[arm-allstar] COS equivalent for other radios

[Kevin Custer]

I'll expand on Marshall's description, and give you a description of 
what a FOB or other radio adapter is looking for...

The COS and PL inputs on the CM108/119 are internally pulled high, to 
3.3 V, and have to be pulled low enough to be valid.  Remember, these 
inputs were "normally" actuated by manual switches in a standard audio 
adapter.  As such they made a good ground when pressed, as that's what 
the other side of the switch was tied to - ground.

Okay, so the CM108/119's GPIO (when used as inputs) are looking for a 
signal close enough to ground to be valid and reliable. This "active 
low" condition is required NO MATTER if the setting in the conf file is 
upright or inverted.

This 'valid' on-state voltage is somewhere around or less than 1.0V for 
optimum reliability.  If you were to use a standard transistor output 
logic (commonly available in many radios) the voltage drop can be as 
high as .7V for active low.  If we were to use a silicon diode for the 
series protection, this adds another .7V, and now we are at 1.4V.  Using 
a Schottky diode (like the BAT43) with a lesser voltage drop (.3V max) 
gets us back down to that 1.0 volt range.  Thankfully, the saturated ON 
state of a BJT transistor with little current sourcing, such is the case 
when driving the GPIO inputs of the CM1XX chip, will result in a voltage 
drop of a few tenths of a volt or less.  And - since the current 
necessary to change states on the GPIO is very little, the drop across 
the Schottky diode is also very little - like a few tenths of a millivolt.

Logic circuits can do many things, but most of them either give an 
active high or active low.  Generally, an active high circuit can source 
a good bit of current from a positive standpoint, but can lack the 
ability to pull a circuit low.  In contrast, an active low circuit can 
source ground, but may have no ability to source a positive voltage.  If 
you are using an active high - open collector transistor stage to feed 
the COS and CTCSS logic inputs of a radio adapter, the active high 
voltage can be anything - even 12 volts or higher.  The blocking action 
of the diode doesn't allow the pin to be driven high, only pulled low.  
The key here is the line has to be pulled low enough to become active.  
Sometimes that requires a pull-down resistor with active high open 
collector stages.  Something around a 1 to 10k usually works.  Of 
course, you could build a one transistor logic shifter changing the 
active high to active low.  A 2N2222 or similar NPN switching transistor 
with the emitter grounded and a 1k resistor to the base from the active 
high COS, usually works well.  The collector becomes the active low to 
feed the radio adapter, satisfying the need for the CM1XX to be pulled 
to ground to change states.  It's also worth noting that changing the 
setting in the config file to the "invert" state doesn't change the 
requirement that these pins need pulled low.  It just means that the 
opposite action of asserting the control state (or not) happens when the 
pin is pulled low.

In summary, you need a ground when the receiver is hearing a valid 
signal capable of opening the noise squelch setting, OR you need a 
ground when it's not.  Likewise, you need a ground when the receiver is 
decoding a CTCSS tone, OR you need a ground when not. Then, adjust the 
settings in the conf file to match the action delivered by your logic.  
The reference to "invert" in the conf file does NOT change the logic 
from active low to active high, or vice versa, it simply changes when 
"active low" is delivered to these inputs.

Kevin W3KKC



On 3/15/2018 1:49 PM, "Marshall via arm-allstar" wrote:
> Fred,
>
> COS stands for Carrier Operated Switch and used to be called COR. (Short for Carrier Operated
> Relay)
>
> The function of the COS is to tell the repeater controller (in our case Allstar software), that
> your Allstar nodes receiver squelch is open, and that there is a signal there.
>
> The COS signal is usually a voltage that swings high or low to give either a + voltage or
> ground with the receiver active.
>
> Every radio is different where the COS signal is, most radios that have a packet data port have
> the COS signal on one of the pins.
>
> Older radios you are going to have to open the radio up and dig around with a volt meter to
> find the COS signal.
>
> Google the model of your radio along with the words "COS signal" you can sometimes find the
> information you need.
>
> Marshall - ke6pcv