I-O Display Systems Basic I/O Product - Manual (Page 25)

Setup & Installation 2
2-8(Vol.1)
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 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 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 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 2-4 Network Bias Resistor Locations