Not Recommended for New Installations. Please contact Technical Support for more information. RS-422/RS-485 Line Tester Model 485T Documentation Number 485T1995 This 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.
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 ..........................................................
INTRODUCTION The Model 485T Tester can be used to check the condition of signals at any node on an RS-422 or RS-485 network. The tester determines if the maximum permissible negative or positive voltages are being exceeded. If these voltage levels are exceeded it is possible to damage the RS-422/485 receivers and drivers. The tester also checks to see if minimum required differential voltages exist on the lines.
high and vice-versa. Also, the drivers are capable of driving a wire with as much as 25 times more capacitance before the signal is badly distorted. Most system designers try to minimize the number of signals used because these standards take two wires per signal. This is one reason most RS-422 and RS-485 systems are either two-wire with ground or four-wire with ground. There are two major differences between RS-422 and RS-485. The first is that with RS-422, one driver can only drive 10 receivers.
Documentation Number 485T1995 Manual B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 433-5105 3
4 Documentation Number 485T1995 Manual B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 433-5105
Documentation Number 485T1995 Manual B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 433-5105 5
CONTROLS AND CONNECTIONS ON THE 485T A and B Terminals The two signal lines used in an RS-422 or RS-485 network should be labeled A and B. These lines should be connected to the A and B terminals of the 485T Tester. If your lines are not marked (or are marked using another method) you can use a DC voltmeter to determine which is which. With no data being sent, the A line will be negative with respect to the B line. On an RS-485 network, if all of the drivers are off, the voltmeter will read zero volts.
Threshold Pot The large pot in the center of the 485T is the threshold pot. In order for RS-422/485 to work properly, you should have a differential input voltage of more than .2 volts across the two lines at the RS422/485 receiver. By adjusting this pot you can determine the amplitude of the RS-422/485 signals. Start with the knob turned down to zero volts (full clockwise) with data being sent on your network.
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.
RS-422/485 TEST A typical test on an RS-422/485 network might go like this: First start by locating the two wires used for the RS-422/485 network and try to determine which is A and which is B. Also, locate the Signal Ground. This may be the same as the ground on the DC power supply that powers the RS-422/485 device. Connect the Signal Ground to the GND terminal. Connect the A and B terminals to what you think are the A and B wires.
See B&B Electronics' free RS-422/485 Application Note for information on proper wiring and grounding of an RS-422/485 network. If the POSITIVE and NEGATIVE THRESHOLD LEDs are both off, you can then test for proper signal amplitude. With the THRESHOLD pot turned to zero (full clockwise), the DATA LED should blink or stay on when data is sent. When data is being sent on the network, turn the THRESHOLD pot counterclockwise until the LED goes out. If, for instance, the LED goes out when the pot is pointing at .
You can then use the 485T Tester to monitor the data that is being sent or received on your network. If you have a datascope or a computer running a datascope-like program you can connect it to the RS-232 DATA OUT connector on the 485T. A typical program would be BreakOut II which is available from B & B Electronics. The RS-232 DATA OUT connector is wired as a DTE with data output on pin 2. Pin 4 is connected to pin 5 and pin 6 is connected to pins 8 and 20 for handshake loopback.
12 Documentation Number 485T1995 Manual B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 433-5105
