to program the radio English language
23-10-2001 Motorola GP-300 TX out of band limits English language

Radio Programming, 8 to 16 channel expansion
Author: Colin G1IVG -


The Motorola GP300 can be re-programmed quite easily to allow use on our Amateur Radio bands. The radio is made in both a VHF and UHF versions, and makes an excellent 2mtr or 70cms portable radio.

Most of the radio's I have worked on have originally been 8 channel units. These can be modified to operate on 16 channels quite easily.

The programming interface can be made for a low cost. All it consists of is a RS232 to TTL level converter, that connects between the serial port of your PC and the radio.

Amateur Radio Conversion.

The radio requires the Motorola Software which can be found if you ask around or search for it on the FTP sites on the Internet. If you can't find it then try your local Motorola dealer who should be able to help at a small cost.

You also have to make an interface, or purchase one.

To program the radio onto the Amateur band, you can find some versions of software that have been hacked into allowing frequencies to be entered to bring the radio into the amateur bands which normally are just bellow the original working frequency. However this depends on your version of software, if it's a un-hacked version then you have to press the key down when entering the frequency. You must release the key when entering the decimal point. For example to program 433.500MHz you will enter the following:    $££.%))  on a English layout keyboard.   (This is 433.500 with the held down. Fill all of the 0's in the frequency field. This example is the same for both the VHF and UHF GP300's.

16 Channel conversion. (for 8 channel radio's only)

To program the radio for 16 channels you need to first of all, pull the channel selector knob off the radio. On the underside of the channel selector knob you will see a small black "Stop". Cut this off with a sharp knife and replace the knob. You will now find the radio then switches 16 positions instead of the original 8 channels.

You then need to "Hack" a file called GP300.MDF with the help of a Hex editor such as Hex Workshop (see my PC links page to go to the home of Hex Workshop, an evaluation version is available to download for free). The file you need to modify is called GP300.MDF and can be found in the main directory of the Motorola software. Keep a backup of your original GP300.MDF file, just in case you make a mistake.

I have made a small video that shows the Hex Workshop screen during the modification. This should help you understand the procedure of editing the file which when complete will give you 16 channels. Click below to download. (364Kb)


  1. Get a copy of Hex Workshop.

  2. Start the Hex Workshop program and open the file GP300.MDF to begin editing.

  3. Go to "Tools", and the "Generate Checksum".

  4. Make sure "Entire Document" and also "Decimal" are ticked. Then press "Generate".

  5. Look in the "Checksum-16" box, in my version of software for example, this results in a checksum of 53454 (yours may be a different number depending on the version of Motorola software)

  6. Write this number down, as you need it later.

  7. Press "Cancel" to return to the main window and your next step.

  8. Go to "Edit" and then "Find". A pop-up window appears.

  9. You need to enter the model number of your radio. For e.g. P93YPC00D2AB.

  10. When you enter the model number leave off the last two letters(AB on my radio).Make sure "ASCII" is ticked.

  11. When you press "Find Next", you will see your model number highlighted.

  12. If you count 8 numbers after the highlighted section, you should see the number "08" as long your radio is originally a 8 channel set.

  13. Edit the "08" to "10" (note 10 in Hex is equal to 16 in decimal).

  14. You now need to edit the start of the program, to allow the software to run.

  15. In the first few lines look for a Hex number that you can easily take away 8 from.

  16. In my software at address 00000040 you will see the number "78" on the right.

  17. Edit this to be "70".

  18. Now you must do a final checksum and make sure it equals the original value.

  19. If this is the same, then replace you original file with the modified file.

  20. Try to reprogram your radio with the extra 8 channels to give you 16 channels.

Please don't ask me for a copy of the Motorola Software

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RIB-IVG (Programming Interface MkII)
Author: Colin G1IVG -

(with special thanks to Roberto EB4EQA for supplying the drawings below).


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

Download a zip file that contains an AutoCAD 14 file of the images above.

Thanks to Roberto Barrios Sanchez (EB4EQA) for providing the above revised layout drawings

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






Radio Connection (GP300).

To connect to your GP300, all you have to do is temporarily connect your interface to the GND and BUS+ on the back of the radio. these connections are visible when the battery is removed.

You will also need a external 7.5vdc supply to power the radio when you are programming it. This is connected between the 7.5 volt and GND connection on the radio.


If you have problems getting this interface to work on your GP300, then try replacing the two 680 ohm resistors that feed the LED's. If you use 4.7k ohm it should work OK. I found that when using version R01.00.00 software I have no problems with the 680 ohm resistors, but for some reason when I ran the newer software (R08.02.00) I had problems reading the codeplug etc. All I have done is replaced the 680 ohm with 4.7k ohm and it works fine now with both versions of software. You will have to experiment a little and let me know what resistors work for you.

