Contemporary Control Systems AI Active Hubs - Manual (Page 20)

TD675100-0M
J
16
the other is tied to ground. Each resistor has a jumper
associated with it. If the two jumpers are installed, the resistor
tied to +5V is connected to the phase A signal line while the
grounded resistor is connected to the phase B line. This voltage
drop will bias the differential receivers into a defined state
when no differential drivers are enabled. Differential receivers
typically switch at or near zero volts differential and are
guaranteed to switch at +/–200 mV. Through the transition
point, 70 mV of hysteresis will be experienced. Therefore, a
positive bias of 200 mV or greater will ensure a defined state.
We recommend that bias be applied to both ends of the wiring
segment by installing the two end jumpers located at position E1
on the daughterboard. This is to be done for only the AI ports
or NIMs located at the ends of the segment. All other NIMs
will have their jumpers removed.
The termination and bias rules are simple. If the NIM or AI
port is located at the extreme ends of the segment, install all
three jumpers at location E1 on the daughterboard. If the NIM
is located between the two end NIMs or AI ports, remove all
three jumpers. If external termination is desired, remove the
middle jumper at E1 and provide the external termination.
Figure 2-9. DC Coupled EIA-485 Option (-485)