B&B Electronics 485T Manual De Usuario

Documentation Number 485T1995 Manual 

7 

 

 

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. 

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.