Specifications

Functional Overview
ProLinx Communication Gateways, Inc. 6
HART Master Protocol Drive Manual
August 17, 2004
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, an FSK signal causes no
interference with the analog value. The HART devices are powered from this 4 to 20 mA
analog loop.
All HART protocol channels in the Prolinx module generate the Bell 202 FSK signal to
communicate in multidrop mode with up to 15 HART devices and provide up to 300 mA
supply for the analog loop in multi-drop mode. Refer to Appendix B for ProLinx HART
field connection information.
User configured commands determine the HART commands to be issued on each
channel to the HART devices. Up to 100 commands can be defined for each port. Data
read from the devices are placed in the virtual database. Any write requests or device
specific command for the HART slave devices are sourced with data from the virtual
database or from a configured constant data block. In the commands it can be specified
whether 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 all this processing of the address
automatically.
The module can be configured to place slave devices that are not responding to
commands from the master ports at a lower priority. If the module recognizes that a
slave device has failed to respond to a message after the user defined retry count, it will
mark the slave as "in communication failure" and set the error delay time to the user
specified value. Each time that the error delay time expires, the slave will be polled and if
the answer is successful, the slave is placed again in an active status. This facility can
improve communication throughput on the HART network.
In a HART network, it is possible to have two masters. The Prolinx 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 throughput is achieved.
1.2 Module Internal Database
Central to the functionality of the module is the internal database. This database is
shared between all the ports on the module and is used as a conduit to pass information
from one device on one network to one or more devices on another network. This
permits data from devices on one communication port to be viewed and controlled by
devices on another port. In addition to data from the slave and master ports, status and
error information generated by the module can also be mapped into the internal
database.