It was common to hear and read
that AS-Interface was limited to a network length of 100 meter. This statement
dates back a long time ago and is now outdated. With Double Master, Repeater,
Bus Terminator, Tuner, and the recently developed 'Advanced Repeater'
Bihl+Wiedemann now offers several options to implement extensive linear as well
as two-dimensional networks. This article discusses the various possibilities
for network extensions and describes the important criteria to develop the
'right' configuration.
The
classical 100 m network
 |
| Fig. 1: Standard network with a length of 100 m |
In
general, the shorter a network the more stable it will operate. A short network
minimizes its susceptibility to errors. It may sound trivial but is still
correct: Since the mechanisms of AS-Interface and its components have been
primarily constructed for 100 m networks, this is the configuration where the
network is easiest to implement and works with the fewest flaws possible;
assuming components in perfect working order are used. Hence, every time the
100 m limit is surpassed the danger increases that (initially) single telegrams
are not immediately recognized by master or slave and thus have to be repeated,
or that the communication with an individual slave is temporary or permanently
interrupted. A crash such as this usually occurs quite abruptly when exceeding
a relatively exactly defined length limit that depends on the particular
configuration. In general, this is not a concern for a network length under 100
m and it is the reason why the classical 100 m network is so successful (see
'Theory', 4).
Several 100
m networksOne option to operate larger
AS-Interface networks is to divide an extended network into several
sub-networks, each with a length of less than 100 m, and to operate each
sub-network with its own master and power supply or to use a Double Master for
two neighboring 100 m networks.
This
solution has become a de-facto standard. It has all the advantages of a 100 m
network and represents a fast route to a robust, functional layout. An
additional advantage is that resistive cable related losses slaves with high
current consumption are kept small. Furthermore, sub-networks fully support the
modular configuration of many machines, as found for instance in material
handling systems with long conveyor distances and they also simplify system
layout and startup.
Theory: Classical 100 m network versus controlled long
distance
In five steps it is possible to understand how to extend networks
beyond the 100 m limit:
- Transmission mechanism: AS-Interface uses sin²-shaped
positive and negative voltage pulses for data transmissions. Receivers in
master and slaves are designed explicitly to recognize these voltage pulses.
This kind of coding is extremely safe because of its lack of higher harmonics.
Additionally, the receiver inspects the correctness of every single telegram on
the basis of several criteria. This is the root of the exceptional operational
safety of AS-Interface even in difficult environments.
- Limits: Yet, the laws of physics remain valid for AS-Interface
also: Device and cable impedances as well as reflections on the cable ends
deform the voltage pulses on the cable, such that – as soon as the
deformations are too excessive – they cannot be clearly identified by the
receivers in the devices and, thus, cannot be distinguished from interferences.
Consequently, this leads to an increasing number of 'requests' through the
system in form of repeated telegram transmissions and ultimately to failure of
a single or several components. This is exactly where every cable extension
design needs to start.
- Repetitions and errors: As always when talking about
AS-Interface it is necessary to clearly differentiate between 'repeats' and
'errors'. If the receiver in a master or slave has difficulties recognizing a
telegram because of deformed signals due to impedances effects or external
noise, it does NOT mean that faulty data can get to the control system –
AS-Interface is too well protected for this to occur. Instead, single telegrams
are repeated (prolonging the individual cycle by only 150 µs each). When
occurring infrequently these repetitions are absolutely normal and harmless for
the system. If several repetitions are unsuccessful the respective slave will
be removed from the configuration until the master can resume communications
with this slave. The master is required to report an error message at this
point.
- 100 m networks: One of the sacred cows of AS-Interface is the
demand for complete interoperability between all devices in the network.
Regardless of its position in the network, the manufacturer of the counterpart,
the number of devices in the network, and the network length, the receiver in
each device must be able to clearly recognize even distorted voltage pulses in
each telegram. This can only be guaranteed when strict requirements for device
and network impedances are specified. For a device these requirements are
enforced via the certification process. For the network this is accomplished by
limiting the length to 100 m. Goal of these requirements is to guarantee the
uninterrupted operation of each system according to its guidelines and thus
assure the reliability of AS-Interface for the user. Hence, the classical 100 m
network has proven to be successful in tens of thousands of
installations.
