Boiler Efficiency

Boiler Efficiency is a term which establishes a relationship between energy supplied to the boiler and energy output received from the boiler. It is usually expressed in percentage. As a general rule,
Boiler efficiency (%) = heat exported by the fluid (water, steam) / heat provided by the fuel x 100.


Types of Boiler Efficiency

The efficiency of a boiler may be classified into following three major types:

1. Combustion Efficiency
2. Thermal Efficiency
3. Fuel-to-Steam Efficiency

Amongst all the three above mentioned boiler efficiencies, the fuel-to-steam efficiency is considered to give the most accurate representation of boiler efficiency on the whole. This is due to the fact that fuel-to-steam efficiency takes into account, the radiation and convection losses while performing efficiency calculations. Typically, it is the job of the boiler manufacturer to define boiler efficiency so that any type of economic analysis could be done properly.

Boiler

Major Parts

A boiler normally consists of following four major sections:

1. Burner: This part of the boiler is responsible for commencing the combustion process inside the boiler. It receives electronic signals from the temperature sensing devices like thermostats, as and when the heat needs to be generated out of the system. In most cases, a fuel tank is positioned just next to the boiler for providing fuel source. A filter system is employed to pump the fuel to the boiler. This fuel is then sprayed as fine particles via a nozzle mounted on the burner which tends to produce ignition and combustion reaction inside the chamber.
2. Combustion Chamber: The burning of the fuel takes place inside a combustion chamber which is generally constructed of cast iron material. This combustion chamber is designed to experience intense heat energy since the temperature inside the chamber tends to reach very higher limits in a very short period of time. Heat produced inside this chamber is then finally transmitted to the heat exchanger unit.
3. Heat Exchanger: A sequence of flue passages is used to filter the liquid inside the combustion chamber in a boiler arrangement. “The pressurized, boiling water is then pumped through pipes to baseboard heaters or radiators, which give off the heat energy produced in the boiler.”[4]
4. Fuel Sources: For its functioning, a boiler consumes variety of fuels such as heating oil, kerosene oil, liquid propane etc.

Main Features

Key features of boilers include:

• The efficiency offered by a typical boiler system falls between 75% and 85%. However, advanced boiler systems made up of copper heat exchangers have now been designed which are capable of providing as high as 90% efficiency.
• Besides heating applications, a usual boiler system can be employed in many other areas like steam-operated locomotives, external combustion engines and power generation plants.
• In order to get high efficiency out of boiler systems, they must be periodically cleaned and maintained. The maintenance process generally includes activities like making combustion chamber free from useless remains and debris, changing of components such as gaskets and checking of temperature.
• The installation and maintenance of boiler systems must be carried out by professionally skilled and certified personnel. Inappropriate installation and operation of boilers can result in very hazardous effects owing to the severe heat energy involved with them.
• The efficiency of a boiler system also depends upon the total heating surface encompassed by the boiler. More the heating surface more is the efficiency. The heating surface of a boiler is generally represented in square feet.
• As a general rule, horse power rating is used for small boilers and thousands of pounds of steam rating is used for large boilers in industrial applications.

Types of Boilers
In general, there are two major types of boilers available for industrial use:

1. Fire Tube Boilers: In this design, the boiler tubes are surrounded by the liquid which needs to be heated whereas the hot gases produced out of combustion reaction are passed via these boiler tubes.
2. Water Tube Boilers: Their design is just opposite to fire tube boilers design. Here, the hot combustion gases are circulated around the boiler tubes whereas the tubes enclose the water to be heated.

Case Study - Carlo Gavazzi Integration

We integrate Variflex RVCF and RVEF drives on ANY Automation Protocol

CAS specially configured a ‘Gateway’ which connects one or more trunks of drives for conversion to BACnet, Lonworks, Rockwell, Omron, GE, Johnson Metasys N2 and many more.

You may now expand your reach and make your customers happy by offering Variflex drives on ALL protocols.

Take advantage of our special Carlo Gavazzi Variflex config software for quick and effective integration.

The customized configuration software lists parameters and makes selection for gateway mapping onto BACnet (or other protocols) easy.

CAS connects Carlo Gavazzi Energy and Power equipment on ALL Automation Protocols.

