6.20 Pulse output
The BA314NG Tachometer has an opto-isolated open
collector pulse output with following electrical
parameters:
Ron
=
60
Ω
+ 3V
Roff
=
1M
Ω
I max
=
10mA
The output pulse may be a synchronous duplicate of
the input pulse or may be scaled and the pulse length
extended.
The retransmitted RTx annunciator on the instrument
display shows the status of the retransmitted pulse
output. Annunciator activation depends upon the
setting of
5ource
in the pulse output configuration
menu.
5caled:
Annunciator activated each time pulse output
open collector is
on
, i.e. Ron is less than 60
Ω
+ 3V.
direct:
Annunciator
continuously activated
6.20.1 Ex nA certification
The pulse output is a separate opto-isolated Ex nA
circuit with the following input safety parameters:
Ui
=
30V dc
Ii
=
100mA
This allows the pulse output to be connected to any dc
circuit providing that in normal operation the maximum
supply voltage is not greater than 30V dc.
Providing the BA314NG Tachometer is correctly
installed in Zone 2, the pulse output may be directly
connected to any suitably protected equipment
located in Zone 1 or 2 of a hazardous area, or to
equipment located in a safe area. The pulse output
should not be directly connected to intrinsically safe
equipment.
Fig 11 shows a BA314NG Tachometer mounted in
Zone 2 retransmitting a pulse to the safe area. The
current pulse flows through R1 resulting in a voltage
pulse output with an amplitude of about 20V.
To comply with the requirements of
EN 60079-14
Electrical installations design, selection
and erection
, each of the wires entering the
hazardous area should be individually fused and
contain a means of isolation. These two requirements
may be satisfied by installing DIN rail mounted
terminals incorporating a fuse which can easily be
removed as shown in Fig 2 to provide isolation. Clear
identification of, and easy access to the means of
isolation is essential for their effective use. It is also
necessary to ensure that the maintenance procedure
makes sure that unauthorised re-closure of the
switches does not occur. It is not considered
necessary to have a means of isolation or electrical
protection for cable screens.
For some applications Ex nA instrumentation
energised by a current limited power supply or
instrument that can be switched off, is often
considered adequate and to comply with the
requirements of the standard.
6.20.2 System design
The Tachometer’s pulse output is a passive circuit
i.e. not powered, but it is totally isolated from all
other Tachometer circuits. Subject to complying
with Ex nA interconnection requirements, the
terminals P1 and P2 may be connected to any other
instrument with an open collector pulse input.
Fig 11 shows how to produce a voltage pulse in the
safe area that could be used to drive a safe area
counter. The positive terminal of the pulse output
circuit P1 is connected to the Tachometer’s positive
supply terminal 1. When an output pulse occurs and
the open collector output ‘closes’, P2 is connected
to P1 and the pulse current flows through resistor R1
in the safe area. The current flowing in the circuit is
determined by R1 which should be chosen to limit
the pulse output current to less than 10mA. For a
24V supply R1 should be greater than 2,200
Ω
.
Fig 11 Transferring pulse output to the safe area
22
Содержание BA314NG
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