# Transducer Types

## Transducer type descriptions

The standard defines the behavior of three transducer types. Attributes
are then used to further define the characteristics of the transducer.
There is a forth type allowed by the standard but not defined. This is
the general transducer type. The transducer types are:

-   Sensor

-   Actuator

-   Event sensor

-   General transducer

The term transducer as used in this document refers to the physical
transducer and all electronics in the path that connects the transducer
to the communications functions. This is a broader definition of the
term transducer than is normally used. In its basic definition a
transducer converts energy from one form to another. So a sensor would
be a transducer that converts energy form one form, temperature,
pressure,etc, to electrical energy. An actuator is a transducer that
converts energy form an electrical form to some other form. A heater is
a simple example of an actuator. However, for the purposes of this
document we are including the signal conditioning, analog-to-digital
conversion and any other electronics used to interface the
energy-converting element with the Transducer Bus in the term
\"transducer.\" This allows us to add characteristics to the devices
that go far beyond the conversion of energy from one form to another.

In the IEEE standard 1451.2 the definition of the transducer type
defined its characteristics. This is useful but it did not allow as much
flexibility as the 1451.3 working group desired. For example in 1451.2
you could have a sensor or a buffered sensor. The transducer type was
determined by the manufacturer and could not be changed. However, the
basic difference between the two types could be as simple as a single
register. With processor based sensors it could be just a slightly
different set of software. The 1451.3 working group chose to define
fundamental transducer types and to allow attributes to define or change
the characteristics of the transducer. The use of attributes allow a
sensor to be designed and manufactured that can be used in several
different ways by programming the attributes. A device can also be built
that does not allow programming the attributes so it becomes fixed much
like a 1451.2 STIM. Table 1 shows the comparison between the IEEE 1451.2
sensor types with the IEEE P1451.3 sensor type with the applicable
attributes. The IEEE P1451.3 standard allows the attributes to be either
programmable or fixed. That is the manufacturers choice. Note that the
streaming attribute is not available in IEEE 1451.2. However the
streaming operation can be achieved by using a data sequence sensor and
running the appropriate software in the NCAP.

Table 1 IEEE 1451.2 sensor types compared with IEEE P1451.3

  -------------- -------------- -------------- ------------ --------------
  IEEE 1451.2    IEEE P1451.3   Sampling       Buffers      Streaming
  Transducer     Transducer     attribute      attribute    attribute
  Type           Type                                       

  Sensor         Sensor         Trigger        Off          Not available
                                initiated                   in IEEE 1451.2
                                attribute                   

  Buffered       Sensor         Trigger        On           
  Sensor                        initiated                   
                                attribute                   

  Data Sequence  Sensor         Free running   Off          
  Sensor                        attribute                   

  Buffered Data  Sensor         Free running   On           
  Sequence                      attribute                   
  Sensor                                                    
  -------------- -------------- -------------- ------------ --------------

## Data sets

All IEEE P1451.3 transducers operate with \"data sets.\" Three fields
within the Transducer Specific TEDS define a data set. They are the
Channel Data Repetitions, Series Units and Series Increment. The Channel
Data Repetitions field defines the maximum number of individual data
samples in a data set. The actual number of samples in a data set can be
made lower than the number in the Channel Data Repetitions field by the
appropriate control command. However, when reset or powered up the unit
will default to the value in the TEDS. The Series units and the Series
Increment fields are closely related. The Series Increment field defines
the interval between samples. The Series Units define the units for the
Series Increment. For example if you want a sensor to take samples 1000
times per second. The value in the Series Increment would be 0.001 and
the entry in Series Units field would be seconds. However if you wanted
a sensor to take a reading every time the temperature changed by five
degrees Kelvin the value in the Series Increment would be 5.0 and the
entry in the Series Unit field would be degrees Kelvin. This latter
example creates some interesting situations for the system designer. In
traditional data acquisition systems data is sampled either in uniform
time intervals or on command from the controller. This makes it
relatively simple for the controller to know when to read a sample
because both the data sampling sub-system and the controller could use
the same variable, time, to determine when to take a sample. With the
capability as described in IEEE P1451.3 a common variable may not exist
between the controller and the data acquisition sub-system. This means
that the controller does not know when the TBIM has a new sample
available or requires a new sample for output. Exactly how the system
designer solves this problem, if it exists in a system, is not mandated
by the standard. The standard provides status words that allow the
controller to find out what the condition of the TBIM but it does not
mandate how they are used.

