Section 4. Installation
4-2
DIGITAL OUTPUT
The digital paging formats require very accurate timing reproduction of the binary waveform
edges. To guarantee such accuracy, the terminal must be located in close proximity to the
digital transmitter; typically a distance of 50 feet or less. It is also recommended that coaxial
cable with a DC resistance of less than 100 ohms be used to carry the digital output signal.
For connections to remote transmitters, digital modems are required in order to preserve the
high-bandwidth waveform edges. The pager manufacturer should be consulted for exact
requirements, but in general, NEC requires 300 baud (or faster) modems, while POCSAG
and Motorola GOLAY require 600, 1200, or 2400 baud.
MICROPHONE CONNECTION
The audio switching circuitry in the encoder is activated during paging transmission to mute
the external microphone audio and supply the paging tone generator signal to the transmitter.
This assumes that the microphone is connected to input TB1 pin 6. If the microphone needs
to be “live” all of the time, then it will need to be connected in parallel with the encoder's
audio output, TB1 pin 4.
If a low-level microphone (millivolt signal levels) is being used, then the tone output needs
to be attenuated to produce levels comparable to the microphone level. To do this, remove
(clip out) 620-ohm resistor R57 from the encoder circuit board.
PUSH-TO-TALK
Radio transmitter keying is performed with the normal “push-to-talk” (PTT) signal. The
encoder uses a DPDT relay to provide its PTT output. On TB1, the PTT connections are
found on pins 10 (N.O.), 11 (COM.), and 12 (N.C.). A second set of PTT connections are
found on TB2; pins 20 (PTT COM.) and 21 (PTT SW.). Whether TB2 pins 20 & 21
constitute a normally open or normally closed pair of contacts is determined by the position
of jumper JP2. The jumpers JP4 and JP5 can be installed to internally ground the PTT
Commons, TB1 pin 11 and TB2 pin 20 respectively.
A very typical configuration would be that a contact closure to ground is used for the
transmit PTT signal. Connect the encoder's normally open contact at TB1 pin 10 to the
transmitter's PTT input, then install a jumper across JP4 of the encoder. Should an open
connection be required for transmit PTT, connect the normally closed output at TB1 pin 12
to the transmitter's PTT input, and then either install JP4, or connect the PTT common output
at TB1 pin 11 to the transmitter's other PTT connection. All of the relay contacts are
provided so that other connection combinations may be made if needed.
When a desk microphone push-to-talk switch is used in conjunction with the encoder, it is
normally connected in parallel with the encoder's PTT connections so either closure will key
the radio.
