Barnett Engineering B1474 User Manual

RA DIO MO DEM

B1474

OP ER A TION MAN UAL

April, 1998
Manual Revision 4.1
(for Version 2.1 Hardware)

TA BLE OF CON TENTS

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FCC RE QUIRE MENTS ...................2

1 IN TRO DUC TION......................3

2 SPEC I FI CA TIONS .....................5

3 NO TA TION ........................7

4 OP ER A TION........................8

5 SETUP PRO CE DURES ...................12

6 IN DI CA TORS .......................16

7 CON FIG U RA TION ....................17

8 CON NEC TIONS......................19

9 BOARD LAY OUT .....................20

10 WAR RANTY STATE MENT .................21

11 AP PEN DIX A

OP ER A TION WITH A GE TRUNKING SYS TEM .......22

FCC RE QUIRE MENTS

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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.

1 IN TRO DUC TION

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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:

Hard ware or Soft ware Hand shak ing

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 De lay

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 De tect

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 De lay

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

Level Se lec tion

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.

In di ca tors

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

2 SPEC I FI CA TIONS

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GEN ERAL

Trans mis sion Rate: 1200 Baud (Bell 202 sig naling) or V.23 if fac tory in stalled

Trans mis sion For mat: 8 data bits, no par ity, 1 stop bit if mo dem buff er ing is se lected;

oth er wise trans par ent

Power Re quire ment: Op er ates from 12 to 30 VDC. Cur rent drain is 110 mA with the LEDs on,

65 mA with the LEDs off.

Tem per a ture Range: -30oC to +50oC

Phys i cal Size: Approx. 10" x 5" x 1", in side a metal en clo sure

Ra dio and Wire line: Com pres sion screw ter mi nals lo cated in side the en clo sure

Data Con nec tor: RS232-C com pat i ble, con nec tions avail able through a DB-25S socket

lo cated on the side of the en clo sure

In di ca tors: Power ON, TXD, RXD, CD, RTS, CTS, BUSY

Re ceive Au dio: Sin gle ended, AC cou pled, -30 dBm to 0 dBm

Trans mit Au dio: Sin gle ended, AC cou pled, -20 dBm to 0 dBm

Squelch De tec tion: Sin gle ended, 1M ohm im ped ance, DC cou pled, de tec tion range

0 to 10 volts, po lar ity is DIP switch selectable

PTT: Clo sure to ground for ac tive, 0.5 A @ 50 volts max.

4 Wire Re ceive: Bal anced, 600 ohm im ped ance, -30 dBm to 0 dBm (op er ates full or

half du plex)

4 Wire Trans mit: Bal anced, 600 ohm im ped ance, -20 dBm to 0 dBm (op er ates full or

half du plex)

DIP SWITCH SET TINGS

Trans mit ter Level: -20 dBm to 0 dBm

Re ceiver Level: -30 dBm to 0 dBm

Squelch Po lar ity: Se lects high or low go ing squelch mon i tor sig nals.

Car rier De tect De lay: 5 or 50 msec

As sert RTS: Forces RTS on.

RTS/CTS De lay: 20, 35, 50, 100, 200, 300, 500, or 1000 msec. Al lows the trans mit ter

time to reach full power be fore data is trans mit ted.

PTT Drop De lay: 0, 10, 20, 50, 75, 100 or 250 msec. Keeps trans mit ter keyed long

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enough to en sure squelch tail noise does n’t in ter fere with data at the
re ceiv ing ter mi nal.

Hand shak ing: Spec ifies the type of flow con trol used.

2/4 Wire: Se lects 2 or 4 wire ca ble op er a tion.

LED Con trol: Dis ables LEDs to save power.

Equal iza tion:

Full/Half Du plex:

3 NO TA TION

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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

4 OP ER A TION

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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:

Hard ware Hand shak ing

Basic operation and timing using hardware handshaking are as follows:

• The send ing de vice as serts its RTS and waits.

