Linking analogue repeaters

[Q] I have a question how to connect SLR8000 with Motorola CM360 to create a radio link that is to say connect SLR8000 in analogue with motorola CM360 analogue. Tell me the pins of the SLR8000 relay accessory.

[A] The CM360 was discontinued some years ago. I might be wrong but as far as I can remember, this radio was not approved for fixed installations. Technically it will probably work okay but you might run into problems with the local regulator, if they decide to check.

I don't have the software for the CM360 anymore. I doubt it will run on Windows 11 and my memory of this product is not that good anymore.

You can use a "link radio" to extend the coverage of an alogue repeater to another repeater or onto a simplex channel as shown below. The link radio basically retransmits whatever goes out over the repeater and anything received on the link channel gets transmitted on the repeater.

The first thing you would need to take care of, is to ensure that the link frequency and repeater frequencies do not interfere with each other. Ideally the link channel should be in a different frequency band (or if not possible, a good few megahertz away).

The antennas would need to be positioned so as to minimise the risk of desense.

The way this works is:

Whenever someone is transmitting via the repeater, it will produce received audio and carrier detect on the accessory connector. Received audio is basically what the repeater receives over the air. Carrier detect is a logic level where high means the repeater is receiving something. The carrier detect output and received audio are both sent to the link radio.

So in the above diagram, Radio A can talk to Radio B via the link radios connected to both repeaters. The repeaters are linked via the "link radio" which can also serve Radio C.

Carrier detect from the repeater can usually be fed directly into the PTT input of the link radio. Sometimes you might need to configure the repeater to produce a logic low whenever carrier is present - this can be done in the CPS (Accessories set).

The link radio also produces received audio and carrier detect. This would be passed to the repeater. As with the link radio, carrier detect from the radio can go straight into the PTT input of the repeater.

Carrier detect is called CSQ Detect in the CPS. I think it has the same name in the CM360 CPS but I haven't used that software for over fifteen years now. If you are using CTCSS, I would suggest to rather use PL Detect (or whatever it's called in the CM360 CPS).

The GPIOs on both the repeater and radio need to be configured to provide/accept CSQ Detect and PTT. This is done in the CPS. You need to pay special attention to the logic levels so that when CSQ Detect goes high, PTT is triggered.

The tricky part is the audio. The CM360 probably outputs around 300mV RMS on the receive audio output. The SLR8000 expects around 80mV.Also, the impedance of the receive audio output on the CM360 is probably around 2kΩ. This means that on the audio line between the CM360 and SLR8000, you will need to employ some impedance matching and attenuation.

By default, the receive audio output on the CM360 has a 300Hz-3kHz band pass and 6dB/8ve deemphasis applied to it. Both of these need to be disabled in the CM360. This is done in the CPS. If I remember right, the fields were called Disable De/Pre-emphasis and RX Flat Audio.

The same is true going the other way, the level and impedance of RX Audio on the SLR8000 is 300mV RMS and1kΩ (nominal) and microphone audio input on the CM360 is also probably 80mV at 600Ω. The SLR8000 will also output filtered audio by default so this too will need to be disabled.

SLR8000 codeplug settings.

The above shows suggested settings for the SLR8000. Note that I am using debounce on GPIO1 (2). This presents accidental triggering of the repeater PTT. When setting Audio Type to Flat Unsquelched, the RX Audio output will produce white (brown) noise whenever there is no signal present - this is normal, Also, Analogue Accessory Emphasis gets set to None, which is what you would need.

The interconnecting cable would look something like this.

In addition to the above, the ground on both the radio and repeater must connected to one another. This is the ground on the accessory connector, not the chassis ground.

Although not shown in the diagram, all audio cables must be screened (coax). The potentiometers and resistors should also be mounted in a shielded box. I would go as far as mounting them all on a PCB - possibly with a 16-pin DIP socket which would allow the board to sit on the back o the mobile radio accessory connector.

 

You can find a listing of SLR8000 pinouts here.

I don't have any CM360 documentation but I'm sure you'll find something on the internet.

The SLR8000 uses an industry standard Dsub 25 connector. For the CM360 you'll need to buy a HLN9457A and use a matching crimp tool.

If this were a digital system, there is a more elegant way to link repeaters that only only passes voice but also control messaging and data - see here.

In principle you could use any repeater and almost any mobile for this. 

If I remember anything else, I will add it in the comments.

Some useless trivia: I had some small involvement in the workstyle photography for the CM series brochures. The radio needed to be installed into vehicles but we couldn't do any drilling or use any screws since the cars were on loan. The solution: lots of Prestik (a.k.a. Blue Tack, Pattafix; Sticky Putty). For the taxi photoshoot, I removed the control head and stuck it over the hidey hole on the dashboard.