Micromod MOD: MODBUS RTU Communications Guide - Manual (Page 18)

MODBUS RTU
COMUNICATIONS GUIDE
14
MODBUS WITH COMMANDER SERIES INSTRUMENTS
SCALING ANALOG VALUES
The first Commander instruments to support MODBUS handled scaling of analog values in a different way
than later instruments. The C200, C300, C1900 and PR100 use a 12-bit register with a raw range of 0 to
4095. The host must scale the value to obtain the desired engineering unit value.
Newer models, such as the C100, C150, C250, C500 and PR250, place an engineering unit value into one or
two 16-bit registers. If two registers are used, the data type is considered a long integer. Values are
represented as positive or negative integers without a decimal point. If a decimal point is used in the
instrument, additional scaling must be performed at the host.
COMMUNICATIONS CONFIGURATION
The Commander must be configured from the face of the instrument via the Serial Data Communications
page. If this page is not accessible, the communications option may not be installed. Some Commanders use
a security switch to access the configuration mode. The parameters on this menu group must be set to match
host or Modbus Master device. The 3 Line type refers to 2-wire (half-duplex) with a common, where 5 Line is
4-wire (full-duplex) with a common. The number of Start and Data bits are fixed at 1, 8 respectively. The
number of Stop Bits is not configurable and can be set to 1 or 2 at the host. Parity can be set to none, odd or
even and must match the Host setting. The Ident or Addr entry configures the Modbus Slave address.
CONNECTION WITHIN THE NETWORK
One additional subject that must be addressed, is the electrical stabilization and electrical connection of the
RS-485 network. By definition, the Modbus Master device is to supply the pull-up and pull-down resistors to
stabilize the network. If the interface device does not have that capability, you may select the "Linked In"
jumper setting on the Commander. If the stabilizing is performed at the Commander, and the Commander
fails or is removed from the network, all communications may be stopped. Depending on the length of your
cable, you may also need termination resistors, which may be installed externally for 3-wire or by a jumper for
5-wire configurations.