The size of a data set may be lowered from the number in the Channel
Data Repetitions field on the Transducer Specific TEDS. What this field
defines is the amount of memory available in the TBIM that can be used
to store data samples of this variable. This number is fixed by design
and cannot be readily changed in the field. The Series Units field is
also something that is designed into the TBIM and is not normally
changed in the field. If time is the Series Units variable a clock
source is identified and the appropriate hardware or software is
incorporated to use the output of this source to determine the interval
between samples. However, if the Series Units are degrees Kelvin a
temperature measurement is included within the TBIM and the output of
that device is used to determine when to take a sample. Since a clock is
not normally used to measure temperature once the TBIM is manufactured
with Series Units of time it cannot be changed to have Series Units of
temperature or vice versa. Thus while these three TEDS fields define the
limits of a data set at least some of them can be modified by the
appropriate control commands or the use of virtual actuators. [See ??
for the details of the control commands.]{.mark}

## Enabling and disabling transducers

Transducers may exist in one of two states. They can be either enabled
or disabled. A transducer that is disabled is not acquiring or applying
data sets. When it is in this state it will not respond to read or write
data commands or triggers. However, it will retain its setup and be able
to respond to other commands such as status reads and read or write TEDS
commands. It will also accept setup commands. A transducer that has been
enabled will respond to all commands. However, it will not normally
begin acquiring or applying data until after the first trigger. How it
responds after that is determined by the attributes.

## Sensors

A sensor measures a single physical parameter and return a digital data
set containing values representing that parameter. In general the sensor
shall start the collection and storing of a data set within the TBIM on
the receipt of a trigger. The timing of the individual samples in the
data set is controlled by the TBIM and is a function of the operating
mode of the sensor and the TEDS fields described in 1.2. A sensor
responds to a read command by returning the appropriate data set as
determined by the Buffered Attribute. If a new data set is not available
the sensor responds with the same data that was returned on the previous
read command. The data transmitted in response to a read command may be
transmitted using the Bus Management communications channel, the TBIM
communications channel or a Data Transfer communications channel. An
exception to this general characteristic is for a sensor being operated
in the streaming mode. In the streaming mode the data set is not held in
the TBIM waiting for a read command but is transmitted immediately to
the TBC. This requires that a dedicated Data Transfer Communications
channel be available to the TBIM at the time that the data set is ready
to be transmitted.

## Event sensor

An event sensor is intended to provide information about changes of
state for the input(s) that the event sensor is designed to monitor. For
example lets assume that we have a sensor on a door that detects the
door being opened or closed. Lets also assume that the door opening
gives a rising transition and the door closing gives a falling
transition. If the event sensor is set up to report rising transitions
it will tell the system when the door was opened. It will not tell the
system if the door is open or closed. The same is true if it is
reporting falling transitions. It will tell the system when the door was
closed but not whether it is closed or open. If the event sensor is set
up to report both transitions it will report when the door has been
opened or closed but not if it is open or closed. Assuming that the
system can some how be set up to know the initial condition the system
can infer whether the door is closed or open from the state transitions
as long as no transition is missed. All this is said to emphasize that
all an event sensor reports is that a change of state for some input has
occurred. If it is desirable to know the present state then a sensor
needs to be included in the system that measures the state using a
sensor as described in 1.4. Both sensor types can be included within the
same TBIM and used on the same input but they have different purposes.
Time-of-day sensors or time interval sensors may be used in conjunction
with event sensors to determine the time of occurrence of an event.

An event is defined as a transition of state. There only two allowed
states, one or zero. The event sensor may be configured to report rising
transitions (from zero to one), falling transitions (from one to zero)
or both. The time of the event is the point of completion of a selected
transition type. A control command specified in ?? allows the TBC to
select which transitions the TBIM should report as events, according to
the enumeration in Table 2. The default condition is to report both
transitions.