If a customer wants to buy Carlo Gavazzi UDM60 Panel Meters and Power Quality Analyzer Type WM5-96

• He/She wants to integrate these with his Metasys N2 controller
• But the Panel meters and the Power Quality Analyzer support ONLY Modbus RTU

No worries, just make the sale!!! We will do the protocol integration.

Why Us?

• Safe Hands: We have over a decade of experience in protocol integration
• We have been closely working with Carlo Gavazzi and know their equipment very well
• Custom tools Specially for Carlo Gavazzi equipment: our long experience with Carlo Gavazzi has enabled us to develop special config tools for their equipment

Ultrasonic Level Sensors - Main Features

Key features of ultrasonic level measurement devices are listed below:

• These sensors use frequencies in the tens of kilohertz range; transit times are ~6 ms/m. The speed of sound (340 m/s in air at 15°C (1115 fps at 60°F) depends on the mixture of gases in the headspace and their temperature.
• The speed of sound waves traveling via the medium which is normally air is prone to get affected by changes in the working temperature. In order to compensate for these changes in temperature and resulting changes in sound wave speed, the level measurement system must include a temperature sensing device. This will help in correct distance calculations and hence accurate level detection results.
• In cases where heavy foam is found on the surface of the process fluid, the use of ultrasonic level measurement techniques are usually avoided since this foam work as a sound absorbent. Consequently, the sound wave will get scattered resulting in non reception of the exact signal by the sensor. This will cause improper functioning of the measurement system.
• Excessive surface turbulence of the fluid can result in wide fluctuations in level measurement results. To avoid this issue, one may employ a damping correction or a response delay with the device.
• Good level measurement requires that the reflected echo from the fluid surface returns back in a straight line to the sensor. Besides, it calls for proper installation of ultrasonic transmitter over the tank. The transmitter should be mounted in such a way that the inner composition of the vessel or tank doesn’t get in the way of the signal.
• In level measurement fields where sound waves get influenced by factors like foam and vapor etc., one can connect a beam guide to the sensor for improving performance of these devices.
• Ultrasonic level measurement technique proves to be quite costly when employed for point level measurement applications.
• In case of fluids which are less viscous, one can execute point level measurement via a technique called ultrasonic gap technique. In this method, a transmit crystal is activated on one side of a “measurement gap” and a receive crystal listens on the opposite side. The signal from the receive crystal is analyzed for the presence or absence of tank contents in the measurement gap.
• To overcome barricades encountered in the vessel or tanks, a technique called tank mapping has been introduced. Tank mapping lets the operator take a "sonic snapshot" of an empty vessel. The transducer transmits a sound burst and the echo is recorded as a signature of the tank. Any obstructions in the vessel will send an echo and create a profile. Later on, this signature or profile is locked into the ultrasonic unit’s memory so it will not respond to echoes created by these obstructions.

Displacer Switches

In a typical displacer switch design, a spring is provided which is burdened with weighted displacers. The displacers having weights greater than the process fluid gets submerged in the liquid resulting in a buoyancy force change. This will cause a variation in the net force operating on the spring. In general, the spring will compress with the raise in buoyancy force. Just like the float level switches, a magnetic sleeve and a non-magnetic barrier tube is also incorporated in displacer switches. The magnetic sleeve is attached to the spring and it moves according to the spring movement resulting in activation of switching mechanism. An in-built limit switch is provided in the design which proves useful in level surge conditions since it keeps a check on the over stroking of the spring. The operating principle of a typical Displacer switch is illustrated in the figure below.



Displacer switches are most commonly employed in oil and petrochemical fields as level transmitters and local level controllers. These switches offer extremely correct and consistent measurement results in applications where clean liquids having stable densities are concerned. They are particularly not appropriate for slurry or sludge type applications since coating of the displacer causes a change in its volume and a resulting change in its buoyancy force. Temperature adjustments should also be done for these switches, specifically in areas where changes in process temperature can significantly affect the density of the process liquid.

