[arm-allstar] 60 second noise burst

Kevin Custer kevin at kc-wireless.com
Sat Sep 22 12:21:50 EDT 2018

On 9/22/2018 10:20 AM, "Doug Crompton via ARM-allstar" wrote:
>    The COS signal from the radio must go down close to 0 v for a low and above 2v for a high.

This is VERY GOOD information to remember.  Why?  Not all logic signals 
are created equally.

The dissertation that follows explains how the CM108 and CM119(A) COS 
and CTCSS logic inputs work, and what logic the IC is really looking 
for.  The CM1XX chipsets were not intentionally made to be AllStar radio 
adapters, but rather simple USB audio adapters for laptops and 
desktops.  Steve Henke W9SH added functionality to AllStar to allow 
these cheap audio components to be converted to inexpensive but reliable 
radio or repeater interfaces.  That part of the code is called XPMR^tm.

The COS and PL inputs are internally pulled high, to 3.3 V, and have to 
be pulled low enough to be valid.  Remember, these inputs (volume-up and 
volume-down) were "normally" actuated by manual switches in a standard 
audio adapter.  As such they made a good ground when depressed.  These 
inputs are not intended to act upon the presence of voltage - only the 
existence of ground.  Most radio adapters use protection diodes 
(BAT-43's) that don't allow voltage to be sent into the CM1XX.  So - it 
doesn't matter what voltage is present on the COS or CTCSS pins at the 
DB9, because voltage on these pins doesn't make anything happen.  In 
fact, without the protection diodes, if voltage greater than 3.3V is 
sent into these pins, the IC will fail.

Okay, so the CM108/119's logic inputs are looking for a ground to be 
valid.  This "active low" condition is required NO MATTER if the setting 
in the conf file is upright or inverted.  So - the setting in the 
configuration file doesn't change the fact that the adapter needs an 
active low to be valid and assert the condition.  All this software 
setting does is change if the low condition exists when the radio is 
hearing a valid signal, or when it's not.  Using a DMM, you can read the 
voltage on the CM1XX side of the BAT-43 diodes to see if the logic level 
is properly changing from 3.3V to 0.0 (or a few tenths of a volt).  I 
have found that the voltage needs to be lower than .9V to be valid, and 
above 2.0V to be invalid.

Not all radio logic signals are created equal.  Some logic circuits can 
source current, but lack the ability to sink.  Sometimes active high 
circuits (circuits that provide a voltage when the state becomes active) 
don't have the ability to pull to ground very well. These circuits may 
not have the capability to pull the COS and CTCSS low enough on the 
radio adapter to become active/valid.  Sometimes the conversion of 
active high to active low is required.  A pull-down resistor can help, 
but nothing beats a real active low circuit.  A 2N2222 (or similar BJT) 
with the emitter grounded usually works well.


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