How Does Trunking Work on a PRO-92?

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Premium Subscriber
Sep 15, 2006
Fair Oaks, CA
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,


Feb 21, 2006
Try posting this in the Ca. forum. There has been lots of Sacramento talk lately cocerning a new system. You will probably find your answer there.

Doug K
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