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  Making a Programming Cable
That Works With a CM300 Radio

By Robert W. Meister WA1MIK
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You acquired a CM300 mobile radio and want to program it. You notice there's a standard modular microphone jack on the front of the control head, so you fire up CPS, plug in your standard trusty modular programming cable, and the radio appears dead. No communications at all.

Some quick research verifies that the same two pins (SCI and GND) are used for programming. You've used the cable to successfully program MaxTracs and other modular jack-equipped radios before, so why won't this one work?

Well a bit more research might lead you to the CM300 Detailed Service Manual, which states:

The HOOK line (J802-6) is used to inform the uP which type of microphone or SCI lead is connected to the microphone socket. The voltage of the HOOK line is monitored by the uP (port PE0, MIC_SENSE) through a resistor divider on the main board. When the HOOK line is grounded (on hook condition) or floating (2.8V nominal), the uP sets the mux (U803) for keypad operation to allow the use of microphones with a keypad. When the HOOK line is connected to 9.3V, the uP sets the mux for SCI operation. This mode is also used to select low cost mic operation where the gain of the microphone path is increased (on the main board) to compensate for not having a pre-amp in the low cost mic.

So it seems all you need to do is connect the HOOK line to a source of 9.3V, but where can you get that from?


It turns out that the front panel MIC jack has assigned signals to the previously spare pins on older radio MIC jacks, including a convenient source of 9.3V, just what you need. Here are the signals on the various pins. At some point Motorola changed the ordering of the pin numbers; in the past they started with pin 8 (RX Audio) at the bottom or left. When you look into the front of the CM300 radio with the notch (for the locking clip) on the bottom, RX Audio is still on the left but now they call that pin 1. The signals are still physically on the same pins; only the pin numbering has been reversed.

PinSignal Name
1RX Audio
3Mic. PTT
4Mic. Audio

The stock modular programming cable only uses pins 2 (SCI) and 5 (GND). Due to numbering differences, these correspond to pins 7 (SCI) and 4 (GND) on the MaxTrac, Radius, MaraTrac, GM300, CDM, and GTX radios. So all you have to do is add a jumper to your existing modular programming cable, inside the hood of the DB25 end, such that it connects the wires going to the modular plug pin 8 to pin 6. These wires should otherwise be unused. You should use an ohmmeter to verify the proper pins and connector orientation.

The original cable only made use of modular pins 2 and 5, and DB25 pins 1, 4, 11, and 15. The new and improved programming cable is now wired up thusly:

Mod. PinSignal NameDB25 PinNotes
1RX AudioNone 
2SCI15Stock cable
3Mic. PTTNone 
4Mic. AudioNone 
5GND1Stock cable
6HookNoneConnect to pin 8
89.3VNoneConnect to pin 6

Note: Jumper: DB25 pin 4 must still be connected to DB25 pin 11.

It seems simple, and it works great for the CM200, CM300, and PM400 radios, and probably a bunch of other commercial and professional radios made since the mid-1990s, and it will still work well with older radios.

Cautions With Other Radios:

MaxTrac, Radius, GM300, DeskTrac, SM50, SM120, M1225, R1225, CDM, and GTX radios either don't use pins 7 and 8 (what some of them refer to as pins 1 and 2), or supply only 5-8V or have a 27V Zener diode on the Hook line, so the added jumper will have no effect.

However, you WILL have a problem if you use this modified cable on a MaraTrac. This is because the MaraTrac has full battery voltage (13-14V) on that pin (they number it pin 1) at basically unlimited current (well, until something burns up) and the Hook input on the logic board has a 10V Zener diode protecting it, unlike the PTT input which has a 27V Zener diode protecting it. To prevent burning anything out, I recommend that you use a 100 ohm 1/4 watt resistor instead of plain wire for the jumper between the modular plug pin 8 and pin 6.

Other Wiring Changes:

While you're inside the DB25 end of your programming cable, you can also connect a wire from the modular plug pin 4 (Mic. Audio) to your DB25 pin 12 (SWB+ power input) so the radio will supply power to the RIB incase the battery should run low during a programming session. The Mic. Audio line has about 8VDC imposed upon it to power the preamplifier built into most Motorola microphones, so it could help keep a RIB running too.

Contact Information:

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

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This page originally posted on Tuesday 16-Jul-2013.

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.