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ROC827 Instruction Manual
Issued Mar-06 Communications 5-16
MVS modules are isolated on the field side. Be aware that you can
induce ground loops by tying commons together.
5.11 HART Interface Module
The HART
®
Interface module allows a ROC827 to communicate with
HART devices using the Highway Addressable Remote Transducer
(HART) protocol. The HART module can receive signals from HART
transmitters or receive and transmit signals from HART transducers. LEDs
provide a visual indication of the status of each HART channel. Refer to
Figure 4-21.
Note: The ROC827 currently supports the HART module only when
installed in slot 1, 2, or 3 of the ROC827 base unit.
The HART module has four analog channels. When configured as an
input, you can configure the channel for use in point-to-point or multi-
drop mode and typically connects to some type of transmitter, such as for
a temperature reading. When configured as an output, you can configure
the channel for use in point-to-point mode only. The output supports a
Digital Valve Controller (DVC).
Point-to-Point Mode
In point-to-point mode, digital communications are superimposed
using the Frequency Shift Keying (FSK) technique on the 4 to 20
milliAmp analog signal (which can still measure the process variable).
This mode allows communications with one HART device per analog
channel.
Multi-drop Mode
In multi-drop mode, you can connect up to five HART devices (in
parallel) to each analog input channel. As with the point-to-point
mode, digital communications are superimposed on the 4 to 20
milliAmp signal. However, the analog signal is used only to measure
the current consumed by the multi-drop loop. When all four analog
inputs are in the multi-drop mode, the ROC827 can support a
maximum of 20 HART devices. The number of devices per channel is
limited by the static current draw of the devices.
A ROC827 equipped with a HART module is considered to be a HART
Host (primary master) interface with a Class 1 Conformance classification.
Most Universal and some Common Practice commands are supported. For
a list of the commands, refer to the HART Communication Module
specification sheet (6.3:HART). The supported commands conform to
HART Universal Command Specification Revision 5.1 and Common
Practice Command Specification Revision 7, (HCF SPEC 127 and 151).
Refer to www.hartcomm.org for more information on the specifications.
The HART module polls the channels simultaneously. If more than one
device is connected to a channel in a multi-drop configuration, the module
polls one device per channel at a time. The HART protocol allows one