Page 243 - Instrumentation Reference Book 3E

Fieldbus 227

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d
Figure 13.34 Point-to-point mode; with provision for one 4-20 mA device and up to two masters, e.g., one management
system and a hand-held terminal.

Gateway Bell 202-
Modem

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Process mana

ir

Field level

Figure 13.35 With multi-drop mode, installation costs are considerably reduced. As manyas15field devices can be
operated from one auxiliary power supply. Management systems and hand-held terminals can be used.

can be used. Beyond this, distances up to 3000 m Layer 2, the link layer, establishes the format
can be covered using single, shielded, twisted for a HART message. HART is a master/slave
0.5 mm2 pairs. protocol. All the communication activities origin-
A total resistance of between 230 0 and 1100 0 ate from a master, e.g., a display terminal. This
must be available in the communication circuit, addresses a field device (slave), which interprets
as indicated in Figures 13.34 and 13.35 by Rg. the command message and sends a response.