[arm-allstar] Yet another USB FOB Modification

Kevin Custer kuggie at kuggie.com
Sun Nov 16 11:47:02 EST 2014

On 11/16/2014 1:34 AM, Doug Crompton wrote:
> I suppose any of those items could be true in about 5% or less of the 
> situations encountered by the average Allstar user.

Since you said user, and not operator, let's put that into prospective.  
Most every transmitter on WAN uses direct FM, the same goes for the WIN 
system.  These two systems make up a whole lot more than 5% of the 
AllStar users.  You are generalizing and comparing apples to oranges.  
Even if you really meant operators, your math isn't close to reality.

> For the average user a standard FOB, whether it be DMK or modified 
> Chinese, works just fine.

Again, that depends on what exactly the average user is doing.  If he's 
using MIC audio only - yes.  If he's also using channel B and dumping PL 
into the modulator to get good sub-audible, then a standard FOB job 
isn't fine.

> I don't want people to get the idea this is going to do something 
> special for them and they need to spend the extra money and buy it.

And I don't want people to think they can get by with a $2.00 FOB when 
they need the additional circuitry of a better device. Education is what 
is needed here, not some half-cocked response that doesn't take into 
account all of the circumstances of the situation.

> Most all radios have TX timeouts so that point is taken care of.

Time out timers in radios?  You aren't serious?  I don't know of any 
situation were I would knowingly enable a TX timer 'on the radio'. That 
surely wouldn't fly on our network.

> The other issues don't apply to most radios.

Again, you are generalizing.  There are plenty of folks that hack into 
the radios circuitry to get adequate deviation, especially on PL tones.

> I don't understand the cos/ctcss one because like PTT this is SW 
> switchable.

Just because the logic state is switchable in software doesn't guarantee 
that it will be properly recognized by the hardware.  The two logic 
states are logic high and logic low.  The FOB requires a logic low (a 
pull to near ground potential) because the logic the device is looking 
for was supplied by a hardware switch to ground. The maximum voltage on 
the GPIO's is 3.3 V, that's why the Schottky diodes exist.  The thing 
you have to remember is that many radios supply a positive logic source 
and they may not have the ability to source a low.  This situation 
happens where the device in the radio is a PNP transistor or other 
active high circuit, and while it has the ability to pull to a high, but 
cannot source a ground without a pull-down resistor.  Pull down's can be 
a problem and is why Mark Herson commented on why he prefers to have a 
pull to +5 so when the system is protected by an incorrect COR condition 
if the radio is turned off or otherwise disconnected.

> Most hams are cheap and building, modifying etc. are a large part of 
> our hobby.

You are generalizing again.  Many (most?) hams cannot do the 
modifications to the FOB's.  Out of the 13 core members of the WAN 
system, 3 of them have the ability to do FOB modifications.  The others 
wouldn't even consider it.

> We usually don't put dollars and cents on our time. It's the fun of 
> doing it. Anyone can buy something and throw it on the air. Everyone 
> has to make the decision what's best for them.

Misinformation destroys a hobby.  Education is best, but getting people 
to read and understand what it takes to make an educated decision isn't 
easy.  Believe me, I know.  I have authored hundreds of articles on this 

> The FOB mod I described in the how-to on the web page is really a 
> piece of cake. All you need to do is solder some wires on the FOB and 
> build a minimal amount of external circuitry. I made it harder 
> (mechanically) because I put it in a nice box which is not necessary. 
> For $10 or less worth of parts and a few hours of your time you have 
> an interface.

While I agree that you have an interface, that interface doesn't take 
into account every installation.  The URI and RIM handle the extra 
LPF'ing needed when driving a modulator directly.  The RIM has buffer 
circuitry on the COS and PL inputs to take whatever the radio can give 
it, and convert it to a solid pull to ground, just the the mechanical 
switch the CM108/119 was designed for.  Your $2 FOB will not work with 
all types of radios.  You have to know what you need and source the 
equipment accordingly.

The RIM isn't some poorly thought through copy of a URI.  It has 
additional circuitry for those instances where they are required.  I 
have nothing against the URI.  I have more of them deployed than any 
other radio interface.  I simply got tired of having to put a resistor 
divider in the discriminator line to get the audio in range of the 
software, adding amplification beyond the ability of the URI to 
sufficiently drive the modulator, and putting transistor switching in 
between the radio and URI so the hardware inputs worked reliably.  
Nothing more, nothing less.

Whether a user of this technology needs the special things the 
commercial interfaces provide depends on the situation, and making 
generalizations doesn't help.  I'm sure you are proud of your work 
converting a FOB, but understand there are fewer hams than you might 
expect with the skills and equipments needed to do a successful FOB job.

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