[App_rpt-users] DMK URI COS Level / Kenwood TKR-850

[Kevin Custer]

On 5/9/2015 1:49 PM, John wrote:
> Is the 0-5 Volt COS swing on a Kenwood TKR-850 sufficient to switch 
> the COS on a DMK URI? Can't seem to get my system to repeat. I'm out 
> in the 'boondocks' with very few parts and a long way from a Radio 
> Shack. Just hoping I don't need to buffer the COS somehow to get it to 
> switch.

All FOB's, including the URI, but the exception of a RIM, require a 
logic signal capable of sinking to near ground potential on the COS and 
CTCSS inputs (active low).  No matter if you choose a upright logic or 
inverted (in software) the hardware needs to provide a signal that goes 
nearly to ground.  In other words, it needs to sink to ground 
potential.  Many radios provide a voltage when the logic state is valid 
(active high), these don't interface to a FOB or URI well.  A pull-down 
resistor can help in these situations, but it's best to use the right 
type of logic circuitry to feed these logic inputs.

Since the CM119 IC is internally pulled up, changing its logic state 
requires a ground.  This was originally done by mechanical switches in a 
FOB, but must now be done with transistors or whatever logic is 
available from the radio.  Protection diodes on these inputs help to 
complicate the situation because they have a voltage drop.  The hardware 
inputs of the CM119 require that the voltage drop to about 1V or less to 
become valid.  While Schottky diodes keep this to a minimum (because 
their voltage drop is about 2/3 that of a silicon diode), you still need 
a solid pull to ground logic signal for a valid COS or CTCSS signal for 
a URI or simple FOB.

Some radios are difficult to properly interface the COS and CTCSS logic 
to a FOB or URI.  I have a board that I designed just for the occasion:
http://www.repeater-builder.com/w3kkc/products/squelch/sc50.html

My suggestion is to use a NPN transistor or FET to invert the active 
high TKR-850 logic for a solid pull to ground.  A 2N2222 or 2N7000 can 
work well.

Kevin
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