Service manual

33
Theory of Operation
DCB, DCFCB and DCFCTB Models
POCSAG Decode
All POCSAG messages contain a Receiver Identity Code (RIC) or CAP code. This
indicates which unit or group of units a message is intended for. The unit must have
its RIC configured before it responds to a message.
The Standard Format for Federal Signal POSAG codes consists of a five (5) character
numeric message preceded by a bracket “[“ and followed by a bracket “]” for a total
of seven (7) characters. The first three (3) numbers are the unit number. Any unit
number less than 300 activates only that unit number. A unit number of 300 activates
all units. Unit number of 301 to 316 activates all units in zones 1 to zone 16
respectively. The unit must be configured for the zone in which it is to function.
The last two numbers in the message are the function number (01 through 06), which
activates functions 1 through 6. Function numbers 96, 97, 98 and 99 activate Poll All,
Reset, Quiet Test or Cancel respectively.
Receive audio from the receiver module enters on JP7, pin 8. This audio is direct
coupled and passes through lowpass filter U1A. R18 and C7 average the DC voltage
present on U1A pin 1 and pass it to U1B pin 6. U1B acts a comparator converting the
audio present on pin 5 into squarewaves. This is fed to the processor for decoding.
Digital
The receive audio is fed to the digital decoder IC (U8). U8 converts the alternating
tones in the digital code to a string of highs and lows forming serial data. The data is
sent to the processor’s RXD port to be decoded.
Encoder Section
The transmit audio is generated by U7 (the DTMF encoder), U8 (the digital encoder)
and the CTCSS board (if present). PTT is generated by the processor taking pin 26
low. This gates Q9 on allowing the transmit audio to pass out to the transmitter.
The PTT also sets the output of U12F high, which allows R65 to turn on Q8 thru
R49. Q8 pulls the PTT line low and activates TRANSMIT LED D7. U12F, being
high, also allows R57 to begin charging C34. When this exceeds 5 volts, U6A forces
its output to ground and shuts off Q8. This provides a time-out timer to prevent the
transmitter from getting stuck in transmit.
The PTT signal also sets the output of U12F high turning on Q13. Q13 sends a
ground to the CTCSS board, telling to encode rather than decode.
Relay Outputs Section
There are four (4) relay outputs that are controlled by the processor. They are driven
through opto-isolator U25 and spike protected to prevent voltage spikes from
affecting the unit. The outputs appear on a connector at the bottom of the board and