Instrumentation & Measurement Magazine 25-7 - 9

Fig. 3. Error handling of a transducer channel as defined by the IEEE 1451.0
Standard.
longer met), the corresponding event bit keeps equal to '1' until
it is explicitly reset by the external computer. The concept is
quite simple and can be done easily with a SR latch.
Another interesting feature of the IEEE 1451.0 Standard is
the definition of status bits specifically for transducers (Table 1).
Bits 1, 11 and 14 are good examples: Bit 1 is activated if the transducer
electronic data sheet (TEDS) has changed, which happens
if the transducer is replaced by another; Bit 11 is activated if the
calibration period of the transducer has been exceeded; and Bit
14 is activated if the correction engine is disabled. All of these
conditions can lead to inaccurate measurements and therefore
need to be detected and handled properly.
Table 1 - Status bits of a transducer channel as
defined by the IEEE 1451.0 Standard
Bit
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17-23
24-31
Meaning
Service request
TEDS changed
Reserved
Command rejected
Missed data or event
Data/ event
Hardware error
Not operational
Reserved
Data available/ data processed
Busy
Failed calibration
Failed self-test
Data over or under range
Corrections disabled
Consumables exhausted
Not-the-first-read-of-this-dataset
Reserved
Open to manufacturers
TEDS: transducer electronic data sheet
October 2022
Status Bits and Safety Locks
The automation industry has a long record of developments
in process control, process digitalization, and
process safety. A key development was the release of
digital communication networks for process control, commonly
known as " fieldbuses. " These networks started
reaching the factory floor during the 1980s and remain
as one of the pillars of industry 4.0. Examples of well-established
fieldbuses are: Profibus, Foundation Fieldbus,
Modbus and DeviceNet. In the following paragraphs, we
will use Foundation Fieldbus (FF) as the guide example,
but similar resources can be found in the competitors as
well.
The FF is a communication technology for process control
supported by the FieldComm Group [4], [5]. It is used
to connect instruments and execute real-time control loops,
with the advantage of providing digital data from the primary
transducer up to the supervision layer. The physical
layer is composed by a two-wire, multidrop bus that supplies
power and carries digital data. Communications follow a sophisticated
protocol that conciliates real-time messaging with
non-critical data exchanges. Above the protocol layers, a programming
interface based on pre-defined functional blocks
allows the development of agnostic applications that run on
any certified equipment.
Fig. 4 shows an example of the most common FF application:
the basic control loop. In this example, the goal is to
control the water level in an open tank, the process variable
is the water level, and the manipulated variable is the flow of
water entering the tank. The FF application comprises three
function blocks:
◗ The analog-input (AI) function block reads the process
variable from the pressure transmitter (the water level is
implicit in the weight of the water column).
◗ The proportional-integral-derivative (PID) function
block runs an enhanced version of the classical PID algorithm
[6].
◗ The analog-output (AO) function block writes the manipulated
variable to the control valve which regulates the
input flow of water.
The FF is very flexible because any instrument can host any
function block, and any device can play the role of bus master.
In this case, a good choice to reduce bus traffic is to have the AI
block running in the pressure transmitter and the PID and AO
blocks running in the control valve. The typical choice for the
bus master goes to the FF card installed in the programmable
logic controller (PLC). The bus master runs the FF application
and determines " who talks to whom, and when, " according
to the diagram represented in Fig. 4b [7]. The arrow from the
AI block to the PID block refers to data transferred over the
bus; the arrows between the PID block and the AO block refer
to local data transfers, inside the control valve, without loading
the bus.
The data transferred between blocks are data structures
composed of status and value. The status field comprises three
parts:
IEEE Instrumentation & Measurement Magazine
9
Instrumentation & Measurement Magazine 25-7

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