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  Using the Channel Select Inputs
on a Motorola 16-channel
GM300 Mobile Radio

Transcribed by Robert W. Meister WA1MIK
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All 16 channels on a 16-channel GM300 can be remotely selected through 5 pins on the 16-pin accessory connector. It would seem that if you've modified the software to allow 32 channels, this scheme will also work to select up to 31 of those channels. Your radio MUST have the expanded four-layer logic board with the firmware in an EEPROM for remote channel selection to work. The 9th character of the model number must be "C" (for expanded), not "D" (for masked).

There are two parts to using this: software (configuring the radio) and hardware (wiring the circuit and connecting it to the radio).

Software Section:

You must program the accessory connector using the Radio Service Software. Follow these steps:

  1. Read the code plug from the radio and save it.
  2. From the Main Menu, press F4 (Change/View).
  3. From the Change/View Menu, press F2 (Radio Wide Configuration).
  4. From the Radio Wide Configuration screen, set ACC. External to "General I/O".
  5. Press F9 (Other Accessory).
  6. On the Accessory Connector Configuration screen, set pins 6, 8, 9, 12, and 14 to the values shown in the table below.
  7. Write the code plug to the radio.

Pin #DescriptionData Dir.DebounceAct. Level
6Channel Select 1 InputNoLow
8Channel Select 2 InputNoLow
9Channel Select 3 InputNoLow
12Channel Select 4 InputNoLow
14Channel Select 5 InputNoLow

Hardware Section:

Insert wires into positions 6, 8, 9, 12, and 14 of the accessory connector and wire them as shown on the schematic below.

The diagram depicts all 16 channels remotely selectable over 5 wires, using diodes to activate the desired channel select inputs of the radio. For applications requiring fewer channels, just delete the unneeded portions of the circuit. For example, if only two channels are required, this can be accomplished without any diodes by connecting Channel Select input 2 (pin 8) to ground through an SPST switch. When the switch is closed, channel 2 is selected; when the switch is opened, channel 1 (or whatever channel was previously set from the front panel) is selected. When none of the Channel Select input pins are grounded, channel selection may be accomplished normally using the radio's front panel up/down buttons. When a channel selection has been made remotely, by grounding one or more of the Channel Select input pins, the radio's front panel up/down buttons are inoperative. The channel selection condition must remain grounded; if you use a momentary connection to select a channel, the radio will revert to the front panel channel when the ground signals go away.

If you have configured and programmed your radio to accept 32 channels, this scheme can be expanded by using a bigger rotary switch and adding more diodes to select the additional channels. Just remember to activate the Channel Select 5 pin for channels 16 and above.

You can connect a repeater controller or other digital interface to the same Channel Select input pins. These should be open-collector and active pull-down to a low voltage (under 0.7VDC) to select the channel. The Channel Select inputs operate in a simple binary code according to the table below (L = Low or grounded, H = High or floating). Note that you can't actually select channel 32 remotely, but you can select channel 32 via the front panel up/down buttons, then when all of the Channel Select inputs pins are high, channel 32 will be selected.

ChannelCS5CS4CS3CS2CS1
Front PanelHH HHH
1HH HHL
2HH HLH
3HH HLL
4HH LHH
5HH LHL
6HH LLH
7HH LLL
8HL HHH
9HL HHL
10HL HLH
11HL HLL
12HL LHH
13HL LHL
14HL LLH
15HL LLL
16LH HHH
17LH HHL
18LH HLH
19LH HLL
20LH LHH
21LH LHL
22LH LLH
23LH LLL
24LL HHH
25LL HHL
26LL HLH
27LL HLL
28LL LHH
29LL LHL
30LL LLH
31LL LLL

You may have noticed that the data pattern is simple hexadecimal encoding. There are rotary switches that will output this sequence directly. You can then eliminate the diodes and all that wiring and connect the switch directly to the Channel Select input pins to choose your channel. If you select the highest channel from the front panel, you can then access all channels with the rotary switch. Hex switches come in various sizes and number of positions, so if you don't need the full capacity of the radio you can get a smaller switch.

Wiring Diagram:

Click on the image for a larger view.

gm300-info/remote-channel-select.jpg

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.


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This article first posted 23-Oct-2015.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.