• Upon re ceiv ing the RTS, the as so ci ated mo dem per forms the fol low ing func tions:

– checks the chan nel, if this fea ture is en abled. If the chan nel is free, it as serts its PTT to

power up the ra dio’s trans mit ter.

– pres ents a mark tone (car rier) onto the chan nel.

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

than the car rier de tect de lay at the re ceiv ing end.

• The mo dem at the re ceiv ing end senses car rier on the line and as serts its CD. The re ceived

data is muted for 5 or 50 msec to en sure that tran sients do not af fect the re ceived data.

• At the send ing end, once the RTS/CTS de lay timer has ex pired, the mo dem re turns a CTS to

the send ing com puter.

• Data is trans mit ted; any for mat may be used.

• Once all the data has been trans mit ted, the send ing com puter drops its RTS.

• Upon sens ing the dropped RTS, the send ing mo dem does the fol low ing:

– drops CTS im me di ately.

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

• Once the PTT drop de lay timer has ex pired, the send ing mo dem re leases the ra dio trans mit -

ter, turns off the mo dem car rier and re turns to re ceive 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 1 MODEM TIMING USING RTS AND CTS

Soft ware Hand shak ing Using XON/XOFF

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Basic operation and timing using XON/XOFF software handshaking are as follows:

• The send ing com puter be gins send ing data. The for mat must be 8 data bits, no par ity,

1 stop bit.

• Upon de tect ing data, the send ing mo dem per forms the fol low ing:

– checks the chan nel, if this fea ture is en abled. If the chan nel is free, it as serts its PTT to

power up the ra dio’s trans mit ter. If the chan nel is busy, it re turns XOFF to the send ing
com puter, which halts the trans mis sion of data. When the chan nel be comes free, the
mo dem sends XON to the com puter, which re sumes send ing data. The mo dem buff ers
the data while it con tin ues with the fol low ing:

– pres ents a mark tone (car rier) onto the chan nel.

– sets its RTS/CTS de lay timer (from 20 to 300 msec). RTS/CTS de lay is a mis no mer in

this case as soft ware hand shak ing does not use RTS and CTS sig nals. It is ac tu ally a
buff er ing time be tween when the mo dem starts re ceiv ing data from the com puter and
when it starts trans mit ting. This de lay must be greater than the car rier de tect de lay at
the re ceiv ing end.

• The mo dem at the re ceiv ing end senses car rier on the line and as serts its CD. The re ceived

data is muted for 5 or 50 msec to en sure that tran sients do not af fect the re ceived data.

• At the send ing end, once the RTS/CTS de lay timer has ex pired, data is trans mit ted.

– The send ing mo dem’s PTT drop de lay timer is started at the end of the last trans mit ted

char ac ter.

• Once the PTT drop de lay timer has ex pired, the mo dem re leases the ra dio trans mit ter, turns

off the mo dem car rier and re turns to re ceive mode.

This configuration can buffer 500 msec worth of characters. To prevent internal data buffer
overflow, it is important that the RTS/CTS delay timer be set to a value less than 500 msec and
that the computer respond to XOFF immediately.

For software handshaking, set XON/XOFF signaling on, assert RTS (continuous) off, and
buffering on. Do not connect hardware RTS and CTS. Set RTS/CTS delay and PTT drop delay
as required.

No Hand shak ing, Buffered

Basic operation and timing with data buffering (no handshaking) are as follows:

• The send ing com puter be gins send ing data. Data for mat must be 8 bits, no par ity, 1 stop bit.

• Upon de tect ing data, the send ing mo dem per forms the fol low ing:

– checks the chan nel, if this fea ture is en abled. If the chan nel is free, it as serts its PTT to

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power up the ra dio’s trans mit ter. If the chan nel is busy, it buff ers the re ceived data un til
the chan nel be comes free.

– pres ents MARK tone (car rier) onto the chan nel.

