Interview Questions.

Question 1. Why Is The H1 Bus Speed 31.25kbps, Not Something Different, Suitable For Closed Loop Digital Control?

Answer :

The H1 bus speed isn't always suitable for equipment control, but it's miles truely properly for closed loop manipulate in method automation, where manipulate cycle times of 0.25 to 1.0 seconds are generally used. A very tough estimate says you could get 50 transactions in keeping with 2nd. If you go away half of of these for unscheduled messages, you could have four samples in line with 2d for up to six manage loops, or 1 sample according to 2d for up to twenty loops (which is probably greater than you would want to position on a single bus segment).

The specific determine of 31.25kbps changed into chosen as a binary fraction of a 1MHz crystal oscillator (divide by way of 32).

Question 2. What Sort Of Waveform Is Used In Manchester Coding? Is It Square Or Trapezoidal? How It Is Generated?

Answer :

Logically, Manchester coding produces a rectangular wave. For H1, IEC61158-2 specifies a modern sign of approximately +/- 9mA into a 50-ohm load. But to lessen the bandwidth of the transmitted sign, IEC61158-2 specifies a trapezoidal waveform with upward push and fall times among zero.12 and 0.25 of a chunk time.

If the device is bus-powered, it sincerely modulates its DC contemporary draw with this signal. Non-bus-powered gadgets may take 10mA, in reality to offer a base for this modulation. It is allowed for a device to take much less than 10mA, then ramp up to 10mA while it desires to transmit. In exercise, right now, all bus-powered gadgets take greater than 10mA.

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Question 3. Is The Digital Signal Susceptible To Noise?

Answer :

Yes, but no longer a lot as an analogue sign. The fieldbus pair is balanced approximately ground, and (using right cable) is twisted, and shielded. All those features reduce the extent of caused alerts into (or out from) the bus. Unlike analogue signalling, low tiers of interference don't have any impact whatever on the accuracy of virtual transmission. Interference take a look at standards are detailed within the IEC61158-2 standard, and endorse undetected mistakes rates of simplest 1 message in 4 months, despite rather severe interference degrees.

Question four. Can You Reuse The 4-20 Ma Wiring Of An Existing Plant?

Answer :

Yes, surely. Although existing wiring was now not designed to carry excessive-pace virtual statistics, the H1 signal is robust sufficient to be carried on typical current wiring.

However, there are  greater considerations:

distance is confined to 400m for a twisted pair, 200m for untwisted cable
unshielded cable won't be usable if neighborhood interference is a problem.
The distance limit is a end result of the cable’s function impedance being unspecified, so reflections might also occur on the ends of the cable, despite the fact that terminators are used. (For quick cables, any reflection isn't a whole lot not on time from the authentic sign, so does not cause severe distortion of the waveform.)
Question 5. Do Iec 61158-2 And Isa S50.02 Allow Redundancy In The H1 Bus?

Answer :

Yes. But up until now, manufacturers have selected no longer to enforce this selection.

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Question 6. What Kind Of Redundancy Is Possible For Fieldbus System?

Answer :

According to the IEC 61158 specs, redundancy is possible in H1 bus segments. However, producers have chosen now not to put into effect this functionality, on the grounds of fee, bearing in thoughts that the quantity of devices on a single phase can be pretty confined in vital method applications (maybe most effective 4 or 6 – one or two control loops-really worth).
If devices had been to provide redundant fieldbus connections, there would additionally be dialogue approximately how a ways up the protocol stack the redundancy need to move in the device; even to the extent of duplicating the inner microprocessor and maybe the sensor or actuator. It may be very difficult to solve those questions in a way first-class for a huge variety of applications. It can be argued that if a selected dimension or actuator is so crucial, it's miles better to in reality upload a entire duplicate tool, and join it to a exceptional fieldbus segment.
(Note that screw ups inside a fieldbus tool have to no longer prevent continued verbal exchange via different devices on the phase.)
It become constantly expected that the high-velocity H2 buses might use redundant cabling, considering the quantity of data carried is a good deal greater, and therefore more vital to shield in opposition to loss. The HSE (excessive-velocity ethernet) bus now proposed rather than H2 will typically be redundant, simply as in industrial networks.
Question 7. What Are The Various Modes Of Redundancy For Different Topologies Of Fieldbus System?