The performance of displacers can be influenced by non-stability in process density in view of the fact that the displacement i.e. the weight loss of the material is equivalent to the weight of the liquid dislocated. As soon as the specific gravity of the process varies, the weight of the displaced material also varies accordingly, resulting in a change in the calibration. Due to this, one can specifically face problems in cases of interface level detection between two liquids having different densities, where the relative signal depends upon the difference between two densities. An important requirement while working with displacers is that even after commissioning, the liquid being detected must retain its density for getting good repeatability.


Advantages

Following are the major advantages associated with the use of floats and displacers:

• They perform extremely well with clean fluids.
• Use of these level sensors proves to be very accurate.
• They are flexible to extensive changes in density of the medium.

Lighting Controls

Section 9.1/Compliance: Interior Lighting

If the lighting system is not installed in the building interior, then the company is disqualified for the Commercial Building Tax Deduction.

If it is, a few more questions about the functionality and the location of the lighting must be answered by yes, in order to be exempted from Standard 90.1. Otherwise, follow the applicable Standard 90.1-2001 and prescriptive interior lighting provisions 9.2 + 9.3. If the code application is required, follow all other applicable state and local code requirements.

(click on the image for a better view)

Section 9.2.1.1/Mandatory Lighting Control Provisions: Automatic Shut-off

The very first question being asked is if the building where the interior lighting installed is larger than 5,000 sq.ft. If the building is smaller, then the building is exempted from mandatory automatic shut-off requirements, and the company can proceed to Section 9.2.1.2/Space Control directly.

If the building is larger than 5,000 sq.ft, the lighting must be utilized for non-stop operation so the lighting will be exempted from automatic switch-off requirements and the company can then proceed to Section 9.2.1.2/Space Control. Otherwise, all interior lighting is required to have automatic switch-off control. Choose one of the three switch-off control methods (based on Time Program, Occupancy Sensor or Unoccupied Signal) before proceed to the final step, which is Section 9.2.1.2/Space Control.


Section 9.2.1.3/Mandatory Lighting Control Provisions: Additional Control

Each type of lighting must have an individual control unit. If the building is a hotel or motel, a master control unit must be installed at the main entry of each hotel guest room to control all fixtures and switched outlets.
Task lighting control must be integrated with light fixture and mounted on wall where it is conveniently and visibly located.
If the above requirements are all met, then proceed to Section 9.2.4/Installed interior Lighting Power and Section 9.2.5/Luminaire Watage.

New - QuickServer - Low Priced Gateways

Good news! Buildings and Industrial automation integrators can now use the new, high performance and low-cost QuickServer to interface devices with ease!

FS-QS-1010-XXXX                                                     FS-QS-1011-XXXX

There are two QuickServer designs.

FS-QS-1010 is BTL Marked and it includes two serial ports (up to 115K baud) and one Ethernet port (10/100 RJ45). This allows it to communicate serial to serial, serial to Ethernet or Ethernet to Ethernet.

FS-QS-1011 includes one serial port (up to 115K baud), one Ethernet port (10/100 RJ45) and one FTT-10 LonWorks port. This allow s it to communicate serial to serial, serial to Ethernet, Ethernet to Ethernet, serial to LonWorks or Ethernet to LonWorks.

Key Features:

• Multi-configuration capability; specific configurations selectable via DIP switches or software.
• Ability to interface up to 250 points of common BAS protocols.
• Same firmware as our industry proven devices.
• BACnet COV support for fast data communication while reducing traffic over a BACnet network
• Capacity of 250 LonWorks network variables - almost 5 times more than competition - FS-QS-1011 Series
• Flat panel mount standard, DIN rail mount option
• Free BACnet Explorer worth $595 with purchase that include a BACnet driver.

QuickServer can configure a combination of drivers such as Modbus RTU, Modbus TCP, BACnet/IP, BACnet MS/TP, LonWorks, JCI Metasys N2 and SNMP.

