Honeywell RS-485, Руководство по установке, Страница: 12 / 18

12
www.honeywell.com
Figure-11
Multi-Drop System
Shield Grounding & Signal Common:
Of particular importance for system integrity and optimal performance is the attachment
of the cable shield to an earth ground connection. As mentioned earlier in this text, only
one end of the cable should be attached to earth to prevent ground loops. The shield
connection wire becomes accessible upon removing a portion of the cable jacket from the
end. This will be a simple non-insulated wire that runs the length of the cable. An earth
conductor wire will need to be attached, and the wire gauge is not critical here.
Making reference to the earlier illustrations 6 & 7, it can be seen that a connection
between the 'signal commons' is established, albeit through series resistors. The term
'signal common' here refers to the common ground system present in the electrical
device, and is not to be confused with earth grounding. It is desirable to maintain some
equilibrium between the signal commons of all the communications devices so as to
prevent them from 'floating away' in terms of electrical potential.
At the same time, ground loop currents could potentially develop when units are
separated over extended distances. Ground currents can be minimized however by use of
a simple resistor in the circuit path. Resistor R3 on the RS-485 adaptor board provides
this current limit function. Additionally, the resistor is mounted in miniature sockets so
that the value can be easily changed if necessary. The default value for R3 is 47k ohms,
and can be "stiffened" if necessary to perhaps 1k or 100 ohms. Circumstances specific to
each field installation will ultimately determine if the R3 value should be adjusted or not.