Documentation Number 485T1995 Manual
7
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 433-5105
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 RS-
422/485 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. The Data LED should be blinking with the data providing
that the network is not running at too high of a baud rate. At higher
baud rates the LED may be on. Turn the threshold pot up
(counterclockwise) until the Data LED stops blinking or turns off.
The reading of the pot at that point is the differential input voltage.
This reading should be .2 volt or more. If the LED does not stop
blinking or turn off with the pot all the way up (full counterclockwise)
the reading is above 1 volt. Readings above 1 volt are not available
since it would 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 RS-485 the
maximum is 12 volts. The Positive Threshold Exceeded LED will
light if these limits are exceeded. This reading is taken with respect
to Signal Ground so the GND terminal must be connected properly
for the reading to be correct.
NOTE:
The RS-422/RS-485 switch is used to control this
threshold. It must be in the proper position for these readings to
be correct.
Negative Threshold Exceeded LED
Both the RS-422 and the RS-485 Standards state that the
network should not have a negative 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 minus 7 volts.
The Negative Threshold Exceeded LED will light if this limit is
exceeded. Again this reading is taken with respect to Signal Ground
so the GND terminal must be connected properly for the reading to
be correct.