Answer :

The use of redundancy in a fieldbus machine may be motivated by means of its topology, but a good deal extra by way of the character of the utility. In particular, the consumer may need to do not forget redundant electricity substances (particularly if one strength supply feeds several bus segments). Host device fieldbus I/O channel hardware could be redundant (providing two unbiased paths to the equal bus cable), for improved reliabilty on each segment. Similarly, gateways between segments might be made redundant, if their endured operation is regarded as vital.

In a fieldbus the usage of control within the discipline, an vital feature is the LAS (link energetic scheduler) which controls bus traffic. The host (if there is one) usually offers this LAS characteristic. To live to tell the tale loss of the host system, it'd be well worth making sure that a lower back-up LAS characteristic is to be had in one or extra of the sector devices.

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Question 8. Can Separately-powered And Bus-powered Devices Be Mixed On The Same Segment Of An H1 Bus In An Intrinsically Safe (is) System?

Answer :

Yes, no problem. The one at a time-powered devices would use explosion-proof or different safety techniques, however would also offer a blanketed IS connection point for the fieldbus.

Question nine. How Is Half-duplex Communication Possible, When Two Instruments In A Segment Want To Send Data Simultaneously In Opposite Directions?

Answer :

The guidelines for get admission to to the bus save you two gadgets ever transmitting at the equal time. Even when two gadgets have some thing to say, they ought to every wait until they're requested for a selected item of information, or acquire the token from the LAS to permit an unscheduled message.

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Question 10. How Can You Connect Or Disconnect A Fieldbus Instrument From The Bus Without Affecting The Performance Of The Rest Of The Instruments? What Sort Of Disturbance Is Introduced During Addition Or Disconnection Of An Instrument? How Can It Be Detected?

Answer :

IEC61158-2 specifies that connecting or disconnecting a tool might also disturb the bus electrically for no longer more than 10ms. That means that one message might be corrupted. Each message consists of a sixteen-bit error-check-polynomial code, which truly ensures detection of the corruption by the receiver. (Depending on the sort and use of the message, the receiver would possibly ask for a repeat, or would possibly surely forget about the message and await it next time around. Where the sender needs to understand approximately this, an “acknowledged” message type may be used, and the receiver might honestly now not renowned the misplaced message.

Question eleven. What Will Happen If The Trunk Or A Spur Is Short-circuited?

Answer :

Bus-powered devices will lose energy, so will no longer characteristic. (Valve actuators need to routinely undertake a secure function.) On return of strength, they need to restart routinely, with or without operator intervention as desired by means of the consumer.
Communication could be misplaced (even to non-bus-powered gadgets). On elimination of the fast circuit, the network will get better automatically.
The energy supply should encompass present day restricting, so it might not be broken, and will get better while the short is removed.
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Question 12. Why Has The H2 Bus Standard With 1mbps And 2.5mbps Been Withdrawn? Is It Only To Make It Compatible With a hundred Mbps Fast Ethernet And Tcp/ip?

Answer :

Basically, yes! The purpose is to take benefit of the commercial development of ethernet, which has produced very low-cost, excessive-overall performance hardware. (But be careful for the gap limits on ethernet, unless you operate fibre-optic cable.)

TCP/IP can be the premise of the HSE fieldbus, but isn't always adequate as it stands; improvements are needed to aid the desired predictability of performance.

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Question thirteen. What Is The Difference Between A Sensor Bus And A Fieldbus? Can Control Instruments (transmitter Or Control Valve) Be Placed On A Sensor Bus?

Answer :

The time period “sensor bus” is used for less difficult buses, where the main function is sincerely the switch of a single variable representing a dimension or output valve function. On/off (contact) I/O is well dealt with, also simple transducers.
A complete “fieldbus” is designed to allow the switch of a lot more, and more complex, facts, inclusive of statistics structures with greater than just a unmarried variable, and tool management capabilities. Any tool needing great configuration for a specific application is better served by using a fieldbus.
Transmitters and valves may fall into either category. A simple temperature transmitter or valve positioner can actually be designed for a sensor bus. But the more complex functionality predicted in a present day transmitter or valve (inclusive of the inclusion of diagnostics, or manage functions) truely needs a complete fieldbus.
Question 14. What Is The Point Of Removing Several Layers Of The Osi 7-layer System?