Simon VK4TSC encountered a problem where the GP300 being programmed/read will fail to pass data to/from the computer with an Error #2 (Serial Bus Error). After checking the BUS+ line with a CRO it was found that the GP300 could not properly switch the data bus, this being caused by the led/resistor combination on the data bus drawing a little more current than the radio could supply. By changing the series resistor to 1K ohm instead of 680 ohm the problem was overcome

The better solution maybe to use a spare gate from the 7407 to drive the led separately. Disconnect led cathode and link it to pin 4 of 7407, isolate pin 3 of 7407 from 5V and link it to pin 2 of same chip. The original 680 ohm series resistor can be left as is. BUS+ is now isolated from the data led.

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 this page for info etc.)

Maxtrac 840 - 800 MHz Truncking Radio (Try for connections).

Please let me know if the Interface works ok on your radio. I am also interested what other radio's this will work on. please e-mail your results.

Sandy Ganz's RIB Card PCB.

You can buy the RIB Card PCB direct from Sany Ganz in the form of a bare PCB. It cost me $20 plus $5 Overseas shipping. For further detailed information you can e-mail Sandy on the following address : All you need to do is purchase the components locally. Sandy also includes a parts list and information on a supplier for the components if you can't find them locally.

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How to program the radio
Author: Thijs - PE1RLN -

It is not difficult to program your radio and if you followed the above mods from the other amateurs, it will be very easy.

Let's assume you have a radio for VHF use and 16 channels.

  1. Make a list of the channels and decide wich frequencies you want to put in. I made channel 16 a scan-channel that scans all the other 15 channels.

  2. Read the codepulg data from the radio. When you do this, you are sure that you can read and program the radio.

  3. Save the file to disk and make an extra copy of it on a different directory. If you f*&ked up the data, you can always reload the backup. I have experience with that...

  4. If you have Hex-editor or something like that, you can easily change the radio's serialnumber! Just open the file you just saved and change the serialnumber. Now you have your own. I put my name in it...

  5. Go to the programming section in GP-300 and press F2 to change the use of sidebuttons etc. I made SB-1 the "Nuicance delete" button. When scanning the other channels, it is possible that the radio stops at a channel where there is only a carrier. When pushing this button, it won't stop there anymore, until you re-activate the scanningmode.

    Also set "Alarmtones on" if you want the extra sound-features. When doing this, you will get a tone when turning power on, activating scanmode and (optionally; check the other field below) when battery is low. If, during scanmode, the radio stops at a busy channel, you can't ofcourse see which one it is. When you leave scanmode and by turning the rotaryswitch you pass that channel where the radio stopped, you'll hear a beep. This is to identify that it is the busy channel you were looking for. Very handy! Haven't seen that on HAM-radios before...

    Unfortunately you'll get a beep everytime you release PTT. But, there is a way to remove the beep.

  6. Go to the menu where you can set several operation modes and identification modes. Here you have to add a mode which has "phone" in it. I don't know the exact name anymore... There you can set wether you want a PTT sidetone or not. Set these fields to "no" and there will be no sidetone for channels with this PTT-mode. Ofcourse, you will have to set every channel with this mode, otherwise every channel has default no mode, so you'll get a sidetone... and you don't want that!

  7. Go to the mode-menu and there you can set your channels with the different mode we set at point 6.

  8. On channel 16 you can set a scan channel. Make the channel non-conventional "scan" and press "Scan List". Now you will get the scanlist of channel 16 where you can see 15 available channels and channel 16 n/a.
    Now, go to the first field and make it n-pri. This means that it is not a priority channel. I made channel 13, my local repeater, PRI, so when the radio stopped at a different channel when scanning, it will still be looking to channel 13 every second and when channel 13 is busy, it will switch to that channel and leave the previous. This may be very useful.

Now, the rest should be no problem for you. I will put my GP300 file on my site and you can download it. In that case you will only have to change the frequencies.

I hope you will enjoy this great radio. I carry it with me all day and I use it a lot. I made my programmer my self and put 3 clamps on it to put on the back of the radio. I made a little peace of wire on the BUS-strip so that I could always clamp it. It works perfectly!

Have any questions? I like to help. Mail me at

Have fun!

73 de Thijs, PE1RLN

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Motorola GP-300 TX out of band limits
Author: -

Motorola GP-300 comes in several different model numbers. The best model number for use on the 2-meter ham band and the public safety band only is set by the factory to only TX down to 146.000.

Easy fix.

To enter a frequency below 146.000, just hold down the shift key, and enter your frequency that way.

Example- 145.170 would display !$%.!&). When you press tab to go to the next field in the mode setup, the correct frequency will be displayed.

I have successfully programmed my radio down to 136 MHz, TX/RX. This also works some other Motorola radios, I have used it on a M-1225 with success, and a UHF GP-300 also with success.

You will just have to try it on yours.

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