– sets its RTS/CTS de lay timer (from 20 to 300 msec). RTS/CTS de lay is a mis no mer in

this case as hand shak ing is not used. It is ac tu ally a buff er ing time be tween when the
mo dem starts re ceiv ing data from the com puter and when it starts trans mit ting.
RTS/CTS de lay must be greater than the car rier de tect de lay at the re ceiv ing end.

• The mo dem at the re ceiv ing end senses car rier on the line and as serts its CD. The re ceived

data is muted for 5 or 50 msec to en sure that tran sients do not af fect the re ceived data.

• At the send ing end, once the RTS/CTS de lay timer has ex pired, data is trans mit ted.

• Re ceipt of any char ac ter from the send ing de vice, or trans mis sion of any buf fered char ac t er,

re starts the send ing mo dem’s PTT drop de lay timer (from 0 to 250 msec).

• Once the PTT drop de lay timer has ex pired, the mo dem re leases the ra dio trans mit ter and

re turns to re ceive mode.

This configuration can buffer 500 msec worth of characters. To prevent internal data buffer
overflow, it is important that the radio system will be available within 500 msec (RTS/CTS
delay plus maximum channel busy time) or the maximum amount of buffered data be less than
50 characters.

For no handshaking, buffered operation, set XON/XOFF signaling off, assert RTS
(continuous) off, and buffering on. Do not connect the hardware RTS and CTS. Set
RTS/CTS delay and PTT drop delay as required.

No Hand shak ing, Non-Buffered

Basic operation and timing with no handshaking, no data buffering are as follows:

• RTS is al ways as serted. Do not con nect the hard ware RTS.

• PTT is al ways as serted, i.e. the ra dio’s car rier is al ways on. There is no RTS/CTS de lay or

PTT drop de lay.

• Data gen er ated by the send ing de vice is passed di rectly through the mo dem with out hand -

shak ing, buff er ing or time de lays. Data may be any for mat.

For no handshaking, non-buffered operation, set XON/XOFF signaling off, assert RTS
(continuous) on, and buffering off. Do not connect the hardware RTS and CTS, and do not set
RTS/CTS or PTT drop delays.

5 SETUP PRO CE DURES

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The semiconductors can be damaged by static discharge. Proper handling procedures must be followed.
Do not touch any of the devices inside the modem, and work only in a static free, clean, and dry
environment.

(1) Re move The Top Cover

Remove the two center screws on the top cover of the modem to release it from the base. Pull
straight up on the cover to expose connections and internal DIP switches.

(2) CON NEC TIONS

Line In ter face

Connect the audio wires in accordance with the configurations below:

4 wire ca ble: Con nect the two trans mit wires into pins 4 and 5 of P1. The re ceive wires con -

nect into pins 7 and 8 of P1. SW3.3 and SW3.4 must be OFF.

2 wire ca ble: Con nect the two wires into pins 4 and 5 of P1. En sure that SW3.3 and SW3.4 are

ON.

Ra dio

If the radio is equipped with a 600 ohm balanced interface, connect the transmit and receive
audio lines as shown above for a cable interface.

For a single-ended radio interface, connect P1-3 to the transmit line of the radio and P1-6 to the
receive line. Connecting the receive input to an unsquelched source in the radio will result in
extraneous characters at the data port, unless the squelch detector is set properly.

PTT: The B1474 pro vides an open col lec tor driver for the ra dio PTT at P1-10.

Squelch: The ra dio’s re ceive car rier op er ated switch (COS) can be mon i tored to de tect

when the ra dio is re ceiv ing a sig nal by con nect ing the COS line to P1-7. This sig ­nal is used to in hibit trans mis sions over a busy chan nel.
The in put is ca pa ble of mon i tor ing a level from 0 to 10 VDC.
See “Squelch Po lar ity” un der DIP switch set tings to set the COS po lar ity and
level, or to dis able this fea ture.

