Barnett Engineering B1474 Manual de usuario

RADIO MODEM

B1474

OPERATION MANUAL

April, 1998

Manual Revision 4.1

(for Version 2.1 Hardware)

TABLE OF CONTENTS

FCC REQUIREMENTS ...................2

1INTRODUCTION......................3

2SPECIFICATIONS .....................5

3NOTATION ........................7

4OPERATION........................8

5SETUP PROCEDURES ...................12

6INDICATORS .......................16

7CONFIGURATION ....................17

8CONNECTIONS......................19

9BOARD LAYOUT .....................20

10 WARRANTY STATEMENT .................21

11 APPENDIX A

OPERATION WITH A GE TRUNKING SYSTEM .......22

B1474

BARNETT ENGINEERING LTD.

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FCC REQUIREMENTS

This equipment generates, uses, and can radiate radio frequency energy and, if not used in

accordance with the instruction manual, may cause interference to radio communications.

It has been tested and found to comply with the limits for a Class A computer device pursuant

to sub-part J of Part 15 of FCC Rules, which are designed to provide reasonable protection

against interference when operated in a commercial environment. Operation of this equipment

in a residential area is likely to cause interference, in which case the user, at his own expense,

will be required to take whatever measures may be required to correct the interference.

This device was tested using a shielded cable connected to the data port. The unit must be

operated using a shielded cable on the data line, with the shield grounded.

B1474

BARNETT ENGINEERING LTD.

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1 INTRODUCTION

The B1474 Radio Modem provides a reliable method of sending 1200 baud data signals over

radio or cable facilities. Digital information is converted to tone format for transmission over

the communication channel. Received tones are converted back to RS232 digital format.

Hardware handshaking using RTS and CTS signals allows the radio channel to be transparent

to the connected equipment. Warm-up and power down delay times required by the radio do

not affect the data signals. Alternatively, XON/XOFF or data buffering may be used where

hardware handshaking is not available.

The communication channel can be a single pair cable, two pair cable or two-way radio.

Conventional land mobile radio equipment can be used or, if specified at time of order, GE

trunking radios may be used.

Power and audio signals are connected to compression terminals inside the modem; the digital

interface is through a DB-25S connector. The B1474 requires 12 to 30 VDC, negative ground,

and approximately 60 mA to operate.

Features available in the B1474 are:

Hardware or Software Handshaking

Either RTS and CTS hardware signals or software XON and XOFF characters can be used to

pause the transmitting station while waiting for the radio system to respond. If short data

streams are to be sent, full data buffering may be used to render the radio system transparent.

RTS/CTS Delay

This adjustable delay allows the radio to warm up before data is sent. The delay is active

regardless of whether hardware handshaking or data buffering is used.

PTT

A low going push-to-talk signal is active whenever the modem is transmitting data.

Squelch Detect

If the radio channel is currently being used, the modem will not return the CTS signal until the

channel is available. If XON/XOFF signaling is selected, XOFF will be returned to indicate a

busy channel; then XON will be issued when the channel is free. (This feature may be disabled

using a DIP switch.)

PTT Drop Delay

This adjustable delay prevents squelch tails from disturbing the received data at the other end

of the system.

Level Selection

Transmit levels are adjustable from 0 to -20 dBm. The receive section has selectable gain of

10 dB, if required, to boost low incoming signal levels.

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Indicators

Modem status condition is indicated by front panel LEDs. The LEDs may be disabled to

conserve power.

B1474

BARNETT ENGINEERING LTD.

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2 SPECIFICATIONS

GENERAL

Transmission Rate: 1200 Baud (Bell 202 signaling) or V.23 if factory installed

Transmission Format: 8 data bits, no parity, 1 stop bit if modem buffering is selected;

otherwise transparent

Power Requirement: Operates from 12 to 30 VDC. Current drain is 110 mA with the LEDs on,

65 mA with the LEDs off.

