[arm-allstar] Unpotted USB audio FOBs --- THEY WORK!!

[Kevin Custer]

Jim,

You may want to use Schottky diodes in the future - depending. Why?

The COS and PL inputs are internally pulled high, to 3.3 V, and have to 
be pulled low enough to be valid.

Okay, so the CM108/119's GPIO's for COS and PL are looking for a signal 
close enough to ground to be valid and reliable.  Remember, these inputs 
were originally manual switches in the FOB's.  As such they didn't 
provide any voltage and made a good ground when pressed, as that's what 
the other side of the switch was tied to - ground.

I have been cautioned that the 'valid' on-state voltage is somewhere 
around 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 about .7V for active low.  If you use a silicon diode for 
the series protection, this adds another .7V, and now we are at 1.4V.  
Using a Schottky diode with a lesser voltage drop (.3V) gets us back 
down to that 1.0 volt range.  I have found that many FOB's will work at 
1.4V but fail to switch at 1.5V. Some NPN and FET switches are less than 
.7 when switched "on" and you can get away with silicon protection 
diodes. YMMV.

If you are using an open collector transistor stage to feed these 
inputs, the active high voltage can be anything - even 12 volts or 
higher.  The diode keeps this pull-up voltage from blowing up the 
CM108/119.  The key here is the line has to be pulled low enough to 
become active.  Sometimes that requires a pull-down resistor with PNP 
type open collector output stages.  Something around a 1 to 10K usually 
works.

Kevin





On 10/25/2014 8:39 PM, Jim Korenz wrote:
> Hi All,
>      I bought 5 of these FOBs also. I had a couple of days this week to mod one. I have a link radio working with the DMK URI, so I haywired my FOB to a DB25F connector to match my current setup. Everything works like it should, PTT, COS. I had to add a resistive divider to get the TX audio in range. But, bottom line, everything sounds good. I used 1N4148 signal diodes in the COS and CTCSS lines instead of the schottky diodes, and I used a 2N7002 FET for the PTT. (I used it because it didn't need the 10K resistor.) The logic signal from the CM108 is 2V, but the FET keys up my TK805 just fine. Thanks for the heads up on the USB audio FOBs. Now all I have to do is upgrade to the new BBB release :-)
>     JimK N8PXW
>