If us ing the Op tion 002 trunking soft ware, con nect the ra dio’s RFCTS into the
squelch in put.

Dig i tal Con nec tion

A DB-25S socket on the side of the unit is used to interface with the computer equipment.

The RS232 interface provides a DCE interface using the following signals:

GND (Chassis) Pin 1

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TXD Pin 2
RXD Pin 3
RTS Pin 4 (connect only if using hardware handshaking)
CTS Pin 5 (connect only if using hardware handshaking)
GND (Signal) Pin 7
CD Pin 8

The cable and housing on the DB-25S socket must be shielded to conform to FCC, part 15-J/A standards
for emissions. Using a non-shielded cable may result in undesirable radio interference.

(3) DIP SWITCH SET TINGS

The following section describes the function of each DIP switch on the modem. The settings are
summarized in a table in Section 7 - Configuration.

LED Con trol

The LED indicators may be disabled to conserve power. Set SW1.1 OFF to disable the LEDs.

RTS/CTS De lay

When the sending device requests transmission, a preset delay is inserted before the modem
transmits. This delay allows the two-way radio to warm up, or, on a cable system, allows the
tones to settle on the channel before data transmission begins. When hardware handshaking is
used, the modem pauses the sending device by delaying return of the CTS signal. When
XON/OFF or no handshaking are used, the RTS/CTS delay specifies the amount of data which
will be buffered. Set DIP switches SW1.2, SW1.3, and SW1.4 for the desired delay, from 20 to
1000 msec, as specified in Section 7 - Configuration.

PTT Drop De lay

When transmission is no longer requested by the sending computer, a selectable delay is
inserted before the PTT on the radio is released. This delay prevents the squelch tail at the
receiving radio from interfering with the data. Set DIP switches SW1.5, SW1.6, and SW1.7 for
the desired drop delay, between 0 and 500 msec, as specified in Section 7 - Configuration.

XON / XOFF Con trols

SW1.8 controls whether the modem will send XON and XOFF to the sending computer when
the radio channel is busy (software handshaking). If the sending computer does not
understand these characters, the switch must be OFF.

Squelch Po lar ity

If the modem is connected to a radio, the channel can be monitored for RF carrier. SW1.9 is
changed depending upon the polarity of the carrier operated switch (COS) signal. If the COS
goes high when carrier is present, and low when the channel is free, set SW1.9 OFF. If the
opposite occurs, set SW1.9 ON. While the channel is busy, the modem will wait, sending only
when the channel is clear.

Adjust R12 so that the modem waits when the channel is busy, and transmits when the channel

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is free. When the BUSY light is ON, the modem is waiting for the channel.

To disable this feature, leave P1-9 unconnected and SW1.9 OFF.

If Option 002 trunking software is used, set SW1.9 according to the polarity of the RFCTS signal
connected into P1-9. Set SW1.9 and R12 so that the data is only transmitted once the channel
has been assigned.

Send/Re ceive

When using the Option 002 trunking software and data buffering, use SW1.10 to configure the
modem for send or receive mode. Normally, all modems within a system should be set for send
mode, meaning that they will key the trunking radio and wait for RFCTS when data
transmission is required. Setting SW1.10 ON sets the modem into receive mode. In this mode,
the modem will only hit PTT if the channel has already been established by another modem.
Having one modem in send mode and another in receive mode will ensure that both modems
do not try to acquire the channel at the same time.

Set SW1.10 to OFF if using hardware handshaking or conventional radios.

Trans mit Level

To adjust the output level of the modem to between 0 and -20 dBm, set DIP switches SW2.1,
SW2.2 and SW2.3 as specified Section 7 - Configuration.

The output levels given are referenced into 600 ohms. Should the output be driven into a high
impedance, the level will be approximately 6 dB higher than expected.

If a high level is required into a single-ended input, connect P1-5 to ground, and use P1-4 to
drive the input.