Temperature Range: -30oC to +50oC

Physical Size: Approx. 10" x 5" x 1", inside a metal enclosure

Radio and Wireline:Compression screw terminals located inside the enclosure

Data Connector: RS232-C compatible, connections available through a DB-25S socket

located on the side of the enclosure

Indicators: Power ON, TXD, RXD, CD, RTS, CTS, BUSY

Receive Audio: Single ended, AC coupled, -30 dBm to 0 dBm

Transmit Audio: Single ended, AC coupled, -20 dBm to 0 dBm

Squelch Detection: Single ended, 1M ohm impedance, DC coupled, detection range

0 to 10 volts, polarity is DIP switch selectable

PTT:Closure to ground for active, 0.5 A @ 50 volts max.

4 Wire Receive: Balanced, 600 ohm impedance, -30 dBm to 0 dBm (operates full or

half duplex)

4 Wire Transmit: Balanced, 600 ohm impedance, -20 dBm to 0 dBm (operates full or

half duplex)

DIP SWITCH SETTINGS

Transmitter Level: -20 dBm to 0 dBm

Receiver Level: -30 dBm to 0 dBm

Squelch Polarity: Selects high or low going squelch monitor signals.

Carrier Detect Delay: 5 or 50 msec

Assert RTS: Forces RTS on.

RTS/CTS Delay: 20, 35, 50, 100, 200, 300, 500, or 1000 msec. Allows the transmitter

time to reach full power before data is transmitted.

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BARNETT ENGINEERING LTD.

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PTT Drop Delay: 0, 10, 20, 50, 75, 100 or 250 msec. Keeps transmitter keyed long

enough to ensure squelch tail noise doesn’t interfere with data at the

receiving terminal.

Handshaking: Specifies the type of flow control used.

2/4 Wire: Selects 2 or 4 wire cable operation.

LED Control: Disables LEDs to save power.

Equalization:

Full/Half Duplex:

B1474

BARNETT ENGINEERING LTD.

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3 NOTATION

Throughout this manual, abbreviations will be used. These abbreviations are explained here for

clarity.

CD Carrier Detect

COS Carrier Operated Switch

CTS Clear To Send

DCE Data Communications Equipment

DTE Data Terminal Equipment

FCC Federal Communication Commission

GND Ground

PTT Push To Talk

RFCTS RF Clear To Send (used in trunking systems only)

RTS Request To Send

SQL Squelch

SW1.8 This signifies a DIP switch. The given example describes the eighth

position on DIP switch number one.

XOFF ASCII 13h sent by a receiving station instructing the sending station

to halt any transmission

XON ASCII 11h sent by a receiving station instructing the sending station

to resume transmission

B1474

BARNETT ENGINEERING LTD.

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4 OPERATION

The B1474 operates from 12 to 30 VDC to provide an interface between two computer devices

and the communications channel. In the descriptions below it is assumed that the sending

device uses a DCE configuration.

Four different types of operation may be configured:

Hardware Handshaking

Basic operation and timing using hardware handshaking are as follows:

•The sending device asserts its RTS and waits.

•Upon receiving the RTS, the associated modem performs the following functions:

–checks the channel, if this feature is enabled. If the channel is free, it asserts its PTT to

power up the radio’s transmitter.

–presents a mark tone (carrier) onto the channel.

–sets its RTS/CTS delay timer (from 20 to 1000 msec). RTS/CTS delay must be greater

than the carrier detect delay at the receiving end.

•The modem at the receiving end senses carrier on the line and asserts its CD. The received

data is muted for 5 or 50 msec to ensure that transients do not affect the received data.

•At the sending end, once the RTS/CTS delay timer has expired, the modem returns a CTS to

the sending computer.

•Data is transmitted; any format may be used.

•Once all the data has been transmitted, the sending computer drops its RTS.

•Upon sensing the dropped RTS, the sending modem does the following:

–drops CTS immediately.

–sets its PTT drop delay timer (0 to 250 msec).

•Once the PTT drop delay timer has expired, the sending modem releases the radio transmit-

ter, turns off the modem carrier and returns to receive mode.

For hardware handshaking, set XON/XOFF signaling off, assert RTS (continuous) off and

buffering off. Set RTS/CTS delay and PTT drop delay as required.

Refer to Figure 1 for a summary of the modem timings.

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BARNETT ENGINEERING LTD.

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B1474

BARNETT ENGINEERING LTD.

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FIGURE 1MODEM TIMING USING RTS AND CTS

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