Answer :

OSI layers three to six are lacking in maximum fieldbuses (and in HART). These are the Network, Transport, Session and Presentation Layers. The services furnished with the aid of these layers in a complete protocol stack are worried with functions of a complete wide location community, where (logical and physical) connections are made and broken, addressing and routing features ought to cover a couple of networks, statistics packets can be corrupted or misplaced, and information can be carried in more than one codecs. In preferred, these offerings aren't wanted, or may be greatly simplified, in a fieldbus where a set set of gadgets is completely connected to a single community.
Fieldbuses generally offer only a bare minimal of those functions, and consist of them inside the ones layers which are completely applied. Different fieldbus protocols range within the volume to which such capabilities are provided (specially, assist for multi-phase community addressing and segmented record switch). Features which are not furnished within the protocol stack can be implemented inside the device or host application itself, however that is rather much less first-class, on the grounds that one of a kind implementations won't inter-function nicely.
Leaving out unwanted capability consequences in less complicated embedded software program or hardware. Since every layer needs to be specified both by means of its feature and its interfaces with the layers above and under it, this additionally simplifies the protocol specification.
Question 15. What Are The Different Standards For Different Layers Of Fieldbus?

Answer :

User layer
Function Blocks: FF-890, 891, 892
Device Description Language: FF-900
Application layer: IEC61158-five, 6
Datalink layer: IEC61158-3,four
Physical layer: IEC61158-2
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Question sixteen. What Is The Difference Between The Standards Iec 1158-2, Iec 61158-2 And Isa S50.02?

Answer :

There is no difference! IEC lately renumbered their requirements, so 1158 have become 61158. The ISA SP50 work has carefully paralleled the IEC fieldbus task, and the ensuing requirements are same in technical content.

Question 17. How Are Mtbf And Mttr Calculated In A Fieldbus System?

Answer :

For the communication route MTBF, you will want to realize which device failures ought to have an effect on it, and their probability. You additionally need to determine whether or not continued verbal exchange with a bunch machine is necessary, or whether the machine can keep to operate without it. Common-factor screw ups together with a non-redundant electricity supply might be the predominant item. With redundant strength substances, the chance of physical damage to the bus might be more crucial.

MTTR need to be estimated on the idea of alternative of failed devices, rather than repair. If a spare tool is available, this can be performed within a few seconds (as some distance as the bus connection and verbal exchange capabilities are concerned; physical installation is probably to be the limiting factor).

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Question 18. Can Switches Or On-off Valves Be Connected On The Same Bus As Continuous Measurement Or Control Devices? If This Is Possible, How Is The 0 And 1 Signal Encoded?

Answer :

Yes, they can. In the IEC and FF protocols, discrete variables are encoded as eight-bit unsigned integers. I assume the fee might be either zero or 1. As for all measurements, an eight-bit fame cost is continually sent with it.

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Question 19. Is It Possible To Place A Thermocouple Or Rtd Directly On Fieldbus?

Answer :

No. You could always want transmitter electronics to enforce the communication protocol. (However, a head-set up transmitter may be possible.)

Question 20. How Can A Plc Be Integrated With A Fieldbus System? Can The Programming Station For The Plc Be The Same (single Device) As The Engineering Station For Fieldbus?

Answer :

For IEC or FF protocols, the expectancy is that a PLC would be linked at the H2 or HSE (high velocity ethernet) level, because it would probably be dealing with a big amount of facts. A fieldbus engineering station may be directly related to an person H1 bus section, or (better) it could be included into a host (“DCS”) tool, serving many segments. When HSE is used, this identical connection may also be used to application the PLC.

However, despite the fact that unique software and communication paths must be used, with Windows NT or a few comparable multi-tasking working system, there is no motive why a unmarried person interface need to no longer offer both capabilities.

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Question 21. Can A Plc Be Interfaced Directly With H1 Bus?