Table 2---Enumeration of event sensor configurations

  -------------- ------------- --------------------------------------------------------
  **Value**      **Meaning**   

  0                            reserved

  1                            rising transitions reported

  2                            falling transitions reported

  3                            both transitions reported

  4-255                        reserved
  -------------- ------------- --------------------------------------------------------

The input to an event sensor can be as complicated or as simple as the
manufacturer wants it to be as long as it can be boiled down to
something that can be defined as a change of state for a single state
variable. The example above for a sensor that detected the door being
opened or closed could be a switch and this could be the input to the
event sensor. The designer would need to include some way of eliminating
switch bounce but that is a relatively simple task. It could also be a
temperature-sensing device with a threshold being crossed whenever the
temperature went through the threshold. More complex devices are also
possible. For example a device could be built to report a change of
state every time that the temperature changed by five degrees. Or
perhaps it is desirable to report an event only if two or more of three
inputs report that the threshold has been crossed. The complexity of the
conditions that define the state are not the subject of the standard
they are a problem for the designer of the event sensor. The definitions
in the standard of what constitutes a change of state are useful for
simple event sensors but are not intended to limit the designs that can
be implemented.

The input to an event sensor may physically consist of one or more
digital or analog inputs. For a digital event sensor, a rising
transition is when the inputs match a specific digital pattern without
change for a fixed period of time, determined by the manufacturer. A
falling transition is when the inputs cease to match the pattern for
some fixed time. For an analog event sensor, upper and lower thresholds
are defined. The lower threshold differs from the upper by a quantity
called hysteresis. A rising transition is when the analog input value
passes successively through the lower and upper thresholds without
recrossing either. A falling transition shall be when the analog input
value passes successively through the upper and lower thresholds without
recrossing either.

There are two types of information that may be obtained using an event
sensor. One is the fact that an event has occurred and the other is the
time of the event. When an event occurs on a triggered event sensor the
TBIM will transmit a frame of data. As a minimum the frame of data will
contain the source address of the event sensor. The frame may include
other data such as the output of an associated Time Interval or
Time-of-Day sensor. If the TBIM does not use a Time Interval Sensor or a
Time-of-Day Sensor, and the time of the event is desired, then the TBC
must be able to detect the time of arrival of the frame of data and use
that to determine the time of the event. If the time of occurrence of
the event is to be determined by the TBC from the time of arrival of the
data frame, the TBIM should have immediate access to a communications
channel. This communications channel should be dedicated to this one
task at the time of the event otherwise variations in the time of
arrival of the signal at the TBC will occur.

If a dedicated data transfer communications channel exists in the TBIM
an event sensor may be operated with the streaming attribute active. In
the streaming mode (see ??) it will transmit a data frame each time an
event occurs after it is triggered until it is disabled.

If there are only a few event sensors on a bus event reporting could be
accomplished by assigning a dedicated data transfer communications
channel to each TBIM containing an event sensor. This would allow the
event sensor to be operated in a triggered mode or in the streaming
mode. In the triggered mode the event sensor will report one event for
each trigger it received. In the streaming mode it would report every
event it detected after an initial trigger. However, if there are a
large number of these devices on a bus, assigning a dedicated data
transfer communications channel for each TBIM quickly drives up cost and
complexity. To allow multiple event sensors to acknowledge events using
a common data communications channel requires that collision detection
be to put into the TBC. The TBIM would expect an acknowledgement of some
sort, possibly a data or status read, as a result of sending out the
event. If it did not receive that acknowledgement in a predefined period
of time it would retransmit the event. In the case of a collision the
TBC would detect a garbled message and broadcast a NAK that could cause
a back-off and retransmit in the TBIMs. However with this approach a
TBIM that is close to the TBC might override the transmission from a
more distant TBIM and the TBC might not detect the collision. In this
case, since one got through, the TBC would not transmit the NAK but the
distant TBIM would not receive the expected acknowledgement so it would
retransmit.

Virtual actuators and sensors within a TBIM may be associated with an
event sensor to allow changing analog setpoints, hysteresis, or reading
the value being sensed. For example, virtual actuators could be used to
set the trip point and hysteresis for an event sensor. The advantage of
using virtual actuators for this purpose rather than control commands is
that a virtual actuator has TEDS associated with it. This association is
communicated to the TBC through the grouping information in the Module
Meta-TEDS. This is useful when the operator wants to change the
set-point or hysteresis. If the TBIM contains a calibration TEDS this
allows the operator to specify the changes in engineering units and the
system, or even the TBIM, can convert the engineering units values to
the appropriate form using the information in the TEDS. Another
advantage of using virtual actuators is that they can be set up at one
time and then applied by the application of a trigger.