Re ceive Gain

DIP switch SW2.5 is used to boost the received signal. To insert 10 dB of gain, turn SW2.5 ON.
This will amplify all incoming signals (including all valid data tones).

Equal iza tion (Mode 1)

To enable the internal amplitude equalizer, set SW2.6 ON. To leave this feature disabled, set
SW2.6 OFF. Use equalization when there is a large discrepancy in levels between the two tones.

Car rier De tect De lay (Mode 2)

When the receiving modem detects carrier, it mutes the RXD line for a short time to ensure that
transients do not affect the received data. Set SW2.7 OFF to delay the received data for 5 msec,
ON to delay the received data for 50 msec. Carrier detect delay should be less than the
RTS/CTS delay at the sending end.

Full/Half Du plex (Mode 3)

To operate full duplex, leave SW2.8 OFF. To operate half duplex, set switch SW2.8 ON. If a

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2 wire circuit is used, the modem will only operate half duplex.

RTS Con trol

SW3.1, when ON, will assert RTS continuously. This causes PTT to be asserted and transmit
tones to be generated. The switch should be on for a no handshaking, non-buffered
configuration. The continuous RTS is also useful for testing purposes.

Buffer

If hardware handshaking is desired, connect the RTS and CTS signals to the DB-25S connector
and set SW3.2 OFF. If XON/XOFF control (software handshaking) or no handshaking with full
buffering is desired, set SW3.2 ON and remove connections from the RTS and CTS positions in
the DB-25S connector.

2/4 Wire

Set SW3.3 and SW3.4 OFF if 4 wire communications are used. Set SW3.3 and SW3.4 ON if
2 wire communications are used. If a 2 wire circuit is used, the modem will only operate half
duplex.

(4) Re place The Top Cover

Place the top cover back onto the base and tighten the two center screws.

(5) Ap ply Volt age

Apply the proper voltage to the unit. If the LEDs are enabled, the run lamp will illuminate,
indicating that power is available at the modem.

6 IN DI CA TORS

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The LED indicators on the front case are used to indicate modem status:

RUN Power is on the modem.

BUSY The modem is waiting to transmit but is held back by the squelch detector.

RTS Request To Send from the computer is present or characters have been

re ceived into the buffer.

CTS Clear To Send is active.

CD Carrier is detected on the incoming channel.

RXD On when a mark is received and off when a space is received.

TXD On when a mark is transmitted.

The LEDs may be dis abled to con serve power by set ting SW1.1 off.

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7 CONFIGURATION

SWITCH 1

SWITCH 2

LED CON TROL

1

LEDS OFF

X LEDS ON

RTS/CTS DE LAY

3 msec2

4

20

35X
X 50
X 100X

200X

300XX
X 500X
X 1000XX

PTT DROP DE LAY

6 msec5

7

0

10X
X 20

X

X 50

X

75

X

100X
X 250X

XON/XOFF

TX LEVEL (INTO 600 OHMS)

2 3

1

Ra dio (dBm)

-4

X -6

X
X

X -10

-8

X -12
XX -14

X

X -20

X

XX -24

Bal ­0

-2

-4

-6

-8

-10

-16

-20

RX GAIN

5 dB

0

X 10

MODE 1 (EQUAL IZA TION)

6

NO RE CEIVE EQUAL IZA TION

X RE CEIVE EQUAL IZA TION

MODE 2 (CD DE LAY)

7

CD DE LAY = 5 msec

X CD DE LAY = 50 msec

8

NO XON/XOFF

X USE XON/XOFF

SQUELCH PO LAR ITY

9

LOW = CHAN NEL FREE

X HIGH = CHAN NEL FREE

SEND/RE CEIVE (TRUNKING ONLY)

10

PTT RE GARD LESS OF RFCTS

X PTT ONLY WITH RFCTS

MODE 3 (DUPLEX)