Answer :

It is quite in all likelihood that PLCs will use H1 fieldbus as an I/O path to size and actuator field devices. I suppose it's far much less in all likelihood that a PLC could offer an H1 connection to a host, with the PLC itself appearing as a discipline tool. HSE (high velocity ethernet) would generally be greater suitable at that degree.

Question 22. How Can A Fieldbus System Be Implemented In An Existing Hart System?

Answer :

HART and FF have one amazing feature in common – they each use Device Descriptions (DDs). This makes it feasible to mix HART and FF in a single device and get equal capabilities for device configuration, diagnostics and instrument control.

Hardware for a mixed gadget wishes some care. Either, a bunch should provide both varieties of I/O system, or the host can use only FF, with a HART-to-FF gateway to connect with the existing HART devices. But be careful of the speed difference! In some packages, HART verbal exchange isn't always fast enough to use for the control variables, mainly if multidrop HART is used. A gateway might also, consequently, must consist of analogue I/O, with conversion to virtual form for the fieldbus connection. In addition, it isn't always but clean (at least, to me!) how a HART tool have to be “disguised” as an FF device through translating its statistics structures, preferably based on its HART DD. As a ways as I understand, no HART-to-FF gateway but exists.

Question 23. How Is ‘test Time’ Defined In A Field Bus System?

Answer :

Communication to and between enter and output devices in an FF bus is controlled, millisecond via millisecond, through the LAS (link energetic scheduler). The sequence of conversation activities, which the LAS manages, is set up with the aid of the machine engineer at some point of design or commissioning. A ordinary configuration tool will will let you set up a cycle time for every manage loop (whether or not it resides in the host, or in the field gadgets, or partly in each), and will mechanically set up the required verbal exchange paths and schedules.

 Because of the way the LAS works, all cycle times on one bus phase ought to be fitted right into a fundamental “macrocycle”. Probably, in most cases, this may additionally be the overall experiment cycle time for manipulate loops on that section.

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Question 24. How Is Speed Affected When All Instruments In A Segment Start Talking To Each Other?

Answer :

For scheduled traffic, such as any used for control purposes, the timing is pre-configured and usually occurs as anticipated, beneath the control of the LAS. This is completely unaffected via any unscheduled visitors. In fact, gadgets don’t just “begin speaking”! What occurs is that the LAS gives each device a hazard to talk, in flip, all through the time, in each cycle, allocated for unscheduled messages. (If there isn’t enough time before the subsequent scheduled message, they ought to wait until later.)

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Question 25. When Are ‘scheduled’ And ‘unscheduled’ Communications Used?

Answer :

Scheduled communications are repeated on a normal pre-configured cycle, and encompass the ones messages wearing measurements or outputs used for manipulate purposes.

Unscheduled communications are fitted into idle times within the scheduled message cycle, and are used for almost the whole thing else: operator show updates, tuning parameter and mode changes, technique alarms, fashion reports, configuration modifications, diagnostics, time distribution. (Unscheduled messages can be prioritised; for instance, alarms can be treated urgently.)

Question 26. What Are The Points To Take Care Of, When Placing Instruments In A Particular H1 Bus Segment?

Answer :

geographical format
effect of loss of the phase
electricity intake (if bus-powered)
Question 27. Who Controls The Traffic (token) On The Bus?

Answer :

The LAS (hyperlink active scheduler). In the occasion of its failure, a again-up LAS (if present) takes over routinely.

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Question 28. Where Is The Physical Location Of The Las (link Active Scheduler)?

Answer :

This is a machine dressmaker’s (and tool manufacturer’s) preference. If there may be a number DCS, the LAS will normally be within the fieldbus interface card. If not, it can be in a field device (however only “hyperlink master” devices offer this capability).

Question 29. How Can Mcc Status Be Accessed On A Fieldbus System?

Answer :

Any discipline device functionality you may imagine, can be carried out in a fieldbus tool. Access to any particular object or items of data is described in the Device Description (DD), which a bunch need to use to interrogate the tool and interpret the response.

Some common tool kinds have “profiles”, the usage of a trendy DD, to lessen the paintings involved in improvement, and to attain a degree of interchangeability. But I don’t agree with an MCC profile has but been posted.

Question 30. How Do You Calibrate A Fieldbus Instrument?

