Campbell Scientific MD485 RS-485 Multidrop Interface - Manual (Page 19)

MD485 RS-485 Multidrop Interface
7. Call-back
Datalogger initiated communication, commonly referred to as “call-back,” is
possible through MD485s in transparent mode. In order to do call-back,
LoggerNet Setup, MD485 Configuration and Hardware should be done as
described in the Transparent Communication section of Appendix A with two
changes in the LoggerNet Setup. The ComPort must have “Call-back Enabled”
checked and the datalogger must have the “Call-back ID” entered to match the
Call-back ID in the P97 instruction of the Edlog program.
For information on how to program the datalogger for call-back, see the
appendix covering Call-back or Datalogger Initiated Communication in the
datalogger manual.
8. Wiring Specifications
8.1 RS-485 Line Length
The EIA/TIA RS-485 communications standard, an upgrade of RS-422,
supports 32 devices (driver/receiver pairs) in a party line or multi-drop mode,
on a cable of up to 4,000 feet.
The standard specifies that each device has a “unit load” of not more than 12k
ohm. It does not specify cable type or data rate.
The MD485 has a 1/8-unit-load receiver input impedance (96k ohm) that
allows up to 256 transceivers on the bus. Practical network design will be more
of a limitation than the electrical load limit of 256 nodes for the MD485.
The RS-485 data transceivers used in the MD485 feature fail-safe circuitry,
which guarantees a logic-high receiver output when the receiver inputs are
open or shorted. Because of this no “biasing resistors” need to be used. They
also feature reduced slew-rate drivers that minimize EMI and reduce
reflections. Because of this, termination resistors do not need to be used for
most applications.
8.2 Grounding
The MD485 has a ground lug. Connect this ground lug to earth ground with an
8 AWG wire. This connection should be as short as possible.
The differential signaling of RS-485 does not require a signal ground to
communicate. The standard allows for a common-mode voltage (Vcm) of -7 to
+12 V. As long as the MD485 local grounds do not exceed this common-mode
voltage limit, the RS-485 communication will work fine.
Over a distance of hundreds or thousand of feet, there can be significant
differences in the voltage level of “ground”; it can easily extend beyond the
common-mode voltage limits of RS-485. The signal ground wire serves to tie
the signal ground of each node to one common ground, which is within the
common-mode voltage of the RS-485 specification.
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