8

FULL DUPLEX

X HALF DUPLEX

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SWITCH 3

RTS ON

1

EX TER NAL RTS

X RTS ON AT ALL TIMES

BUFFER

2

USE RTS/CTS CON TROL

X USE BUFFER

RTS ON

4

3

4 WIRE CA BLE

X 2 WIRE CA BLE

X

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8 CON NEC TIONS

P1 AU DIO

PIN SYMBOL DESCRIPTION

1

3

4

5

6

7

8

9

10

TX RADIO

RX RADIO

P3 DIG I TAL

PIN

1

SYMBOL

+V

GND2

TX WL

TX WL

RX WL

RX WL

SQL

PTT

GND

INPUT VOLTAGE

GROUND

TRANSMIT OUTPUT, SINGLE ENDED

TRANSMIT OUTPUT, BALANCED

}

RECEIVE INPUT, SINGLE ENDED

}

RECEIVE INPUT, BALANCED

SQUELCH DETECTOR INPUT

PUSH TO TALK OUTPUT

CHASSIS GROUND

2

3

4

5

7

8

SIG GND

TXD

RXD

RTS

CTS

CD CARRIER DETECT

TRANSMIT DATA

RECEIVE DATA

REQUEST TO SEND

CLEAR TO SEND

SIGNAL GROUND

9 BOARD LAY OUT

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Barnett En gi neering Prod uct War ranty

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WARRANTY STATEMENT

Barnett Engineering Ltd. warrants that all equipment supplied shall be free from defects in
material or workmanship at the time of delivery. Such warranty shall extend from the time of
delivery for a period of one year. Buyer must provide written notice to Barnett Engineering
Ltd. within this prescribed warranty period for any defect. If the defect is not the result of
improper usage, service, maintenance, or installation and the equipment has not been otherwise
damaged or modified after delivery, Barnett Engineering Ltd. shall either replace or repair the
defective part or parts of equipment or replace the equipment or refund the purchase price at
Barnett Engineering Ltd.’s option after return of such equipment by the buyer to Barnett
Engineering Ltd. Shipment to Barnett Engineering Ltd.’s facility shall be borne on account of
the buyer.

(1) Consequential Damages

Barnett Engineering Ltd. shall not be liable for any incidental or consequential damages
incurred as a result of any defect in any equipment sold hereunder and Barnett Engineering
Ltd.’s liability is specifically limited to its obligation described herein to repair or replace a
defective part or parts covered by this warranty.

(2) Exclusive Warranty

The warranty set forth herein is the only warranty, oral or written, made by Barnett Engineering
Ltd. and is in lieu of and replaces all other warranties, expressed or implied, including the
warranty of merchantability and the warranty of fitness for particular purpose.

11 AP PEN DIX A - OP ER A TION WITH A GE MARC V TRUNKING SYS TEM

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When ordered with the Option 002 trunking software, the B1474 will handle the special timing
and control requirements of a GE Marc V trunking system. The following sections describe the
connections to a trunking radio, and the slightly different operation of the radio modem.

CON NEC TIONS

The modem connects into the Marc V system the same as it would to a conventional radio, as
described in section 5, using the PTT, RX data and TX data pins. The only difference is that the
squelch monitor is tied to RFCTS within the radio to determine when the channel is assigned.

To ensure that these connections are made correctly and the proper signals are supplied to each
unit, order the Option 001 interface cable from Barnett Engineering Ltd. Otherwise, refer to the
manual for the Ericsson GE radio to determine the correct interface points.

OP ER A TION

The trunking modem operates with some slight timing differences from the conventional
modem.

Hard ware Hand shak ing

The sequence followed in obtaining a trunked channel and transmitting the sending computer’s
data using hardware (RTS/CTS) handshaking is described below. Refer to Figure 2 for an
illustration of the timing relationships.

1. The sending computer asserts its Request to Send (RTS), causing the modem to
key the radio and wait for a channel to be assigned.

2. When the channel is assigned, the trunking radio returns RF Clear to Send
(RFCTS) to the modem. The signal is connected to the SQUELCH input on the
modem.

