Communicating With The Transmitter; Ma Hart - Honeywell SLG 700 User Manual

1.4 Communicating with the Transmitter

It is possible to remotely monitor and configure a transmitter using either the HART
FOUNDATION
transmitter can be monitored using the analog current, and with both interfaces, can be
configured using the three-button interface and display.
The protocols are not interchangeable. Each protocol uses
Note: significantly different terminal and communication boards that are
installed before shipping.

1.4.1 4-20 mA HART

The output of a transmitter configured for the HART protocol includes two primary modes:
•
Point-to-Point Mode: one transmitter is connected via a two-conductor, 4-20mA current
loop to one receiver.
•
Multi-Drop Mode: several transmitters are connected through a two-conductor network to a
multiplexed receiver device.
The major difference between the two modes is that in Point-to-Point mode, the average
value of the loop current represents the current value of an analog signal representing the
process inside the tank. In multi-drop mode, the average value of the loop current is fixed,
usually at 4mA. Therefore, in Point-to-Point mode, an external control system can read the
Primary Variable (PV) through an analog input without HART messaging, whereas in multi-
drop mode, the PV can only be read as a digital value using HART messaging.
Note:
SLG 700 supports HART version 7 and its associated backward compatibility. The analog
signal is modulated by Frequency Shift Keying (FSK), using frequencies and current
amplitude that do not affect analog sensing at the receiver. The accuracy of the analog level
must be precisely controlled for accurate sensing. HART communication will not bump
process variables. In multi-drop mode, theoretically up to 16 devices in HART 5 (addresses
0-15) or up to 64 devices in HART6/7 (addresses 0-63) can exist on the two-conductor
network. Practically, the number of devices in a multi-drop installation is limited due to
design constraints. When installing into a multi-drop network, consider that the SLG700
requires a minimum startup current of 17mA and a minimum terminal voltage of 11V during
startup. After this initial startup period (approximately 0.5 seconds), the loop current will be
fixed at 4mA, and the minimum terminal voltage is 14V. The power source, wiring, intrinsic
safety barriers, and other devices in the network must also be considered.
Note:
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TM
Fieldbus (FF) protocols. Alternatively, with the HART
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In the HART system, the abbreviation PV is used to denote the
Primary Variable which may be only one of a number of process or
device variables that may be available.
The SLG700 requires a minimum startup current of 17mA, even
when configured in multi-drop mode. The minimum terminal voltage
is 11V during startup. After startup, the loop current will be fixed at
SLG 700 SmartLine Level Transmitter User's Manual
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or
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option, the
Revision 7