I have been playing around with CTSS (continuous tone-coded squelch system) programming the GM300 and M100 Radios to accept RX Squelch Code CTCSS. There is also TX Squelch Code but for the purpose of this article I will not go into TX Squelch Code programming. The service software manuals describe this proses in detail and can be consulted for more information.
What is CTCSS? Most two-way radios have 38 CTCSS codes which stands for Continuous Tone Coded Squelch System. Many two-way radio manufacturers have different names for this feature. These are some of the manufacturers and their names: Motorola: "Private Line (PL)," GE/Ericsson: "Channel Guard", E.F. Johnson: "Call Guard", RCA: "Quiet Channel", and some simply call it "tone squelch," " privacy codes," or "interference eliminator codes". The most proper name is CTCSS. These codes enable a two-way radio to keep the radio squelched (quiet) if the chosen code is not received with a transmission. In other words, the speaker of the intercom is not turned on even though a transmission is received. Technically speaking, a CTCSS tone code is a sub-audible tone/code which allows your radio to ignore (not hear) calls from other parties who are using the same channel. With CTCSS it may seem like you have your own private channel but other parties can still hear your calls if they set up their radio with the same tone code or no tone code at all. Both receiving and transmitting radios must be set to the same interference eliminator/CTCSS code or it will appear that a radio is not receiving transmissions. While CTCSS is said to transmit sub-audible tones (can't be heard on the radio's speaker), a radio is really continuously transmitting an audible tone below 300 Hz during a conversation. Each radio contains a filter to filter out anything below 300Hz so the tone is not heard even though it is there. Encoded tones are sent by the transmitter and decoded tones by the receiver to un-mute its speaker.
In short I want the receiver to unmute if a carrier with the specific sub-audible code (tone) is present. This would come in useful in hopefully curb commercial break-thru by preventing opening the receive audio on both the above radios. Now the programming of the radios is fairly strait forward but there are a few hurtles to overcome to get this to work properly. I am not going to go into great detail here as the radio service software manuals contains all the necessary information except for the few important hints that I am going to post here that might be of help to others.
IMPORTANT HINTS:
1. Establish the model and what type of Logic Board your radio has. The two leading web-sites that has information on Logic Boards are listed HERE , HERE and HERE. Please note that only the extended logic board gives you the option to program pin functions for the expanded 16 pin accessory connector at the back of the radio.
2. Please note that there is a difference between the GM300 8 channel radio and the 16 channel radio 16 pin accessory connector at the back of the radio. Instructions for the Expanded Accessory Connector in the image below.
(Click on images for larger view.)
3. The Motorola M100 Radius Radio only has a 5 pin accessory connector at the back of the radio which is not programmable.
4. If you want to use the radio for AllStar or Echolink then before you program the CTCSS functions you need to note that the radio has a remote hook switch on the microphone that is used when the radio is in monitor mode. The yellow monitor light on the front panel of the radio will be yellow and on under normal operation if programmed this way. Once you programmed the radio for CTCSS this light will still be on and CTCSS will not function as it should. The receiver will still open and allow any station in irrespective if CTCSS is programmed in the said radio. If you programmed the radio for CTCSS and hook the microphone to a mic-clip the monitor light will go off and the CTCSS will function. Unfortunately this will not work if you use the 8 pin microphone connector for AllStar or Echolink as the microphone will not be connected to the radio. You need to short pins 3 and 4 on the RJ-45. Pin 3 is the "hook" pin. Pin 4 is ground. By shorting pin 3 and 4 we make the radio believe that the microphone is on hook in the mic-clip for CTCSS to work. The other option only works with the extended logic board radios. You can program the accessory connector in wide mode function to use pin 7 and 14 (back of radio). Once programmed you can jump pin 7 (ground) and pin 14 (Remote Hook Switch Input) that will do the same as shorting 3 and 4 on the RJ45. In my case I cannot use the accessory connector as I do not have an extended logic board an therefor not the programming option in the RSS. The only option left is to short pin 3 and 4 on the RJ45. Please ensure that you use the correct pin outs. I will not be held responsible if you use the wrong pins. This will damage your radio!!!
I sure there are other hints for this, but the few listed above did the trick for me.
You need to choose the RX squelch type that you will be using. If no CTCSS then use CSQ. I chose TPL as DPL and INV.DPL has certain limitation or hick-up which I am not going to go into here. Google is your friend if you want to know more about DPL and INV.DPL
What is RX Squelch Code?
RSS LOCATION
DEFINITION
Receiver unmutes if carrier with this specific sub-audible code is present. Receiver unmutes when it detects this code, allowing more users and privacy on a frequency.
Now onto programming :
Programming the Rx Squelch Type
RSS LOCATION
DEFINITION
Determines what form of continuous sub-audible code (along with carrier) is needed to open receive audio.
DEFAULT/CHOICES
Finally: I trust that the above will be of help to other radio amateurs that would like to program Motorola Radios with the CTCSS code function.
Programming Example Images:
