MVI56-HART ♦ ControlLogix Platform
Reference
HART Multi-drop Master Communication Module
User Manual
ProSoft Technology, Inc.
Page 89 of 171
October 13, 2011
5.2.5 HART Channels
The MVI56-HART module supports the HART protocol as a Master on up to
four
channels. Each channel is individually configurable.
The HART protocol uses the Bell 202 standard frequency shift-keying (FSK) signal to
communicate at 1200 baud, superimposed at a low level on the 4 to 20 mA analog
measurement signal. Having an average value of zero, and FSK signal causes no
interference with the analog value. The HART devices are powered from this 4 to 20 mA
analog loop.
Both HART protocol channels in the module generate the Bell 202 FSK signal to
communicate in multi-drop mode with up to 15 HART devices and provide up to 250 mA
supply for analog loop.
Auto-Poll Modes
Each HART channel can be set to operate in three different modes:
Point-to-Point
Multi-drop
User Mode
Using the configuration file, choose the auto-poll mode through the Auto-Poll Code
parameter (P, M, or N). In the first two modes, the module will automatically collect data
from each HART instrument on the channel (auto-poll) and store the data in the module's
database.
In User Mode, the module will only execute the commands in the user command list and
will not automatically acquire data. Refer to the Auto-Polling section for more information.
Mode
Use Auto-Poll
Use Command List
N (None)
N
Y
M (Multi-drop)
Y
Y
P (Point-to-Point)
Y
Y
When configured for Point-to-Point communication, the channel will automatically poll
data from the connected slave address 0.
When configured for Multi-drop, the channel will automatically poll data from the
connected slaves in the network (address 1 to 15). Refer to the Auto-Polling section for
more information about auto-poll mode.
User-configured commands can be issued on each channel to the HART devices. Up to
99 commands can be defined for each port. Data read from the devices is placed in the
virtual database of the module, which is passed between the module and the processor.
Any write requests or device-specific commands for the HART slave devices are sourced
with data from the virtual database, or from a configured constant data block. Within the
commands, it can be specified whether or not to use the HART device's short or long
address. If the long address is selected, the device is polled first with short address to
ask for the long one. Then, the device is polled with the long address. The module does
this processing automatically.
In a HART network, it is possible to have two masters. The module fully supports the
existence of a second master, but it can reduce the throughput on the HART network.
This facility is enabled or disabled in the module's configuration. If the ability to have a
second master on the network is disabled, then the maximum communication throughput
is achieved.
