Quadlogic MiniCloset-5c, Руководство по установке

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Appendix
© COPYRIGHT 2009
Quadlogic Controls Corporation
80
Termination Resistors
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A terminating resistor a resistor that is placed at the extreme end or ends of a cable. The
value of the terminating resistor is ideally the same value as the characteristic impedance of
the cable.
The value of the terminating resistor MUST match the characteristic impedance of the wire or
else reflections will occur when the signal travels down the cable. There are instances where
reflections are bound to happen because of cable and resistor tolerances; however, large
enough mismatches may cause reflections big enough to bring about errors in the
transmitted data.
With this in mind, it is important to match the terminating resistance and the characteristic
impedance as closely as possible. The position of the terminating resistors is also very
important. Termination resistors should always be placed at the far ends of the cable.
Datalink Network
Quadlogic meters and Scan Transponders can be set-up to be a data link communication
network when an RS-485 module is available (see Figure A-2). The data link communication
network can have up to thirty (30) meters which are daisy chained together. The beginning
and end of each 30-meter segment within the network MUST have two (2) terminating
resistors for each pair of wires.
The data link communication network most of the time will have a Quadlogic device with a
Modem/RS-485 module where a dedicated telephone line will be plugged in. It is highly
recommended to put the Quadlogic device with the Modem/RS-485 module at the beginning
of the network. Furthermore, the total wire run of the network MUST not exceed the wire
limit of 4,000 feet.
Occasionally, however, it becomes necessary for a particular meter to be further away from
the “main RS-485 Data Link trunk” than the distance allowed by the Data Link Plug
assembly. In such an event, a longer, extended cable CANNOT be used to connect that
meter to the RS-485 Data Link in an elongated “T” junction configuration. Rather, the RS-
485 must be routed directly into that individual meter and then drawn back out from that
meter to the next meter in the system in one continuous line.
WARNING:
While it would seem reasonable for a branch or “T” connection to run from RS-485 Data Link
and permit the proper functioning of the data gathering, this is not a solution. It is
absolutely imperative that a Quadlogic metering system with RS-485 Data Link never have
branches running from the main line. The twisted, shielded pair wires must “enter” and
“exit” each meter in the system with the exception of the first and last “terminator” meters.
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http://www.maxim-ic.com/appnotes.cfm/appnote_number/763