- Control: In many networks the rigid requirements for the
system impedance lead to inherent reserves; reserves in the form of
"under-utilized" repetitions. Today these reserves can be measured with simple
accessories like Analyzers and Tuners. The innovative new aspect is that the
frequency of repetitions is measured and evaluated. Thus, it is possible to
carefully approach the actual limits of a system and to deal in a controlled
way with deviations from the theoretical objective: a system free of
repetitions. This is the foundation of network extension by 'tuning' the
network impedance: 'controlled long distance'.
|
This solution cannot be used in
the following two cases:
- Data of
individual master (gateways) must be transmitted to the control system: Usually
this is done by employing an upper level bus. In systems without an upper level
bus or if the upper level bus cannot be used, this does not work. Examples for
these systems are controller applications or applications without an upper
level bus.
- Safety at
Work applications are easiest to implement when the Safety Monitor and the
AS-Interface safety slaves are on the same AS-Interface network. However,
workarounds are possible.
Signal
refreshing: The classical Repeater
 |
| Fig. 2: By using the Advanced Repeater a total network length of
up to 3 * 300 m and 2 daisy-chained Repeaters can be realized. |
Another
option is refreshing the network signals, provided that a receiver can
recognize them. A Repeater is placed within the 100 m limit. It divides the
network into two galvanic isolated segments, refreshes the data received at one
segment, boosts them to the specified signal shape, and transmits the data on
the second segment. An additional Repeater can be used at the end of the second
segment to extent the network by another standard length of 100 m for a total
of 300 m. The galvanic isolation into segments minimizes cable loss and
achieves exceptional stability under conditions of extreme electromagnetic
noise even in critical environments. (Even though AS-Interface has excellent
noise immunity, there are limits! See 'Theory', 3)
Since each
master expects the reply of the called slave during a specified time frame
(Tout) a combination of two daisy-chained Repeaters and a network
length of 300 m present the operational limit for the classic Repeater. If the
master does not receive a reply the communication is considered corrupted. Each
Repeater delays the transmission of the master telegram and the slave reply by
a certain small amount of time. Thus, daisy-chaining more than two classic
Repeaters would exceed the allowed timeout Tout. However, it is permitted to
use several parallel operating Repeaters configured in a star shape. This
allows two-dimensional – not linear! – networks of over 300 m
length.
Network
extension using Repeaters leads to the situation where telegrams of all 31 (or
62, respectively) slaves are read by one master that therefore processes the
data image of all inputs and outputs. In this case the use of a controller to
pre-process data and control the process is possible. Safety at Work networks
extending over several segments are also permitted.
Advanced
RepeaterThe
Advanced Repeater is the newest development in network extensions. The
following two features differentiate the Advanced Repeater from the classic
Repeater: Firstly, transmitter and receiver are further optimized and tuned to
each other enabling the clear transmission and recognition of signals even in
difficult situations. Secondly and most importantly is the minimization of the
delay time of the signals when passing through the Repeater. While the
classical Repeater delays each pair of telegrams by slightly less than 14
µs, the Advanced Repeater only causes delays of 9 µs. This
seemingly small difference facilitated through a new concept enables the design
of larger networks (see below).
(Detail:
The classic Repeater uses approximately one positive voltage pulse to interpret
a signal as an AS-Interface telegram and to transmit it to the next segment.
The Advanced Repeater starts this process considerably earlier. Initially, this
introduces the risk to misidentify noise as the beginning of a telegram. But
the Advanced Repeater instantly uses the error detection mechanisms of
AS-Interface to differentiate between telegrams and noise interference. If
interference occurred, the transmission is interrupted immediately, enabling
the next receiver to clearly recognize the transmitted pulses as defective.
This way the accurate transmission of telegrams is guaranteed.)
