reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

•

Radio systems

mination card, channel 1 remote control unit (RCU) to

trial data signals

'

termination card are

the two fixed station transmitters (ITX1 and ITX2)

i n

which use the same channel I frequency. Following

-

1

...on neet

ed

to the LIAR

selection of channel 1, the CPU will load the data

tor

messages onto the data bus and the operator's serial

flc

interface addresses sequentially onto the address bus;

,',■

r.1 bus

and ViCC versa.

the 'channel I selected' indication will then be displayed

on operator controller 1 and a 'channel 1 busy' indica-

\ll

,2

,-, n trol; ,Ind channel selections from the con-

are naken to the relevant UART as 'serial data

tion on all other controllers. On receipt of the channel

od all indications from the CEQ and fixed radio

selected indication, operator 1 will press the SELCALL

.r

buttons to call the required mobile and press the

▪

L ART as 'serial data out'.

send/key-transmitter button. The SELCALL code for

the required mobile will be sent as an eleven-bit data

I he serial data in and out signals (SDI and SDO)

.111 e

word to the operator's serial interface where it will

be loaded onto the address and data bus, under the

cl o en-bit word corn prises a start zero '0' bit,

data bits, a parity bit which can be either '0' or

control of the CPU, and transferred to the SELCALL

la give odd Ts parity, and a stop T bit. The odd

serial interface. The interface will convert the parallel

parity is achieved by choosing a '0' or '1' for the

data to serial data using a UART associated with chan-

nel 1 and the serial data will pass to the SELCAL L.

r,irity

and eight data bits are added together. This

encoder in the channel 1 termination card. The encoder

will convert the serial data to the necessary SELCALL

VF sequential tones which will be passed, together

with the transmitter keying frequency of 2970 Hz

(originating from the local operator's termination card)

,1

nal5 comprise a 2295 Hz tone for logic zero and a

and the CTCSS tone from the channel I RCU, to the

Hz tone for logic one. The eleven-bit tone-burst

two fixed station transmitters.

A 2970 Hz detector in each transmitter will switch

on the RF transmitters and the SELCALL sequential

tones and the CTCSS sub-audible tone will modulate

the RF carrier signal. When the relevant mobile receiver

detects the correct SELCALL code, the audio output

of that receiver will be unmuted and a call tone heard

by the mobile user. The mobile user will press the

mobile press-to-talk (PIT) switch and reply verbally

11 , cd for error detection.

with his name or callsign: a sub-audible CTCSS tone

In order to understand the general signalling

will also be transmitted.

On receipt of the correct RF carrier and CTCSS

tone, either one or both channel I fixed station receivers

will receive the call. If both receive the call, the CTCSS

,1e , cribed.

detector in each will lift the mute on the receiver audio

f he 'select' button for channel I is first pressed on

stage and pass the audible speech signals to the voting

equipment. The better signal will be selected and the

:rain

the controller to the operator's serial interface

output of the voted receiver will be connected by the

receive audio pair into the channel 1 termination card

detect

the operation of the interface UART and pre-

via the receive amplifier in the RCU. At the same time,

pares

the

an eleven-bit serial data word indicating the voted

receiver will be passed from the UART to the FSK

on

the data and address bus. The CPU will decode the

modem in the RCU. The FSK tone-burst, eleven-bit

,i..ILlress

data, identify the associated operator termina-

word will be decoded by the modem in the channel 1

terminating card and an eleven-bit serial data word

passed to the channel serial interface, where channel 1

UART will convert it to a parallel data word. Under

luple.xer (MUX) on the addressed operator termina-

control of the CPU, the parallel data word will be

!on card to channel 1

passed, via the data bus, to the UART associated with

operator I (OP I) in the operator's serial interface.

The UART will convert the parallel data to serial

"aY is

data and this will pass via the cable highway to the

connected from the local operator's controller

local operator controller (0P1), where the voted re-

ceiver will be indicated on an LCD display. At the

Chapter 8

DTMF signalling to operate the terminal unit equipment

and receive standard telephone ringing when called by

the terminal unit.

The DWTS are located at local control points, e.g.,

CCR, works control office and the power station re-

ception desk.

The star button (*) on the keyboard is depressed to

obtain a dial tone. The handportable automatically

searches for the first free channel and transmits the

relevant DTMF code (see Fig 8.16 and Fig 8.54),

On receipt of the DTMF code, the TE2200 stores

the dial tone request until the CPU is free to deal

with the call and simultaneously transmits a continuous

carrier to busy the channel. When the CPU is free to

deal with the call request, the TE2200 transmits an idle

(R) tone.

On receiving the idle tone the handportable transmits

its 3-digit DTMF ID to the CPU. The CPU then

repeats the ID back to the handportable in the form of

a SSFC comprising a pulse of idle tone followed by

each of three tones representing the handportable ID.

On receiving the correct ID, the handportable trans-

CPANNEL 3

mits a pulse of acknowledgement (R) tone and simul-

IED 5 , A7ICA

taneously unmutes the loudspeaker.

HANNEL 3

Receipt of the acknowledge tone by the TE2200

, xED STATIC-

results in a dial tone being sent to the handportable.

CL

The handportable user then keys, on air, the re-

quired mobile or handportable ID or the access code

\ \

for the PABX followed by the telephone number in

, -zyjE!,,LILT ■ F EOUEtiCY

DTMF signalling.

zxEP

On receiving the first DTMF tones, the CPU removes

11` 47A

the dial tone and stores the received code.

:7 E CALL SIGNALLING SYSTEM

If the DTMF ID code is for a handportable or

AS

, ER - =',ANSP.!IT -TER

mobile, the CPU checks that the handportable is not

Fr( 8.52

already engaged on a call. If it is engaged then a busy

tone is returned to the caller. If not, the CPU sends a

preamble idle (R) tone to alert all quiescent hand-

portables which are continually scanning the channels

PREAMBLE

¶1 BITS

that an ID is about to be transmitted. All handport-

ables lock-on to the channel and receive the subse-

also hears the ID which acts as a confidence tone fol-

quently transmitted ID. The calling handportable user

lowing the keying process.

8 BITS DATA

If the called handportable is switched on and re-

ceives the ID correctly, it acknowledges the call by

automatically returning an acknowledgement (R) tone.

At the same time the AF section of the handportable

is unmuted and the acknowledgement (R) tone is heard

in the handportable at full volume, i.e., it by-passes

. 8.53 N187 control system — serial data format

the volume control. This acts as an alerting signal to

the user of the called handportable indicating that an

incoming call has been received.

On receipt of the acknowledgement (R) tone, the

TE2200 sends a ringing tone which is heard by both

handportables at the volume control setting level which

exists at each handportable.

The called party answers the call by depressing the

PIT switch.

Telecommunications

Chapter 8

CALL

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