Scott,
A while back there was a thread on this subject and one of the members posted
the reply below. It really made a lot of sense and I saved it in my archives
for future reference.
Don, KD9PT
Lastly, if you can do it, run your logic level signals as
active low, for two reasons:
1) they are much easier to debug,
2) a blown fuse doesn't create a false signal.
In addition I prefer to run the interfacing between radio and
the controller as open collector since most controllers have
their own internal pull-up resistors, and a open collector
output is therefore universal - it will work with a five volt logic
system, a +12 volt logic system, a relay, a LED (for testing),
or anything else, and all with no modifications. Active low signals
are easy to debug, and easy to interface - with proper design
and component selection you can treat them as if they were
a relay contact that goes to ground when active.
With an active low environment when the signal is inactive, the
transistor is switched off, the collector floats and is dragged
high by the pullup resistor. When the signal is active the open
collector is pulled to ground.
As mentioned above, if you leave the input open (for example
unplugging the cable between the control receiver and the
controller) the pull-up resistor causes the input to go high.
If you program the input for active low and unplug the cable
it doesn't care.
Personally, because of the above reasons, I dislike active high.
I recommend that first you program your repeater controller
COR, PL and logic inputs to be active low, and add a transistor
inverter to your hardware if necessary.
Another benefit of active low interfacing is if a radio loses power
or otherwise fails, the pull-up resistor in the controller doesn't
"pull up" the dead signal to a false active, resulting in the system
being keyed until the timeout timer disables the system transmitter
(you DO have a separate backup timeout timer, don't you?), thereby
rendering the entire system unusable. Also when you are working
on the system you can do a large amount of testing with nothing
more than a clip lead shorting the open collector outputs to ground
without worrying that you're going to blow something up (picture
a site visit some day... PL decode not working? ground the PL
decode lead and see if the PL indicator on the controller front panel
lights up. yes? the problem's gotta be in the decoder itself inside
the receiver chassis!). Also with open collector interfacing you can
shut off the system, disconnect or un-cable the bad radio/device/etc.
and then turn the system power back on and everything else in the
system still works because the pullup resistor in the unplugged open
circuit looks like an idle signal...
----- Original Message -----
From: scott w
To: Repeater-Builder@yahoogroups.com
Sent: Thursday, June 03, 2010 3:55 AM
Subject: [Repeater-Builder] active low COR
Hello to group,
What is the advantage of an active low COR.
Most I have talked to say they have their controllers set that way, so
I set mine that way,plus the Maxtrac I am using as a receiver default is active
low.
I simulated a power failure to the reciever and the line went low and
the transmitter thought it was time to transmit. I also accidently pulled the
cord from the controller off the back of the receiver and again a low status
and the transmitter thought it was time to transmit. Im not seeing a benefit of
a active low COR in those repects. Should I go to active high or since those
things dont usually happen often leave it LOW..
Any advice or ideas appreciated..
73s