It is possible for the TBIM to perform consistence checks on changes in
its setup inputs. If the event sensor is comparing a set of discrete
inputs to a bit pattern then the only check that can be made is that the
bit pattern is a legal input. What constitutes a legal input is up to
the manufacturer to define. For an analog input event sensor with either
the threshold or the hysteresis changeable the situation is a little
more complicated. The following relationship defines the checks that can
be made for an analog event sensor:

maximum sensed input \> threshold \>= (threshold - hysteresis) \>
minimum sensed input value.

The standard requires that if consistency checks are to be made they
should be made after each setting is changed. This leads to the problem
that if both the threshold and the hysteresis are being changed it is
possible that changing the first parameter may cause the hardware error
status bit to be set but the problem will be cleared up by changing the
second parameter. For example suppose that both the threshold and the
hysteresis were to be changed. The new setting for the threshold is such
that subtracting the old value of the hysteresis will put the lower
threshold below the minimum sensed value. This will cause the hardware
error status bit to be set. However, when a new lower value of
hysteresis is entered the problem will no longer exist. Since other
problems could also set the hardware error status bit it cannot be just
cleared when the second input is changed and the problem goes away. The
simplest fix for this problem is that if an analog event sensor allows
both the threshold and the hysteresis to be changed then it should only
do consistency checking on command.

The minimum time between events which can be detected by the transducer
shall be specified in the Transducer specific TEDS field, Transducer
Sampling Period, ??.

## Actuator

In its simplest form an actuator accepts a single data sample from the
bus and stores it in the TBIM. Then after the receipt of a trigger it
applies the new data to its output in the appropriate form. The
\"appropriate form\" being determined by the TBIM designer. However, the
standard allows actuators to work on data sets as described in 1.2. The
data set can contain one or more samples to be applied to the output. If
the series unit for the data set is seconds then the operation is to
apply each sample in the data set after the period given in the Series
increment field until the entire data set is output. Since time cannot
decrease this operation always proceeds in one direction. However, if
the series units are something other than seconds it may very well be
possible to have the direction through the data set reverse itself. For
example if a TBIM was built that used an actuator to implement a
temperature correction circuit. A data set could be sent to the actuator
that defined the temperature correction and then the actuator could
change whatever it needed to change in the TBIM as the temperature went
up and down.

Actuators could also be built that did not wait for the trigger before
applying the data to its output. This would be normal for virtual
actuators that are used for control or setup functions that are not time
critical. These devices would respond to a trigger with a trigger
acknowledge immediately, if a trigger acknowledge is required, and the
trigger would not change anything within the TBIM. Another application
where the data is applied without waiting for a trigger is an actuator
being operated in the streaming mode. In this mode the actuator would
begin outputting data as soon as the first trigger is received and
following that it would output each sample as it was received until it
was disabled. Actuators can also be built that do not require the
receipt of a data set. These devices would have a built-in data set or
sequence of operations that they would perform when triggered. A simple
example might be a display that incremented by one for each trigger
received. A more complex example might be an actuator that implemented
an emergency shutdown sequence upon the receipt of a trigger.

## General transducer

The working group writing the standard did not feel that it had covered
every possible combination of operating modes and timing that
manufacturers might want to build. The general transducer is a way to
allow a transducer to be built that uses many of the features of the
standard but is not compliant with one of the operating modes or the
timing defined in the standard. This type of device cannot be guaranteed
to work with a standard compliant TBC without software changes to the
TBC. It must however be capable of working with a standard compliant TBC
without hardware modifications.

## Attributes

Attributes are used to define and/or control the mode of operation of a
transducer. They may be either built into the transducer and not be
changeable or they can be changed by means of control commands. There
are four classes of attributes defined in the standard. The sampling
attributes define the relationship of the data set and the trigger. The
buffers attribute defines whether or not multiple buffers exist on the
bus interface to the device. The end-of-data-set operation attribute
defines what an actuator does when it reaches the end of a data set. The
streaming attribute identifies whether or not the transducer can be
operated in the streaming mode.

### Sampling attributes

The sampling attributes determine the relationship between the trigger
and the sampling of the data by a sensor or the application of a sample
to the output of an actuator. There are two possible sampling attributes
allowed by the standard. They are the Trigger Initiated attribute and
the Free Running attribute. These two attributes cannot both be active
at the same time. However, it is possible to build transducers that can
be switched between these two operating modes. Table 3 lists the
enumeration\'s that are used in the TEDS to identify the sampling
attributes that the transducer can use.

