Siemens MD-12HD, User Manual, Page: 42 / 74

41 Siemens Industry, Inc.
Communication and Verification - Details
This section is intended to support the commissioning of the MD-xxHD meter by an
instrumentation technician. In many cases, the electrical installation is conducted
ahead of the availability of the RTU or was performed by a different installer. Often
the technician is working in concert with a remote programmer who is confirming the
connectivity with a remote host system. A Digital Multimeter (DMM) can be used to
confirm measurements at the board terminals, if necessary.
It is assumed that the meter is now powered up from the line
voltage. It is safe to touch the meter (including the user buttons)
with the top cover removed ONLY IF THE INTERNAL HIGH
VOLTAGE COVER IS INSTALLED.
Communications settings and real-time data values can be confirmed quickly using the LCD
interface if equipped. When significant setup modifications are anticipated, a computer interface
is recommended.
Physical Connections on an RS-485 Multidrop Network
The MD-xxHD meter uses a 2-Wire Half Duplex RS-485 Implementation.
▪
Termination Resistors
—are NOT included on the MD-xxHD meter. If the MD-xxHD meter is at
the end of a daisy-chain, then connect a 120-ohm leaded resistor between the + and
– terminal at
the connector.
▪
Bias Resistors
—are NOT included on the MD-xxHD meter. Bias resistors are needed if the idle
conditions of the bus are in an indeterminant logic voltage. Bias resistors are usually located at
the master node and are usually 680 ohms for a RS-485 network.
▪
Network Topology
—
RS-485 is designed to be implemented as a daisy chain (series
connections) rather than star or cascade topologies.
▪
Signal Names
—
Some RS-485 devices use the terminology A/B while others use +/-. Note that A
is (-) and B is (+). Many manufacturers incorrectly label the terminals.
▪
Bus Loading
—The MD-xxHD meter is a 1/8
th
unit load allowing up to 256 like devices in parallel.
2-Wire Multidrop Network using Terminating Resistors.