Appendix
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Doc No: UMAN\GWY-300-B\0106
REVISION 0
A
Flexible Applications
1.
HART is principally a master/slave protocol which means that a field (slave) device speaks only when
spoken to by a master. Two masters (primary and secondary) can communicate with slave devices in a HART
network. Secondary masters, such as handheld communicators, can be connected almost anywhere on the
network and communicate with field devices without disturbing communication with the primary master. A primary
master is typically a DCS, PLC, or computer based central control or monitoring system. A typical installation with
two masters is shown in Figure
2.
The HART protocol can be used in various modes for communicating information to/from smart field
instruments and central control or monitoring equipment. Digital master/slave communication simultaneous with the
4-20 mA analog signal is the most common. This mode, depicted in Figure 4, allows digital information from the
slave device to be updated twice per second in the master. The 4-20 mA analog signal is continuous and can still
carry the primary variable for control.