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Documentation Number 485T1995 Manual 

B&B Electronics  --  PO Box 1040  --  Ottawa, IL  61350 

PH (815) 433-5100  --  FAX (815) 433-5105

 

RS-422/RS-485 Switch 

 

This switch controls the positive threshold voltage for the 

Positive Threshold LED only.  When the switch is in the RS-422 
position, the positive threshold is +7 volts.  When it is in the RS-485 
position, the positive threshold is +12 volts.  This switch has no 
effect on the Data LED, the Threshold Pot, or on the Negative 
Threshold  LED. 
 

Termination Switch 

 

In some cases RS-422 and RS-485 pairs are terminated.  This 

termination is usually needed only for long or high speed networks.  
Terminations usually consist of a 100 ohm one-half watt resistor 
located at each end of the network.  In some complicated multi-drop 
networks it may be hard to determine exactly were the “ends” are.  A 
100 ohm one-half watt termination resistor is included in the 485T.  It 
may be helpful if you are testing an existing network with no 
terminations and want to see what happens if you terminate it at one 
point.  Normally, you should leave this switch OFF (down). 

 

Power

 

 

The 485T will work from a nine-volt battery or from 12 VDC 

through the 2.5 mm jack marked DC Power 12VDC IN.  When you 
plug in the external power supply (available from B&B Electronics as 
the Model 485PS) the battery is automatically disconnected. 

 

RS-232 Data Out Connector 

 

This is a standard DB-25S RS-232 connector.  When the DATA 

LED is blinking, any RS-422/485 data on the A & B inputs is 
converted to the RS-232 format and output on pin 2.  You can 
connect this to a device that has an RS-232 port and can read your 
data.  Be sure you turn the threshold pot all the way down 
(clockwise) to zero.  For use on standard ASCII data B&B 
Electronics sells a program called BreakOut II that will turn a PC into 
a serial data monitor.  With it and a PC you can monitor data on your 
RS-422/485 network. 

NOTE: 

 If your data is garbled you may have the A and B lines 

connected backwards or the threshold pot may not be all the 
way down to zero. It is also possible that the RS-422/485 signal 
may be too weak.

Содержание 485T

Страница 1: ... product Designed and Manufactured In Ottawa Illinois USA of domestic and imported parts by B B Electronics Mfg Co Inc 707 Dayton Road P O Box 1040 Ottawa IL 61350 PH 815 433 5100 FAX 815 433 5105 Internet http www bb elec com orders bb elec com support bb elec com 1992 B B Electronics Revised August 1992 Not Recommended for New Installations Please contact Technical Support for more information ...

Страница 2: ...3 5105 TABLE OF CONTENTS INTRODUCTION 1 RS 422 AND RS 485 STANDARDS 1 CONTROLS AND CONNECTIONS ON THE 485T 6 A AND B TERMINALS 6 GROUND TERMINAL 6 THRESHOLD POT 7 POSITIVE THRESHOLD EXCEEDED LED 7 NEGATIVE THRESHOLD EXCEEDED LED 7 RS 422 RS 485 SWITCH 8 TERMINATION SWITCH 8 POWER 8 RS 232 DATA OUT CONNECTOR 8 RS 422 485 TEST 9 ...

Страница 3: ...re usually located close to each other wire length was not much of a factor in the standard RS 232 maximum wire lengths were set at 50 feet RS 232 lines can run farther with special low capacitance wire but there is a practical limit of 300 or so feet The EIA then developed two newer standards to allow serial data communications to go past the 50 foot barrier The first is RS 422 Electrical Charact...

Страница 4: ...a Typically the RS 422 or RS 485 driver will switch between 5 volts and ground Since devices can be long distances apart they can be powered on different power lines or transformers that can force their reference grounds to be at different voltages The RS 422 Standard allows the signal lines to go as high as 7 volts and as low as 7 volts The RS 485 Standard allows 12 volts and 7 volts Another requ...

Страница 5: ...Documentation Number 485T1995 Manual 3 B B Electronics PO Box 1040 Ottawa IL 61350 PH 815 433 5100 FAX 815 433 5105 ...

Страница 6: ...4 Documentation Number 485T1995 Manual B B Electronics PO Box 1040 Ottawa IL 61350 PH 815 433 5100 FAX 815 433 5105 ...

Страница 7: ...Documentation Number 485T1995 Manual 5 B B Electronics PO Box 1040 Ottawa IL 61350 PH 815 433 5100 FAX 815 433 5105 ...

Страница 8: ...m as two separate networks You would first run your tests on one pair of signal lines and then run them again on the other pair NOTE Make sure that you are connected to the correct two lines when using the tester It is possible for the tester to look like it is working properly when one line is connected and the other is floating or connected to a wire in another pair Ground Terminal The GND termi...

Страница 9: ...d make the lower voltage readings difficult to read Readings above 1 volt indicate that differential voltage is more than adequate Positive Threshold Exceeded LED The RS 422 Standard states that a network should not have a positive common mode voltage of more than 7 volts This means that the difference between Signal Ground and either the A or B input should not be more than positive 7 volts For R...

Страница 10: ...network with no terminations and want to see what happens if you terminate it at one point Normally you should leave this switch OFF down Power The 485T will work from a nine volt battery or from 12 VDC through the 2 5 mm jack marked DC Power 12VDC IN When you plug in the external power supply available from B B Electronics as the Model 485PS the battery is automatically disconnected RS 232 Data O...

Страница 11: ...g sent and when the network is idle If either of these LEDs is on or blinking you may have a grounding problem You should first check that the RS 422 RS 485 switch is in the proper position and that the Signal Ground is connected correctly to the 485T Refer to B B Electronics free RS 422 485 Application Note for information on proper grounding of your network NOTE If the positive or negative thres...

Страница 12: ...olt Most RS 422 485 drivers start out with a differential of about 2 volts but the resistance inductance and capacitance in the line can reduce it considerably NOTE The differential voltage you are trying to measure is the RECEIVED voltage You should only make the threshold measurement when the node you are testing is RECEIVING data not when it is transmitting If you have a multiple node system wi...

Страница 13: ... for handshake loopback Pin 7 is ground and there is no connection to pin 3 NOTE If the data you receive is garbled you may have the A B connectors backwards Try reversing them With the 485T connected and the THRESHOLD pot turned to zero full clockwise any RS 422 485 data on the network will appear as RS 232 data at pin 2 of the RS 232 DATA OUT jack If you have a four wire network you can only vie...

Страница 14: ...12 Documentation Number 485T1995 Manual B B Electronics PO Box 1040 Ottawa IL 61350 PH 815 433 5100 FAX 815 433 5105 ...

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