Table 3 Sample initiated attributes

  ------------- ------------------------------------------------------------
  Enumeration   Description

  0             Not applicable (only useful in general transducers)

  1             This transducer is capable of being operated in the Trigger
                Initiated Mode as described in 1.8.1.1

  2             This transducer is capable of being operated in the Free
                Running Mode as described in 1.8.1.2

  3             This transducer can be operated in either the Trigger
                Initiated mode or the Free Running mode. However, these two
                modes are mutually exclusive.

  4 - 255       Reserved
  ------------- ------------------------------------------------------------

#### Trigger initiated attribute

A sensor being operated with the Trigger Initiated attribute true will
begin acquiring a data set immediately upon receipt of a trigger. An
actuator operated with this attribute true will begin applying the
appropriate data set to its output immediately following a trigger. If
the data set contains more than one data sample the timing of the
remaining samples are under the control of the logic within the TBIM. A
typical example of this type of device would be a sensor that uses a
sample-and-hold (S/H) circuit followed by a successive approximation or
flash type A/D converter. When the trigger is received the S/H would be
pulsed to store a sample of the data. At this time the trigger
acknowledge would be sent to the TBC. The A/D would then convert the
data to a digital form and store it in the data set memory in the TBIM.
After an interval specified in the Transducer Specific TEDS the process
of sampling, converting and storing a second sample would occur. This
process continues until all samples required for the data set are
available. At this time the sampling stops until another trigger is
received.

This attribute is useful for sensor systems that require a number of
devices to acquire samples nearly simultaneously. A global or group
trigger applied to a number of sensors would cause them to take a single
sample of data. The TBC can then read the data from each of the sensors
and know that it was taken at approximately the same time. Differences
will occur based in the distance between the TBIM and the TBC (trigger
signal propagation delay) and the difference in response times that
different sensors have to the trigger but for many systems this will be
adequate. It is also useful for actuators that need to operate at the
same time. The data is first written to the actuators and a global or
group trigger is issued to cause the data to be applied. For systems
that use multiple sample data sets the Synchronization communications
channel allows multiple TBIMs to be operated with a common clock.

#### Free running attribute

A sensor with this attribute active measures some physical parameter
autonomously and continuously when enabled. The data being acquired and
converted is discarded until a trigger is received. Once a trigger has
been received the next sample converted is stored in the TBIM as the
first word in the data set. Consecutive samples are stored in the TBIM
until the entire data set is completed. At this point the transducer
returns to discarding samples until the next trigger is received. A
typical example of this type of device would be a sensor that uses an
oversampling A/D converter such as a Sigma-Delta A/D converter. This
type of A/D converter must be operated continuously because the output
is built up by combining many samples in a digital filter. Any
interruption of the sampling will cause erroneous data. When the trigger
is received the sensor waits until the next sample is available from the
A/D converter. At this time the trigger acknowledge is sent to the TBC
if a trigger acknowledgement is required. The transducer will then read
the data from the A/D converter and store it as the first sample in the
data set. When the next sample is available from the A/D converter it
will be stored as the second sample in the data set. This process
continues until all samples required for the data set are available. At
this time the sensor stops storing samples and begins discarding them
until another trigger is received.

An actuator with this attribute active is armed to apply the data set
previous sent to when it receives a trigger. The actual time of
application of the data sample is under control of a clock within the
actuator. The clock can be derived from the Synchronization
communications channel or be independent of any other clock in the
system.

#### Buffers attribute

A transducer that allows a data set to contain more than one sample has
a buffer in it large enough to hold the entire data set. A transducer
that is capable of being operated in the buffered mode must have at
least two buffers available that can hold a complete data set. The TBC
can determine by reading the TEDS if the Transducer has more than one
buffer but not the number of buffers available. A sensor uses these
buffers to store the data from the time that the TBIM acquires it and
the TBC reads it. An Actuator uses the buffers to store the data sets
between the time that they are sent to the TBIM and when they are
applied to the output. Table 4 shows the enumeration used in the TEDS to
define whether or not buffers are available.

A sensor with this attribute active must have at least two buffers
available for storing data sets that have been acquired as the result of
triggers. Normally a sensor operated in this mode will have only two
buffers. One to store the data set acquired as the result of the
previous trigger and a second buffer to store the data set being
acquired as the result of the most recent trigger. However, if the
system cannot always get around to reading the data set before a new
data set is available more than two buffers can be used to cover this
possibility. If more than two buffers are used then it will become more
difficult for a TBC to determine the time that a sample was taken and
most TBC\'s are not expected to have this capability.

