[arm-allstar] 60 second noise burst
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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