Answer :

This is truly no distinct to every other smart transmitter, except that now, you don’t ought to fear approximately the four-to-20mA analogue sign! You simply want to examine out the measurement over the bus with a appropriate host tool.
You want a supply of the method variable: stress, temperature (or ohms or millivolts), pH, or anything. Typically, when you join a acknowledged process cost to the sensor, you may send a unique message to the device, together with the price you examine, and what it must be. The discipline device will then alter a saved parameter to make it so. Two values, close to the ends of the tool’s running range, are commonly enough, though a few devices may additionally use a extra complicated multi-factor calibration inside the manufacturing facility to modify linearity corrections, or even a temperature cycle to modify temperature correction.
Automatic calibrators will step thru this technique with handiest minimum user involvement.
Question 31. What Is The Length Of The Fieldbus Message? Is It Different For Discrete And Continuous Variables?

Answer :

The total message period can vary from eleven to 276 bytes (known as “octets” within the fieldbus international), relying on the message type and facts content. There aren't any more start and stop bits, so at 31.25kbps, every byte takes 256 microseconds. The entire message should consequently take anything from approximately 3 to 70 milliseconds. Reading a unmarried variable, whether or not it is discrete or continuous, could be close to the shorter give up of this range. For performance, several variables can often be collected into a unmarried message.

Question 32. Is Fuzzy Logic Control Introduced As A Standard Function Block In Fieldbus?

Answer :

Not as far as I know. But proprietary types of this characteristic must be to be had  from essential producers.

Question 33. How Can Cascade, Ratio, Split Range, Feedforward And Override Control Be Achieved Through Fieldbus?

Answer :

The first set of trendy FF characteristic blocks include PID with far flung setpoint input and feedforward input, Ratio, Bias/Gain (for break up range output), and Signal Selector (for override manipulate).

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Question 34. Can Htg (hydrostatic Tank Gauging) Be Achieved Using A Single Fieldbus Instrument? If So, How?

Answer :

It need to be viable. Multivariable transmitters are properly-supported by means of the IEC and FF protocols. A fieldbus HTG device may connect directly to multiple sensors (stress, temperature) or might possibly use a neighborhood multidrop HART bus to attach to standard HART transmitters. (These may also be powered from the fieldbus, via an embedded dc-to-dc converter in the HTG device.)

Question 35. How Can Multiple Variables Be Measured Using A Single Fieldbus Instrument?

Answer :

No problem. A tool may have as many sensors because the dressmaker desires. And further derived variables can be calculated from these if preferred. Each measured or derived variable is represented by way of its personal Analog Input block, and is hence to be had to study over the fieldbus. In specific cases, it can be beneficial to mix numerous measurements into a single facts array for extra green messages.

Question 36. Can A Control Block (e.G. Pid) Be Inserted In A Transmitter Instead Of In A Control Valve? If So, How Will Performance Be Affected?

Answer :

Yes, it simply can. There isn’t much difference in performance – there's one more message in line with cycle, to balance the PID output fee to any limits imposed by way of the Analog Output Block inside the valve positioner (to avoid imperative wind-up).
In a cascade manage utility, it's far pretty probably that the primary PID will be positioned inside the transmitter for the number one variable.
Should an I/P converter be built into the (fieldbus well matched) control valve? If now not, how would an I/P (from a extraordinary vendor) be examined as interoperable 
There virtually is no “I/P converter” any extra. I would expect the “digital-to-P” characteristic to be constructed into the fieldbus valve controller, so long as the valve itself stays stress-actuated. The controller might or won't be physically established with the valve itself. (It could probable want position remarks from the real valve stem.)
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Question 37. What Does The Control Hierarchy For A Fieldbus System Look Like (with Control In The Field)?

Answer :

you can placed the regulatory control loops inside the field in case you wish, with benefits of overall performance and reliability. Complex control capabilities will live inside the DCS. Supervisory functions, information analysis and hyperlinks to management systems live where they're now, however perhaps take more facts immediately from subject devices by way of skip-via through the DCS.

The right degree to place any characteristic is the lowest at which all of the important data is available. (For example, it’s in all likelihood not beneficial to place an interactive controller out on a bus segment, if it desires facts from devices on every other bus section.)