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HART Multiplexer Master KFD2-HMM-16
Appendix
Subject to reasonable modifications due to technical advances.
Copyright Fuchs, Printed in Germany
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Multidrop
In contrast to
➝
point-to-point, more than two (field) devices are connected together to one seg-
ment (pair of wires) in a multidrop system. To correspond to each singe device it must have a
unique
➝
address. Because communication can only be established to one field device, cycle
times are increasing proportional to the number of field devices. In HART multidrop operation the
current through each field device is fixed at 4 mA to allow parallel operation of more than one
device (up to 15 devices are possible).
Multimaster
HART allows connection of two masters, a primary and a secondary master. A high level station is
configured as primary master, usually this is the process control system or the main service sta-
tion. A lower level station is configured as secondary master, this may be a hand terminal or a ser-
vice station. The difference between primary and secondary master is the priority of the bus
access: the primary master has a higher priority than the secondary master. Messages sent by the
masters are characterised by a master bit, so that the masters can recognise which responses are
intended for them.
Point-to-point
In a point-to-point communcation system, only two communicating devices are connected together
to one segment (pair of wires). A point-to-point system is for example the master-slave system mul-
tiplexer-field device.
Polling address
➝
Address
Primary master
➝
Multimaster
Primary variable
Process value measured by a field device. The unit depends on the used HART command (see
commands 1, 2 and 3). The primary variable of a pressure sensor could contain for example the
measured process pressure in the unit "bar".
Secondary master
➝
Multimaster
Secondary variable
Additional value (measured in the prozess) of a field device (up to four additional values are sup-
ported by HART). This variable can only be read by HART command 3.
Slave
A device (e. g. transmitter or valve) in a
➝
master-slave system that receives commands from a
➝
master device. A slave is not able to initiate a transaction.
"Smart" field device
Microprocessor-based device that can be programmed, has memory, is capable of performing cal-
culations and self-diagnostics and reporting faults, and can be communicated with from a remote
location.
Tag
Unique tag (designation of the control engineering point) of the field device within the process
plant.
