64
9900 Transmitter
How HART
®
Works
The HART
®
(
H
ighway
A
ddressable
R
emote
T
ransducer)
Protocol uses Frequency Shift Keying (FSK) to
superimpose digital signals on top of the analog
4 to 20 mA current loop. This allows two-way digital
communication to occur and allows additional information
beyond the normal process data to be communicated
to the 9900. This digital signal can contain data such as
device status, diagnostics, etc.
The HART protocol provides two simultaneous
communication channels: a 4 to 20 mA analog signal
and a digital signal.The analog signal communicates the
primary measured value using the 4 to 20 mA current loop.
Additional information is communicated using a digital
signal superimposed on the 4 to 20 mA signal.
Communication occurs between two HART-enabled devices, in this application a Signet 9900 Transmitter and a PLC
or handheld device, using standard wiring and termination practices. The HART Protocol communicates at 1200 bits
per second without interfering with the 4 to 20 mA signal and allows the PLC or handheld device to communicate two or
more updates per second to and from the 9900.
20 mA -
4 mA -
Digital
Signal
Analog
Signal
Time
Note: Drawing not to scale
Frequency Shift Keying
Digital over Analog
Time
As deployed in the 9900 application, HART allows remote verifying, testing, adjusting and monitoring of primary
and secondary device variables. Features available in the 9900 Transmitter with H COMM Module installed:
• Adjust 4 mA:
Allows
fi
ne-tuning to compensate for errors in other equipment connected to the 9900.
Adjust the minimum and maximum current output.
• Adjust 20 mA:
Allows
fi
ne-tuning to compensate for errors in other equipment connected to the 9900.
Adjust the minimum and maximum current output.
• Supports Multi-Drop Mode:
Allows up to four 9900 Transmitters be installed in Multidrop mode.
• Supports all Universal HART Protocol Revision 7.2 commands
• Supports many Common Practice Commands
• Makes Primary and Secondary values available at PLC.
Secondary values are sensor-dependent and
are available with Flow, pH, Conductivity, Resistivity, Salinity and Level sensors.
The HART protocol operates according to the master-slave method. Any communication activity is initiated by the
master, usually a programmable logic controller (PLC) or a data acquisition system. HART accepts two masters:
the primary master - usually the control system (PLC) - and the secondary master - a PC laptop or handheld
terminal used in the
fi
eld.
HART
fi
eld devices - the slaves - never send without being requested to do so. They respond only when they
have received a command message from the master. Once a transaction (i.e., a data exchange between the
control station and the
fi
eld device) is complete, the master will pause for a
fi
xed time period before sending
another command, allowing the other master to break in. The two masters observe a
fi
xed time frame when
taking turns communicating with the slave devices.
HART
®
is a registered trademark of the HART Communication Foundation, Austin, Texas, USA.
Any use of the term HART hereafter in this document implies the registered trademark.
H COMM Module Overview