3. The modem produces tones onto the channel and waits the specified RTS/CTS
delay. This delay is a minimum of 5 msec to ensure that all tones have stabilized
before any data is sent.

4. When the RTS/CTS delay has expired, CTS is returned to the sending computer
and the transmission of data begins. When all of the data has been transferred,
the sending computer drops its RTS signal, causing the modem to drop its CTS
signal.

5. The modem holds the radio’s Push To Talk (PTT) on the radio until the PTT
Drop Delay has expired. The channel assignment will not be lost immediately on
releasing PTT, so the receiving computer can use the same channel to respond.

The receiving computer receives a Carrier Detect (CD) followed by the data some interval later
.

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FIGURE 2 MODEM TIMING FOR A GE MARC V

TRUNKING SYSTEM USING RTS/CTS HANDSHAKING

Soft ware Hand shak ing Using XON/XOFF

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Basic operation and timing using XON/XOFF software handshaking in a trunking system is as
follows:

• The send ing com puter be gins send ing data. The for mat must be 8 data bits, no par ity,

1 stop bit.

• Upon de tect ing data, the send ing mo dem per forms the fol low ing:

– checks the chan nel. If RFCTS has al ready been as signed (the chan nel is avail able), it as -

serts its PTT to power up the ra dio’s trans mit ter. If the chan nel has not been as signed
yet, it sends XOFF to the send ing com puter and as serts PTT to ob tain the chan nel.
When the chan nel gets as signed, it sends XON to the com puter and con tin ues.

– pres ents tones onto the chan nel.

– sets its RTS/CTS de lay timer.

• The re ceiv ing mo dem senses car rier on the line and as serts its CD. The re ceived data is

muted for 5 or 50 msec to en sure that tran sients do not af fect the re ceived data.

• Af ter the RTS/CTS de lay timer has ex pired, data is trans mit ted.

• When ever the mo dem re ceives a char ac ter from the send ing de vice or trans mits any buf -

fered char ac ter, it re starts the PTT drop de lay timer.

• Once the mo dem stops re ceiv ing char ac ters and the PTT drop de lay timer has ex pired, the

mo dem re leases the ra dio trans mit ter and re turns to re ceive mode.

This configuration can buffer 500 msec worth of characters. To prevent internal data buffer
overflow, it is important that the RTS/CTS delay timer be set to a value less than 500 msec and
that the computer respond immediately to an XOFF command.

No Hand shak ing

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Basic operation and timing with data buffering (no handshaking) in a trunking system is as
follows:

• The send ing com puter be gins send ing data. Data for mat must be 8 bits, no par ity, 1 stop bit.

• Upon re ceiv ing data, the mo dem per forms the fol low ing:

– checks the chan nel for RFCTS - if the chan nel has been as signed al ready, it as serts

its PTT to power up the ra dio’s trans mit ter. If the chan nel is busy, it as serts PTT to ob ­tain the chan nel and buff ers the re ceived data un til the chan nel be comes as signed.

– pres ents tones onto the chan nel.

– sets its RTS/CTS de lay timer.

• The re ceiv ing mo dem senses car rier on the line and as serts its CD. The re ceived data is

muted for 5 or 50 msec to en sure that tran sients do not af fect the re ceived data.

• Af ter the RTS/CTS de lay timer has ex pired, data is trans mit ted.

• Re ceipt of any char ac ter or trans mis sion of any buf fered char ac ter re starts the mo dem’s PTT

drop de lay timer.

• Once the mo dem stops re ceiv ing char ac ters and the PTT drop de lay timer ex pires, the mo -

dem re leases the ra dio trans mit ter and re turns to re ceive mode.

This configuration can buffer 500 msec worth of characters. To prevent internal data buffer
overflow, it is important that the radio system will be available within 500 msec (RTS/CTS
delay plus maximum channel busy time) or the maximum amount of buffered data is less than
50 characters.