An actuator with the buffers attribute set has at least two buffers
available to store data sets being received from the bus. The data is
intended to be taken from these buffers in the order they were received
but the standard does not require this method of operation. The order
that the buffers are read and output is under the control of the TBIM.
The TBC can determine by reading the TBIM status that buffers are
available to be written but not which buffers are available or how many.

Table 4 Buffers attribute

  ------------- ------------------------------------------------------------
  Enumeration   Description

  0             The transducer has no more than one buffer available

  1             The transducer has multiple buffers available.

  2 - 255       Reserved
  ------------- ------------------------------------------------------------

#### End of data set operation attributes

These attributes determine the action that an actuator will take when it
reaches the end of a data set. Table 5 lists the enumerations that are
available from the TEDS. The standard allows actuators to be built that
can only be operated in one of these modes. It also allows this
attribute to be programmable. Note that this attribute performs no
useful function if only a single sample is included in the data set.

##### Hold attribute

This attribute may only be used with actuators. An actuator with this
attribute active shall apply all of the samples in a data set and then
continue to apply the last word in the data set until a new trigger is
received. This attribute can be used when it is desired for the actuator
to change to a new state and then hold that state indefinitely. If a
data set is used with more than one entry the change of state can be
made to follow some prescribed waveform when changing states. A type of
actuator that might use this attribute would be a valve controller that
had only two states. It can be commanded to either state and it will
remain in that state until another data set is received followed by a
trigger. However, if the data set contained more than one data point
this same actuator could be caused to change states several times in
some sequence defined by the data set.

##### Recirculate attribute

This attribute may only be used with actuators. An actuator with this
attribute active shall apply all of the samples in a data set to the
output, then return to the beginning of the data set and repeat the
application of that data set, repeating this operation until another
trigger is received. When another trigger is received the transducer
shall switch to the new data set when it reaches the end of the current
data set. This attribute would be useful for actuators that are used to
output a repetitive waveform. The waveform is described in the data set
which the actuator reads and outputs continually until another data set
is received or until it is disabled.

Table 5 End of data set operation attributes

  ------------- ------------------------------------------------------------
  Enumeration   Description

  0             Not applicable

  1             This transducer will hold the last value in the data set
                until another trigger is received as described in 1.8.1.4.1

  2             This transducer will recirculate through the last data set
                until another trigger is received as described in 1.8.1.4.2

  3             This transducer can be operated with either the hold mode or
                the recirculate mode. These two modes are mutually
                exclusive.

  4 - 255       Reserved
  ------------- ------------------------------------------------------------

#### Streaming attribute

A transducer with this attribute active will begin to acquire data or to
apply data when the trigger is received. Additional triggers are not
required to cause more data sets to be acquired or applied. Table 6
shows the enumerations that the TEDS uses to identify the capability
that a particular transducer has with regard to this attribute.

After the first trigger a sensor will transmit a new sample as soon as
it is acquired. This requires a dedicated data communications channel be
available to the TBIM. Multiple sensors within a single TBIM may share
this data channel but sensors in another TBIM cannot use it. A sensor
with the buffers attribute active will collect data sets and transmit
them as soon as they are filled. The buffers would be useful in this
case only if the time required to transmit a data set was longer than
the time between samples. Using data sets with more than one sample in
them avoids the overhead associated with single word transmissions.

An actuator with the streaming attribute active shall apply the data
written to it at the appropriate time after it has been received. An
actuator with the buffers attribute active shall immediately switch to
the next buffer after applying all of the data in the current buffer.

Table 6 Streaming attribute

  ------------- ------------------------------------------------------------
  Enumeration   Description

  0             This transducer is not capable of being operated in the
                streaming mode

  1             This transducer is capable of being operated in the
                streaming mode

  2 - 255       Reserved
  ------------- ------------------------------------------------------------

**Stuff to be moved but still included.**

Transducers and/or TBIMs may be operated synchronously through use of
synchronized clock sources. In this mode, the synchronization
communications channel may be used to generate synchronized clocks in
the TBIMs. The synchronized operating mode is defined in ([paragraph
number tbd]{.underline}).

[Action Item - types#1 (Synchronization Subgroup) Define a method of
operating multiple TBIMs synchronously.]{.mark}