I-O Display Systems Basic I/O Product - Manual (Page 61)

Installation 2
2-7(Vol.2)
Network load V.S. Noise suppression:
In order to improve RS-485 Bus noise immunity, particularly under tri-state conditions, a pair
of “Network Bias Resistors” have been installed in each BASIC I/O AD unit. This design
feature has been implemented in order to satisfy the majority of our customers. If it is
necessary to multidrop more than eight BASIC I/O AD units(but ultimately less than thirty-
two devices), It will be necessary to remove these network bias resistors so as to not exceed
the maximum bus loading. However, in order to retain noise immunity, the network bias
resistors should remain installed in at least one BASIC I/O AD on the network. Figure 2-4
below shows the location of the four network bias resistors.
R34 is the 1.5k from prior +bias resistor
R35 is the 1.5k from prior - bias resistor
R36 is the 1.5k from next + bias resistor
R37 is the 1.5k from next - bias resistor
Figure 2-2 Network Bias resistor locations