[arm-allstar] fob active-high logic diagram ---was: Icom UR8050 connection

[David McGough]

Hi Matt,

The input pin configuration I typically use for active-high logic is 
similar to this diagram (excuse the ASCII art!):

+-----------+
|CM108/CM119|                   +-----------------------+
|GPIO INPUT |------->|----+-----|radio COS (active high)|
+-----------+       /     |     +-----------------------+
                   /      /
  BAT 43 Schottky /       \
       Diode              /  <-- 10K ohm resistor
    (or similar)          \
                          |
                          V (GND)


The purpose of the Schottky diode is to provide some protection to the
CM108/CM119 input pin, in the event that the radio COS logic-high level is
more than 3.3VDC (the max allowed at the input pin). If the radio COS
voltage is above 3.3V, the diode becomes reverse-biased and limits the
maximum voltage seen at the input pin. The CM108/CM119 internally provides 
a weak pull-up resistor which is adequate to turn the diode "on."

The reason for using a Schottky diode versus a standard silicon diode is
the lower forward voltage drop they exhibit (about 0.3V), when compared to
a silicon diode (about 0.6V). This lower voltage drop is important to make 
sure the input pin can be reliably turned "off" by the external signal.

Finally, if you use a URI or RIM type device, the Schottky diode is
already part of the internal circuit, so only the 10K pull-down resistor
is needed externally.

73, David KB4FXC


On Sun, 25 Sep 2016, Matt wrote:

> Thanks David. That's what is happening with mine. If the repeater is off 
> or the fob is disconnected then the node becomes keyed.
> 
> Do you have any more details on where to add the pull-down resistor 
> (from ground to COS?) and what value?
> 
> Thanks,
> 
> Matt.
> 
> > David McGough <mailto:kb4fxc at inttek.net>
> > 25 September 2016 at 20:07
> > Hi Matt,
> >
> > Yes, carrierfrom=usb means COS is active-high.
> >
> > Note that most CM108/CM119 fobs have weak internal pull-up resistors. As a
> > side effect, you may have your node become "keyed" (COS active) when the
> > fob is disconnected from the repeater or the repeater is powered off. If
> > you encounter this issue, one solution is to add an always present
> > pull-down resistor at the COS input on the fob.
> >
> >
> > 73, David KB4FXC
> >
> >
> >
> > Matt <mailto:matt at m0lmk.co.uk>
> > 25 September 2016 at 19:46
> > Hi all.
> >
> > I'm trying to get an Allstar node connected up to an Icom UR8050 
> > repeater. I'm using the RPi3 image which is working well and a 
> > modified USB sound fob.
> >
> > The Icom has a "Busy" output which becomes high when the repeater is 
> > receiving so I guess this need to connect to the COS line on the USB 
> > fob. My question is, what setting should I use in simpleusb.conf?
> >
> > I think I should be using carrierfrom=usb. Can anyone confirm that?
> >
> > Thanks.
> >
> > Matt
> > M0LMK
> 
>