Operating Instructions

Siemens Industry, inc. 48
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.
Communication Verification
Verification includes confirmation of BOTH the physical interface settings (Serial or Ethernet)
and the protocol (Modbus or BACnet) settings.
The LCD User Interface can be used to quickly confirm the settings required for each combination of
interface and protocol. The interface is intuitive and groups together commonly associated registers. The
complete interface is presented in Appendix A as a navigational map. Arrows indicate how to move from
one menu display to the next. A Reverse Contrast entry in the documentation indicates the active menu
item in a list which corresponds to a blinking character in the physical LCD. The ENTER button is used to
select a property and up/down buttons are used to select among the values supported by the meter. Note
that changes to the meter configuration are limited to the communication interface using the LCD. If
additional changes (such as CT type) are required, they must be made using a software interface.
Because the LCD User Interface on the MD-3HD is two lines instead of four, navigating the menu structure
is different from the MD-12HD, MD-24HD, or MD-48HD. Additionally, the MD-3HD features a navigation
thumb wheel which allows the user to scroll up or down and select menu options by pressing the thumb
wheel. While the User Interface allows for checking real time values, it does not allow for advanced meter
configuration. ViewPoint HD is the recommended configuration tool for the MD-3HD.
2-Wire Multidrop Network using Terminating Resistors.