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Operator Manual for TACT TA-4800 in NSW National Parks & Wildlife Service User Mode G/00123B Issue 1 Page 21
PTT AND COS
PRESS TO TALK (PTT) OUTPUT TO RADIO
Description
A relay is provided to apply press to talk (PTT) to the radio when the transmitter is to be keyed. Keying the
transmitter also allows TACT to transmit audio from the telephone to the radio field. Normally a radio will
require a contact closure to ground to activate its PTT (or key its transmitter).
Connection
‘PTT Common’ is on Pin 1 of the ‘
RADIO
’ connector and ‘PTT Normally Open’ is on Pin 2 of the ‘
RADIO
’
connector. The radio’s Ground or 0V should be connected to Pin 1 and the radio’s PTT input should be
connected to Pin 2. If the radio requires a voltage input to activate its PTT then a suitable external voltage
should be connected in series with Pin 2.
CARRIER OPERATED SQUELCH (COS) INPUT FROM RADIO
Description and Programming
TACT requires a COS signal from the base radio or repeater when it is receiving a transmission from a
mobile. It may take the form of contact closure or a voltage.
TACT must be programmed accordingly:
*6703# Range
Default
*75
COS Sense
0,1
0
0 = COS active low
1 = COS active high
000
000
000 = digital (binary) input OR:
*77 COS
Threshold
1-255
128
Voltage reference relative to 5V.
(128 = centre of COS swing 0
5V)
*#
Normally, TACT is supplied configured for a contact closure and su5V via 100K to the radio. ‘COS
Threshold’ must then be set to 0 to denote a binary input, and the ‘COS Sense’ set to 0 if a contact closure
to ground denotes a valid COS condition. ‘COS Sense’ is set to 1 if a valid COS is denoted by an open
condition.
Should the radio present a voltage to indicate the COS condition, the ‘COS Threshold’ must be set to a
number in the range 1 to 255 inclusive. 1 represents near 0V and 255 repr5V. For example, if the
radio presents a voltage in the range +0.5V to +3.0V, then the ‘COS Threshold’ would be set to {[(3.0V
0.5V)/2 + 0.5V]/5V} x 255 = 89, ie. in the middle of the input voltage range. Note that if either limit is outside
the range 0
5V, for example –5V to +10V, then while the input is protected against modest voltage
excess, only the portion from 0
5V is recognised and in such an instance the ‘COS Threshold’ would be
set to 128, ie. half the sensing range. ‘COS Sense’ must then be set to 1 if the upper voltage represents
valid COS or set to 0 if the lower voltage represents valid COS. Additionally, the voltage source in the radio
must be capable of sinking the small current presented by the COS input, ie. about 50 micro Amps. If this is
not so, then resistor R104 must be removed from TACT and re-installed into position R106.
Connection
The COS output from the radio is connected to Pin 4 of the ‘
RADIO
’ connector and is sensed relative to
ground or 0V with Pin 6 of the ‘
RADIO
’ connector connected to the signal ground (0V) of the radio.