	BACnet/IP	BACnet MS/TP	Modbus TCP	Modbus RTU	JCI Metasys N2	LonWorks	SNMP
BACnet/IP		FS-QS-1010-0285	FS-QS-1010-0237	FS-QS-1010-0104	FS-QS-1010-0122	FS-QS-1011-0131	FS-QS-1010-0333
BACnet MS/TP	FS-QS-1010-0285		FS-QS-1010-0419	FS-QS-1010-0367	FS-QS-1010-0309	N/A	FS-QS-1010-0694
Modbus TCP	FS-QS-1010-0237	FS-QS-1010-0419		FS-QS-1010-0030	FS-QS-1010-0117	FS-QS-1011-0104	FS-QS-1010-0248
Modbus RTU	FS-QS-1010-0104	FS-QS-1010-0367	FS-QS-1010-0030		FS-QS-1010-0038	FS-QS-1011-0085	-
JCI Metasys N2	FS-QS-1010-0122	FS-QS-1010-0309	FS-QS-1010-0117	FS-QS-1010-0038		FS-QS-1011-0097	FS-QS-1010-0150
LonWorks	FS-QS-1011-0131	N/A	FS-QS-1011-0104	FS-QS-1011-0085	FS-QS-1011-0097		FS-QS-1011-0337
SNMP	FS-QS-1010-0333	FS-QS-1010-0694	FS-QS-1010-0248	-	FS-QS-1010-0150	FS-QS-1011-0337

StuxNet Virus - How to protect your system:

What is StruxNex Virus?
 
All of the vulnerabilities in the Windows operating system have now been patched. As always, it is important to stay current with security updates. Unix-like hosts, such as Linux, BSD, OSX, etc, are not vulnerable Stuxnet. Going further, disabling USB mass storage on control system hosts (needed for some S7 control functions) would prevent an infected flash drive from loading the malware onto mission critical machines. Segregating SCADA & control system hosts from other infrastructure and using access control lists between zones is a good preventative method, but the most effective protection to this attack is white listing or host intrusion prevention systems. Removing or restricting access to shared network folders is also recommended. Up to date anti-virus definitions can identify Stuxnet, but zipped files must be unpacked in order for scans to be completely accurate. Siemens has provided the Sysclean utility, which can be used to remove the infection from a compromised host. It is also important to apply the SIMATIC security updates. And of course, mission-critical servers/systems should never be used for general web browsing, email, etc.

Modbus Practical Help

The FieldServer Modbus RTU drivers offer several function moves that handle 32-bit integers and 32-bit float values. More importantly, these function moves consider all different forms of byte sequencing.

The following table shows the FieldServer function moves that copy two adjacent 16-bit registers to a 32-bit integer value.

Function Keyword	Swap Mode	Source Bytes	Target Bytes
2.i16-1.i32	N/A	[ a b ] [ c d ]	[ a b c d ]
2.i16-1.i32-s	byte and word swap	[ a b ] [ c d ]	[ d c b a ]
2.i16-1.i32-sb	byte swap	[ a b ] [ c d ]	[ b a d c ]
2.i16-1.i32-sw	word swap	[ a b ] [ c d ]	[ c d a b ]

The following table shows the FieldServer function moves that copy two adjacent 16-bit registers to a 32-bit floating point value:

Function Keyword	Swap Mode	Source Bytes	Target Bytes
2.i16-1.ifloat	N/A	[ a b ] [ c d ]	[ a b c d ]
2.i16-1.ifloat-s	byte and word swap	[ a b ] [ c d ]	[ d c b a ]
2.i16-1.ifloat-sb	byte swap	[ a b ] [ c d ]	[ b a d c ]
2.i16-1.ifloat-sw	word swap	[ a b ] [ c d ]	[ c d a b ]

The following table shows the FieldServer function moves that copy a single 32-bit floating point value to two adjacent 16-bit registers:

Function Keyword	Swap Mode	Source Bytes	Target Bytes
1.float-2.i16	N/A	[ a b c d ]	[ a b ][ c d ]
1.float-2.i16-s	byte and word	[ a b c d ]	[ d c ][ b a ]
1.float-2.i16-sb	byte swap	[ a b c d ]	[ b a ][ d c ]
1.float-2.i16-sw	word swap	[ a b c d ]	[ c d ][ a b ]

Given the vairous FieldServer function moves, the correct handling of 32-bit data is dependent on choosing the proper one. Observe the following behavior of these FieldServer function moves on the known single-precision decimal float value of 123456.00:

16-bit Values	Function Move	Result	Function Move	Result
0×2000 0x47F1	2.i16-1.float	123456.00	1.float-2.i16	0×2000 0x47F1
0xF147 0×0020	2.i16-1.float-s	123456.00	1.float-2.i16-s	0xF147 0X0020
0×0020 0xF147	2.i16-1.float-sb	123456.00	1.float-2.i16-sb	0×0020 0xF147
0x47F1 0×2000	2.i16-1.float-sw	123456.00	1.float-2.i16-sw	0x47F1 0×2000

Notice that different byte and word orderings require the use of the appropriate FieldServer function move. Once the proper function move is selected, the data can be converted in both directions.

