Send me the slightly longer but not completely technical explainer then
How would it pick up a CC from an entirely different system? The only CC's in my ROAM are for MPSCS. They aren't used by any other system at any other location in the state.
Your lucky.
Next, let's get out on the table the meaning of trunked "system", or rather, some confusion about this term.
A trunked system is a unique combination of a SysID and one or more control channels, only one of which is active at any one time. The simplest form of this is a single-site SmartNet "system." However, also included is a multi-site SmartNet simulcast system.
Now let's deal with SmartZone. SmartZone is a way of connecting, for control and selected audio purposes, two or more "zones," where now a "zone" means a unique combination of SysID and single active control channel. Each zone acts independently, within its coverage area. However, sometimes a zone will carry traffic that originates on a different zone, depending on the affiliation of "roamers."
Let me illustrate by an example, which will be simplified a bit: the Massachusetts State Police system (which is also used by other state and local agencies). Originally, it consisted of four independent SmartNet "systems." Each had the same SysID, but each had separate control channels. Subscriber units could monitor only one "system" at a time. If you moved from the territory covered by one to that covered by another, you had to manually switch in your radio.
What SmartZone does for you is two things. First, in theory, it will handle the switching for you automatically. It does this by applying a very complicated, and system-specific, set of parameters based on signal strength, BER and a couple of other factors, all of which are designed specifically for this system and tweaked a bit based on experience. Unless your radio has the same values as everyone else on the system, SmartZone auto switching won't be reliable.
Regardless, a SmartZone mobile or portable only listens to only one "zone" ("system") -- and only one control channel -- at a time.
Now, talkgroups on a SmartZone system come in three flavors. A "statewide" TG will be repeated on all of the "zones", regardless of the "zone" on which the channel grant was initiated. These are resource intensive and there are only two on our Massachusetts system.
A "home only" TG will only be recognized on one "zone." If you pass out of that "zone" to another "zone" while selected on a home only TG, and then try to get a channel grant, you will get an error boop.
Finally, a SmartZoned TG is like a "home only" TG except that it will also be carried on another system if a user happens to be affiliated on that other system and selected on that talkgroup, and only as long as this condition exists. If you're selected on a SmartZoned TG and you re-affiliate to a non-home "zone," you're radio will so advise the controller and temporarily bring traffic on that TG into the non-home zone (until you go back home again).
Back to our simplified example. Let us call the four "zones" A Troop, B Troop, C Troop and D Troop. In each "zone" we will have 3 "patrol" TGs. All of these are "home only" except that A PTL 1 is SmartZoned in C Troop (because the jurisdiction the A PTL 1 cruisers is right next to C Troop).
If you are listening to the C Troop "zone" (i.e., the C Troop SysID and C Troop control channel) you will hear all traffic on C PTL 1, 2 and 3. You will not hear anything on any D Troop TG. You will not hear anything on A PTL 2 or 3, and you will not hear anything on A PTL 1 unless an A Trooper's radio has switched affiliation from the A Troop control channel to the C Troop control channel because the A Trooper is near the line and his radio hears the C Troop control channel better than the A Troop control channel.
(Before any Massachusetts people chime in with all of the exceptions to what I've just stated, let me reiterate: this is a simplified description, for illustrative purposes.)
What do you do if you want to hear A Troop stuff and C Troop stuff, and you're on a high enough hill that you can hear both control channels? Use two radios. Or program one radio with two separate "systems" and scan both. However, if you do that, remember that the radio can only listen to one control channel at a time, so it will act as if it were two radios, with only one turned on at a time. If the radio is listening to A Troop's control channel, you won't hear any traffic on the C Troop system.
The PSR-500 and -600 attempt to duplicate the SmartZone feature, in a simplified way. GRE made no attempt to emulate all of the SmartZone parameters, in part I suspect because of complexity and memory limitations, but probably also because, unless you're an authorized user of the system, you will have no access to the engineered values, and with the "wrong" values, the radio will function in an unpredictable way. Rather, the PSR radios attempt to determine all receivable control channels from the list, and then park on the strongest of them as long as the strongest doesn't dip below a value that you can set via WIN500. This is a valiant effort. In real life, though, because of the high variability in derived signal strength from a portable, they tend to jump around. Given that most TGs are "home only" (since SmartZone and Statewide TGs are terribly consumptive of system resources), you don't know what you'll hear.
Remember, no radio can listen to more than one control channel at a time.
Ergo, the way to maximize what you're going to hear is to program each "zone" as a separate TSYS. You can program all of the TGs in the whole state for each if you please (plenty of memory in most cases and by using the copy feature, you only have to type them once), but doing so is a waste because, remember, most TGs are "home only" to only one "zone." Assign each TSYS to its own scan list. Then, based on your knowledge of how the system is laid out and where you are, select the system appropriate for your location (by selecting the scan list for that TSYS). You'll hear as much as can be heard.
Now, if you like (and don't want to take my advice), you can activate two or more scan lists. However, your radio can only listen to one control channel at a time, so what you'll hear is a bit unpredictable.
Sorry; too long an answer, and I'm not sure how much I've helped. This is not a simple subject, and folks spend years learning the intracacies of SmartNet/SmartZone trunking.