The
alternative: Signal corrections
The alternative to signal
refreshing are corrections on the system impedance and thus on the signal
shape. The rigid requirements for the system impedance (see 'Theory', 5) lead
to reserves in most networks. Violations against the 100 m rule frequently
happened consciously or unconsciously, without causing problems in individual
cases. This action can be compared to 'blindly' crossing a border that was
consciously set earlier. Since Analyzers and Tuners are available as simple
diagnostic devices to monitor the communication quality of the network, these
reserves can be used and the network can be 'tuned.' Consciously changing the
system impedance and automatically controlling the result without compromising
the safe operation of the system can achieve larger networks.
Bus
TerminatorThe Bus Terminator is a fixed
impedance, primarily used to reduce reflections along the cable. Hence, the Bus
Terminator is installed far away from the power supply. In many cases it is
possible to expand the network to about 200 m if a Bus Terminator is
employed.
Tuner and
Diagnostic Tuner
 |
| Fig. 3: Diagnostic Tuner |
When using
the Tuner¹ the network impedance is individually optimized. In a learning
phase the Tuner automatically varies its internal impedance over a large range
while measuring which slaves can be reached at all and how many errors they
produce. At the end of the learning phase the impedance value resulting in the
lowest number of telegram repetitions is held. This value can be permanently
saved for future reference. Additionally, testing results are displayed by
using a 'traffic light' similar to the one used for the Analyzer: the red,
yellow, and green LEDs signal the actual tolerance range of the system. Since
the Tuner is usually installed permanently in the network, the monitoring
process is active all the time. With the Tuner a controlled network extension
of up to 300 m is possible.
Any
possible concerns by the user that the system could become instable when the
remaining reserves have been exhausted are addressed by the LEDs. Instead, the
system becomes even safer because of the optimization of the impedance and the
qualified monitoring of the communication.
The
Diagnostic Tuner² automates monitoring even further: it is used as a slave
sending the results of the continuous monitoring process to the master and
control system. Here the application program can automatically react with
countermeasures or requesting preventive maintenance.
Large
networks: AlternativesBy using the devices described
above a greater variety of techniques for large networks extensions, in
addition to dividing the network into sub-networks as explained in the
beginning, is available. In many cases network extensions using these
components have economical and technical advantages, as long as the following
two requirements are observed:
- Generally,
in critical environments short networks are less prone to interferences than
longer networks. This holds true for AS-Interface also, but is rarely of any
consequence because of its low susceptibility to interferences. Dividing the
network into galvanically isolated sub-networks or segments (utilizing
Repeaters) usually are of theoretical advantage only, but should still be
considered.
- If a system
has a cable length of over 100 m the safe range as described in the original
specifications is exceeded. In general, using the tools described above makes
this possible without endangering the system. But this should be verified since
this control action represents the key to all network extensions (possibly with
the exception of pure Repeater networks). Hence, it is strongly recommended to
either continuously monitor the system with a Tuner or to selectively examine
the system with an Analyser³ installation. Both control techniques prevent
'surprises' from happening ('controlled long distance' see 'Theory',
5).
- Total
length
- Segment length
- Position and protection class of
master, power supply, extender, and Repeater
- Power
consasumption of the slave
- Costs
|
| Fig. 4: Criteria for selecting a configuration |
Observing
these guidelines allows the unproblematic transition to larger networks. Bus
Terminators, Tuners, Repeaters, and Advanced Repeaters can be combined. The
details of the system define the optimum configuration. Figure 4 lists the most
important criteria:
Total
and segment length are the basic data for every system (see fig. 5-9
below). While designing a network it must be remembered that the possible
length for a network using Bus Terminator and Tuner are given as 'up
to'-values: for instance 'up to 200 m' or 'up to 300 m'. For most networks
these values are correct. But there are cases where a Tuner must replace a Bus
Terminator before reaching the 200 m mark and where a (Advanced) Repeater must
replace a Tuner before getting to 300 m. This is the logical conclusion when
the true limits of the system are approached in a safe and unproblematic
fashion. The limits now depend on the details of the specific network (see
'Theory', 5).
