Pepperl+Fuchs KFD Series, Manual

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HART Multiplexer System KFD*-HM*-16
Operation
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8-04
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7.1.1 Number of Channels
The multiplexer master provides 16 channels for connecting field devices or control units that
support digital communication according to the HART specification. A maximum of
15 additional multiplexer slaves can be connected. These multiplexer slaves in turn support
16 channels. With just one multiplexer master, 256 channels can therefore be reached in
maximum configuration.
7.1.2 Multiplexer Table (Module Table)
The multiplexer master and the connected multiplexer slaves must be labeled as present in
a multiplexer table, see chapter 9.1 command 157. Only modules defined as present in this
table are included in communication. The multiplexer table consists of 16 bit, 1 bit for each
multiplexer address. The default is module 0 (multiplexer master) and 1 activated.
7.1.3 Interface
The multiplexer master acts as a transparent gateway between the maintenance station and
the field devices. The maintenance station is typically a computer with appropriate software,
see chapter 3.2. The maintenance station can address up to 31 multiplexer masters via a
RS-485 connection of up to 57600 baud. Since each multiplexer master can address up to
256 field devices, each RS-485 interface communicates with up to 7936 field devices.
7.1.4 HART Communication
As a digital communication system for maintenance and configuration purposes,
the HART protocol is supported by many field devices with conventional analog
4 mA to 20 mA circuits. The HART signal is modulated to the analog current as an FSK signal,
see chapter 3.1. The modulator/demodulator circuit (modem) required is integrated in the
multiplexer device. Only one HART-compatible field device can be connected to each
HART connection of the multiplexer device
–
there is no multidrop functionality.
Special procedures with regard to the HART communication:
•
On the host side (RS-485), the extended address is always used, except in the case of
command 0.
• On the field device side, either the polling address or the extended address is used
depending on the need.
• The primary master and secondary master operating modes and the corresponding
time responses are supported on the field device side.
• On the host side, only the commands 0 to 3 and 11 to 13 are accepted from the
secondary host. Other commands are not accepted or are ignored.
• Commands to connected field devices are only accepted from the primary host.
• Extended messages and messages in burst mode are detected and used but are not
generated by the multiplexer device itself.
• A response buffer is available for a delayed response telegram. A message with an
initiating command that requires a long execution time can be temporarily stored here.