Contemporary Control Systems AI Active Hubs User Manual

Page 21

TD675100-0M

For EIA-485 DC operation, it is very important that all devices
on the wiring segment be referenced to the same ground
potential in order that the common mode voltage requirement
(+/–7 Vdc) of the EIA-485 specification is achieved. This can
be accomplished by running a separate ground wire between all
AIs and computers (pin SH has been provided for such a
connection on the AI) or by relying upon the third wire ground
of the power connector assuming that the DC power return is
connected to chassis ground on all computers (this is the case
with the AI). Another approach would be to connect the DC
common of each computer to a cold water pipe. Connected
systems, each with different elevated grounds, can cause
unreliable communications or damage to the EIA-485
differential drivers. Therefore, it is important that an adequate
grounding method be implemented.

Figure 2-10. Each -485 hub port has provisions for

applying bias and termination. Make sure common mode

voltage (Vcm) does not exceed +/–7 Vdc.

In summary, segments of -485 (-485D) connected NIMs can be
extended through the use of active hubs. Select an AI model
with a -485 compatible port. Connect one end of the segment to
this port following the same termination rules as used for a

+5V

PIP2

TXEN

RXIN

TXEN

PIP2

Vcm