Selection and placement
depend on the protection class and position of the components: power supplies
usually offer protection class IP20 and need to be installed in a cabinet or
junction box. Masters and Repeaters are offered in both protection classes.
When installed close to the power supply, IP20 devices lend themselves to
mounting in a single junction box. While master and power supply can be freely
placed, Bus Terminator and Tuner must be positioned far from the power supply,
possibly at a distant branch of the network. Hence, Bus Terminator and Tuner
are always constructed to meet protection class IP65.
To minimize
cable losses when individual high current consumption slaves are used it
may be necessary to connect the power supply in close proximity to the slave.
This can lead to the separation of master and power supply.
The
costs for different configurations depend on the selected devices, the
design, and the protection measures for the devices. They can vary considerably
even for similar network lengths. Costs are discussed in the following
chapter.
Typical
cases
 |
Fig. 5: Splitting into sub-networks
|
 |
Fig. 6: Signal refreshing with classical repeater
|
|
 |
| Fig. 7: Signal correction with bus termination (up to 200 m) or
with tuner (up to 300 m) |
Networks
with a length of up to approximately 200 mIf the 100 m standard network
(fig. 1) is exceeded four alternatives can be suggested: splitting into
sub-networks (fig.5), Repeater (fig. 6), Bus Terminator (fig. 7), or using a
Tuner. The standard solution, as described above, is splitting the network into
2 sub-networks employing a Double Master (in the middle of the application) or
using two separately installed single masters.
Bus
Terminators result in the lowest device costs. It only costs about 8 % of an
additional gateway with power supply or 15 % of a Tuner or a combination of
Repeater and power supply, respectively. In this case the network should be
examined with an Analyzer during the start-up phase. If a Tuner or an Advanced
Tuner is employed this examination is performed constantly.
A design
with Repeater is only suggested in the following two cases: either if the
network runs through an environment with extreme EMC conditions or if users
with high current consumption are placed in the first and second
segment.
Networks
with a length of up to about 300 mThe same considerations apply to
networks up to about 300 m with the only difference that instead of a Bus
Terminator a Tuner or two Repeaters with additional power supplies must always
be used. A solution using a Tuner costs about 50 % of the design with a
Repeater. A solution '1 Repeater plus 1 Bus Terminator' is possible with the
disadvantage, that the continuous monitoring of the network is then
omitted.
Networks
with a length of more than 300 mTo span a network of more than 300
m at least 1 Repeater and an additional power supply as well as a Tuner or Bus
Terminator or one additional Repeater, depending on the length of the system,
is necessary.
More than
two daisy-chained classical Repeaters or three 300 m segments separated by two
classical Repeaters are not permitted. Signal delay times in the Repeater and
in the extended segment exceed the permitted time-out duration between master
call and slave response at the location of the master (see above). By using the
classical Repeater the network extension cannot exceed 600 m.
Networks of
more than 300 m with Advanced RepeaterThis is the primary field of
operation for the Advanced Repeater. Its delay times are short enough to permit
the design of three 300 m networks with two Advanced Repeaters. This is also
applicable for Safety at Work applications: In this case the resulting delay
times of less than 60 µs are short compared to the guaranteed
Safety-at-Work response time of 40 ms.
This makes
AS-Interface a genuine long distance network. Fig. 9 shows an extended network
of up to 1500 m linear extension (using Tuners) that can be further expanded as
a two-dimensional network if star shaped branches are added. One specific
feature must be observed for the start-up of such a system: During the teaching
process each Tuner can only affect its own segment but evaluates the
information of all slaves. Consequently, this may necessitate starting up the
network segment by segment.
 |
| Fig. 8: Large network with two double daisy-chained Advanced
Repeaters and Bus Termination |
Final remarks: In spite of the
original length limitation of 100 m for AS-Interface, today a network extension
by more than one order of magnitude is easily possible with thorough planning.
This includes monitoring the communication quality to obtain the number of
telegram repetitions. Is this number sufficiently low ('green' LED light on
Tuner and Analyzer) there is no reason to refrain from using large, extended
AS-Interface networks, for instance for conveyor applications and in plant
construction.
|
