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
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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.
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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.
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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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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:
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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
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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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FIGURE 1MODEM TIMING USING RTS AND CTS
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