How Does Trunking work on PRO-92?

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gmclam

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I was advised to post here (instead of RS scanners) .....

I am a product designer, which includes writing firmware, software & hardware design. So I am a little more than curious as to how the PRO-92 "thinks" when it is scanning a trunked system. After some of my own examinations, I thought I'd post here and see if others know more details and/or if their knowledge jives with my hunches.

First some background where I am monitoring.....

Here in the greater Sacramento (California) area, up until several weeks ago the city and county EACH had their own trunking system. They each have a set of assigned frequencies and what was not a very well thought out set of trunking IDs. A lot of city agencies from police & fire to public works and transit workers are on the city trunk. The county trunk has the sheriff and metro fire department as well as county workers like parks & rec.

When the systems were 100% separate, each trunk had a control channel broadcasting. The city IDs were received only on the city trunk frequencies and the county IDs were received only on the county trunk frequencies.

What seems to have taken place is first the trunk IDs have been carefully assigned to not overlap. Each agency, regardless of city or county now has a unique ID. It appears now that sometimes the two trunks operate as one, and sometimes they operate as two separate trunks. Now I am clearly receiving CITY IDs on "county" trunk frequencies at some times of day. During these times there is only ONE control channel broadcasting total.

What I have done is program ALL frequencies for BOTH trunks into a SINGLE bank of my PRO-92. I've learned that if I turn off a control channel, no channels are received (if in closed scan mode). It appears that I must have BOTH a control channel AND the voice channel programmed to make this work. Also, it appears that once the scanner finds a control channel active within a bank, that is what it monitors. That is it will not search nor monitor another control channel in the same bank. What this means is that when both trunks are active separately, I will only receive communications from the programmed IDs of the trunk for the first control channel found. Can someone comfirm or clarify this finding for me?

The manual states:
For Motorola (above 806 MHz range), the scanner monitors the control channel between each transmission to identify talk groups. For some Motorola (under 512 MHz range) and LTR systems, the scanner uses the subaudible data sent with each transmission to identify talk groups.

What I believe actually happens is that the scanner reads the ID from the CONTROL channel when scanning. The trunk ID is only read from the subaudible data when in MANUAL MODE (as the scanner is not at that moment receiving the control channel).​

If I am reading other posts correctly, it seems that some scanners only require the control channels to be programmed. I believe I also read something that suggests programming the Sacramento trunk into TWO banks (I guess so that reception is possible from both trunks when 2 control channels are active). Am I missing something?

Thank you,
George
 

WayneH

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Which version of the 92 is it? Early models didn't use the control channel for determining which talkgroups were on which frequencies, they relied on the subaudible data on the voice channels. The later 'B' version did use the CC, and operates like any other scanner (though probably requires all voice channels).
 

gmclam

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PRO-92B w/ ver 3.28 f/w

Hello Wayne. Thank you for your reply. Since posting this message I've found a lot of links here to the kind of information I was seeking; although there is always more to learn. The scanner I am questioning is a PRO-92B. I confirmed the f/w version with a hidden keycode sequence at power up.

I did not realize that some scanners only required the control channels to be programmed until reading other threads here. I am from the way old school before scanners, then crystal controlled units and onward. To me the whole trunking thing just seemed like programming in all the FREQs of the trunk, and then populating the Trunk ID list with the IDs we want to hear.

I've read the applicable section of my user's manual (and it is the manual for the B version) many times and there is a section that states how it reads the ID from the "current channel" (as quoted before) but from my experimentation I do not believe that is true. The most basic test I performed was to merely lock out the current control channel and I stopped hearing that trunk. Further information I've read from links here at RR have confirmed this.

So what I am finding is:
1. I must program in ALL channels, voice & control
2. The control channel must not be locked out (it uses this channel in SCAN mode to know what channels have communication

It does appear that I can have more than one active control channel in the same scanner bank and receive communication from either trunk. This is because of how the Sacramento trunk works (as explained earlier).

I'd appreciate anything further you can add, or any corrections you might have to my findings. Thank you very much,
George
 

gmclam

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After my inital post, I found lots of links here which were a wealth of info. Now I'd like 2 more things; 1 is source code for the f/w so I can correct bugs (I am a programmer) but I know that is unlikely. 2 is some technical info on the Motorola sub-audible tone protocol. For that, I'll ask in another forum.
Thank you Wayne!
 
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