wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Foundation Fieldbus 35-5
Many designs also endeavor to take into account the criticality of the individual field devices, so
that in the unlikely event of a communications failure, the impact to the control system is minimized.
It is recommended that you follow similar rules as are being used to manage this risk to determine the
assignment of devices to I/O cards today. By using these guidelines, the impact on a single component
failure on a design will be similar or less than for an analog control loop, especially if you have “control
in the field.” With control in the field, as long as the field devices have power, it is possible for them to
continue to control at the last set point, though there will be “loss of view” by the DCS and process
operator. Fieldbus is also designed to “fail gracefully” with the additional options on loss of contact with
the host of holding last position, or moving to a mechanical fail position.
The following formula is used to determine the voltage available at any device or node (V d ) in the
fieldbus network.
∑
( )
Vd = Vp − ⎡ Id + IHH + ISC − I ⎤
Dmin R
⎣⎢
⎦⎥ ×
where
V p is the voltage at power supply
I d is the current required to power device
I HH is the current required to power handheld unit(s), typically 10.mA
I SC is the current load on segment when short circuit is activated
I Dmin is the current demand of device on network with minimum current requirement
R is the resistance of cable
35.7 FFPS—Fieldbus Power Supplies
FF H1 networks require fieldbus power supplies (FBPS)/power conditioners because the fieldbus signal
itself is normally between 0.75 and 1.V (approximately 9.mA) peak to peak. This Manchester-encoded
signal changes every 16 or 32.ms, so a traditional 24.V bulk power supply would be working full time to
absorb this signal to maintain a steady DC voltage. FBPS/conditioners typically source their power from
a 24.V DC bulk power supply, as shown in Figure 35.2.
FBPS or power conditioners must comply with Fieldbus Foundation standard FF-831 “fieldbus power
supply test specification.” After a FBPS has passed the tests outlined in this specification, they will have
the fieldbus “check mark” confirming their suitability for use on H1 networks.
Fieldbus power supply/conditioner
CPU 1
CPU 2
FF I/O
24 V DC
T
12–28 V
80–500 mA
T
FIGURE 35.2
Foundation fieldbus power supply/conditioner.
© 2011 by Taylor and Francis Group, LLC