Honeywell RS-485, Руководство по установке, Страница: 13 / 18

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Summary
When examining the RS-485 standard, it becomes quite apparent this approach offers the
potential for high performance over long distances. With this comes a price, and the price
here is increased complexity in system setup. Many variables exist that must be properly
selected by the user when customizing the system to his/her requirements. In contrast, the
recently developed USB connection scheme offers fast data transfer, multiple node
attachments, and consumer level simplicity, but with very limited cable lengths.
From a practical point of view, application of the CNI product with RS-485
adaptor board is likely to be straightforward in most circumstances. Data transfer rates
are inherently limited by the CNI itself, at 38.4k bps maximum. Multi-drop attachments
are likely to be a rare occurrence. Even most cable runs will be relatively short at most
field sites; certainly less than 4000 feet. Taken together, what this means is that if the
installer neglects to add a termination resistor, or doesn’t use quite the ideal cabling, in all
likelihood the system will still function just fine. The following points are recommended
to help speed the installation process:
• Prepare the equipment for installation to the fullest extent possible while still at
the office. Attaching miniature wires and updating the CNI configuration memory
can test a persons patience when the outdoor temperature is 40 degrees and
blowing rain.
• When possible, short cables runs should be configured for full-duplex operation.
This reduces the possibility that two transmitters will ever “bang heads”. In
addition, trouble-shooting of problems becomes easier since it is well defined
which twisted wire set is the ‘talker’, and which is the ‘listener’.
• Observe proper wire polarities at each end. A reversed connection will not result
in any circuit damage, but the communications link will of course fail to operate.
• Begin the field site installation using the default jumper settings JP1=In, JP2=Out,
JP3=Out, and JP4=A+B. A full-duplex system with only a single transmitter per
twisted wire pair can optionally have JP4 set at B+C to leave it continuously
enabled.
• Bring along a spare CNI unit that is known to be in good working condition.
Having this available can be of great assistance when trying to isolate problems in
a non-functional system.