[TrunkCom] Re: scanner stuff

Tue, 19 Feb 2002 14:05:23 -0500

On Tue, 19 Feb 2002 08:48:00 -0500 "Jerry Kacprzycki"
<[email protected]> writes:
> Motorola Talk Group I.D. s are based on the number 16; I believe.
> Some will only be two digits long, some will be three digits long and
so on.

Not exactly.  Motorola systems use 3-digit hexadecimal talkgroups (some
radios use 6-digit decimal numbers, but these are simply the decimal
equivalent of the hex number with 800000 added to it).  The Trunk Tracker
converts the hexadecimal to decimal and multiplies it by 16, i.e.
Motorola hex 005 becomes Trunk Tracker 80.  All valid talkgroups on a
Type 2 system will be exactly divisible by 16.  If you get talkgroups on
a system that are not exactly divisible by 16, you either have a Type 1
system or a Type 2 talkgroup with the Status Bit activated (see below).

> Status Bits are the Talk group I.D., based on/divisible by the 
> number 16 and the number 1, or the number 2, or the number 3,... 
> added to the Talk Group.  Thus the resulting Talk Group or staus Bit 
> is NOT based on/divisible by 16.

Status Bits are not the talkgroup, they are a status indicator that is
transmitted from the control channel along with the talkgroup
information:

ID+0 = Normal talkgroup
ID+1 = ATG (All Talk Group)
ID+2 = Emergency Talkgroup
ID+3 = Crosspatched talkgroup (to another talkgroup)
ID+4 = Emergency crosspatch
ID+5 = Emergency multi-select
ID+6 = ?
ID+7 = Multi-select (dispatcher simulcast)
ID+8 = DES (digital encryption)
ID+9 = DES ATG
ID+10 = Emergency DES TG
ID+11 = Crosspatched DES TG
ID+12 = DES Emergency crosspatch
ID+13 = DES Emergency multi-select
ID+14 = ?
ID+15 = Multi-select DES TG

First-generation trunking scanners would make the conversation appear on
a new talkgroup when it actuality the talkgroup simply had the Status Bit
activated.  Thus, when listening to (for example) 6704, if they
cross-patch to another talkgroup, the conversation would then appear on
6707 (+3).  Second generation models introduced Status Bit masking that
allowed you to either see the status bits activated or mask them and
continue to show the same talkgroup (i.e. show 6704 even when Status Bits
are activated).  I do not know how the PRO-93 (which the original poster
was asking about) handles them since the manual does not mention the
feature at all.

Here's an exerpt from one of my previous books explaining why a "new"
talkgroup would appear with Status Bits:

-----------------

Now why did the first-generation models add these numbers?  It has to do
with the way the Trunk Tracker (TT) translates the talkgroup ID's on a
Type 2 system.  Motorola talkgroups are hexadecimal numbers, so a typical
Type 2 talkgroup would be a number like 44d.  But the control channel
transmits 44d0, the zero being the "status bit".  The radio users have no
control over this status bit (except when they press their emergency
button which changes the bit to 2), and all Motorola radios on that
talkgroup are affected by this status bit.  For example, if the status
bit is 1, all radios on the same talkgroup go into an "All Talk Group"
mode; if the status bit is 3, the current talkgroup is patched to another
talkgroup, etc.

So why does this status bit affect the TT display?  Because of the
hexadecimal-to-decimal conversion that the TT does.  Convert 44d0 to
decimal format and you get 17616.  But convert 44d3 to decimal format and
you get a totally new number, 17619.  The first-generation TT sees this
as a "new" talkgroup instead of seeing it as the same talkgroup with the
status bit activated.  In other words, 44d0, 44d1, 44d2, all the way up
to 44df are all the SAME talkgroup, but with different status bits.  Yet
the TT instead "sees" them all as different talkgroups: 17616, 17617,
17618�all the way up to 17631.

If you were to enter all of the status-bit talkgroups, you'll hear
NOTHING on them until one of the status bits is activated.  In other
words, if the "normal" ID is 21616, you'll hear nothing on 21618 until
one of the officers presses his emergency button.  Then once it is
pressed, all conversations from 21616 will go to 21618 and stay there
until the emergency is reset, during which time you will hear nothing on
21616.  Another example - a police ID on 6704 is cross-patched to another
ID.  The number on the TT will change to 6707 and NOTHING will be heard
on 6704. 

-----------------

The Scanner Dude
Brian J. Cathcart - KE4PMJ
NEW! South Florida Frequency & Trunking Guide - 6th Edition (On CD-ROM
too!)
(Two books are now ONE)