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Appendix E – HART Communications
Senscient ELDS™
Rev 15
161
GB
14. Appendix E – HART Communications
14.1. Overview
Following the release of ELDS systems at Mod state 2, all ELDS Receivers incorporate a HART
compatible communications modem / link in the 4-20mA(1) output. The HART communications
modem / link provided inside the ELDS Receiver is configured to be a Slave Transmitter device,
with the intention that this be used in a conventional HART Master / Slave loop.
The HART communications modem incorporated into ELDS Receivers supports all Universal
HART Commands as defined in HCF_SPEC-127, Revision 7.1 except for Command 6*. At the
time of writing, Common Practice and Device Specific HART commands are not supported.
* Command 6 – Write Polling Address is not supported because it would provide a means by
which an ELDS gas detector could be made to permanently stop signalling gas readings via the
analogue current on its 4-20mA output. This command is only required to enable HART devices to
be put into loop current mode for use in a multi-drop configuration, a configuration that is not used
in gas detection systems. The existence of a loop current mode would compromise the safety
integrity of an ELDS gas detector’s primary output (4-20mA output), which is why this mode and
command are not implemented or supported by ELDS units.
An ELDS HART Device Descriptor (DD) is available from Senscient, our distributors or agents.
This DD file will facilitate the automatic selection/configuration of ELDS units onto an existing
HART communication system. The ELDS HART DD is not currently included in the DD library
distributed by the HART Foundation.
14.2. Electrical Connections / Configuration
Since the HART communications modem in the 4-20mA(1) output of ELDS Receivers is
configured to be a Slave device working in a conventional HART Master / Slave loop, the electrical
connections / configuration for such an installation should be in accordance with any of the three
following arrangements for isolated (2 wire), source or sink (1 wire):
In order to conform to relevant HART physical layer specifications, the impedance of the
HART Primary Master (Current Input) must be between 230Ω and 600Ω within the normal
frequency band (950Hz to 2500Hz).
The electrical connection diagram shown for an isolated HART loop on 4-20mA(1)
assumes that there is a separate, isolated power supply inside or provided locally to the
HART Master input. This arrangement will provide the most reliable operation when an
isolated, 2-wire HART loop is being employed. However, it is recognized that it is also
possible to energize this loop by making use of the same +24V supply that is powering the
field devices. Such an arrangement would see the +24V supply connected to provide the
voltage source shown in the diagram for the ‘isolated’ HART loop.
Only 4-20mA(1) incorporates a HART modem / link. The 4-20mA(2) output is just an
ordinary 4-20mA output.
The two (2) 4-20mA outputs incorporated into each ELDS Receiver unit are completely
independent and electrically isolated. Consequently, 4-20mA(2) can be configured as
isolated, source or sink. Refer to section 3.3.3 for details of the wiring options which all
remain available for 4-20mA(2) when using 4-20mA(1)’s HART capability.
Summary of Contents for Senscient ELDS 1000 series
Page 58: ...Installation Commissioning 58 Senscient ELDS Rev 15 GB 2 x Mounting Brackets ...
Page 140: ...Certification 140 Senscient ELDS Rev 15 GB 9 1 2 Control Drawing CENELEC ATEX ...
Page 142: ...Certification 142 Senscient ELDS Rev 15 GB 9 1 4 Control Drawing CSA US Canada ...
Page 159: ...Appendix D Manufacturer s EC Declarations Senscient ELDS Rev 15 159 GB ...
Page 160: ...Appendix D Manufacturer s EC Declarations 160 Senscient ELDS Rev 15 GB ...
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