5
3. SYSTEM DESIGN
The BA688CF may be connected to any fieldbus
segment as shown in Fig 2 providing it can
provide the additional 25mA required to operate
the Fieldbus Display.
Fig 2 Fieldbus system
3.1 External switches
For applications requiring operator inputs to be
made by large industrial push buttons, terminals
S1 to S7 allow up to six external switches to be
connected to the Fieldbus Display. When
external switches are connected the front panel
push buttons may be operated in parallel or
disabled – see section 5.3.7
Terminals S1 to S7 are not electrically isolated
from the fieldbus terminals, therefore external
switches and wiring must be isolated from earth
and all other electrical circuits.
For reliable operation it is recommended that the
cable connecting the Fieldbus Display to external
switches is less than 5m long.
3.2 Alarm outputs
Each alarm output is a galvanically isolated
single pole solid state switch output as shown in
Fig 3.
Fig 3 Equivalent circuit of each switch output
The outputs are polarised and current will only
flow in one direction. Odd numbered terminals
should be connected to the positive side of the
supply.
Ron
=
5
+ 0.7V
Roff
=
greater than 1M
Note:
Because of the series protection diode,
some test meters may not detect a closed alarm
output
WARNING
These Alarm Outputs should not be
used for critical safety applications
such as an emergency shut down
system.
When the BA688CF is disconnected from the
fieldbus, or the fieldbus is de-energised all the
alarm outputs will open irrespective of how they
have been configured.