Of the many hex-to-floating point converters and calculators that are available in the Internet, very few actually allow manipulation of the byte and word orders. One such utility is located at www.61131.com/download.htm where both Linux and Windows versions of the utilities can be downloaded.

Once installed, the utility is run as an executable with a single dialog interface. The utility presents the decimal float value of 123456.00 as follows:

One can then swap bytes and/or words to analyze what potential endianness issues may exist between Modbus RTU master and slave devices.

How Real (Floating Point) and 32-bit Data is Encoded in Modbus RTU Messages

The article discusses some of the typical difficulties encountered when handling 32-bit data types via Modbus RTU and offers practical help for solving these problems.
The point-to-point Modbus protocol is a popular choice for RTU communications if for no other reason that it’s basic convenience. The protocol itself controls the interactions of each device on a Modbus network, how device establishes a known address, how each device recognizes its messages and how basic information is extracted from the data. In essence, the protocol is the foundation of the entire Modbus network.
Such convenience does not come without some complications however, and Modbus RTU Message protocol is no exception. The protocol itself was designed based on devices with a 16-bit register length. Consequently, special considerations were required when implementing 32-bit data elements. This implementation settled on using two consecutive 16-bit registers to represent 32 bits of data or essentially 4 bytes of data. It is within these 4 bytes of data that single-precision floating point data can be encoded into a Modbus RTU message.


The Importance of Byte Order

What To Take To Site With You Part II

Modbus Trouble Shooting - What To Take To Site With You Part I

7. Serial Break out box

8. Lcom DB9-Terminal blocks (or similar)
http://www.l-com.com/item.aspx?id=8141 Male and Female

9. DB9 and DB25 make and female connector make-up kits (Solder free)



10. Rx / TX cross cross over.





It is useful to be able to swap the conductors connected to pins 2 and 3. Take a module with you. It is easier than changing the wires.

EG. Ziotek Null Modem Adapter DB25
http://www.cyberguys.com/product-details/?productid=751&rtn=750&core_cross=SEARCH_DETAIL_SIMILAR#page=page-1


11. Terminating Resistors

Take 52.3, 75, 100, 120 and 150 Ohm resistors with you. 0.25 Watt is usually more than enough.

12. Gender Benders






 




 +














13. Ethernet Patch cables

14. Hub

A hub is not a switch. A hub can be used for trouble shooting whereas only as ‘supervised’ switch can. Most switches are not supervised. For more information read Appendix.

Modbus on RS485 Advice #9

Advice #9



What can go wrong with 485?
Let's say you adopted all the best practices for installation of the network but you get intermittent or unacceptable performance because of packet loss, noise, collisions … Then you should consider hiring an expert to resolve your problems because now you are in the ‘Art’ part of RS485. These are some of the things they will look at.

• Reflections.

Without a scope and expertise you won't know this is a factor. It is easy and cheap to eliminate. Look at the cable spec. Find the nominal impedance. Buy two resistors of the same value. At each end of the trunk install the resistors between the Tx and Rx terminals. If you don’t have obvious ends of the trunk (because you created a star) then we recommend re-cabling to form a linear trunk or we wish you luck.

Some devices have terminating resistors built into them. If the vendor did a poor job, the default is to have the resistor active and they must be disabled unless they are the terminating devices on the network. Read vendor doc.

• Biasing, Idle State Biasing, Fail Safe Biasing, Anti Aliasing

There are a whole string of terms uses as synonyms to describe this phenomenon.
To use two wires ( as opposed to full duplex 4 wire) for RS485 each devices transmitter and receiver must be set to an idle state to release the line for others use. Releasing the line means allowing it to ‘float’. It must not be allowed to float at any voltage level so devices have pull up/down resistors to pull the line to an allowable ‘floating’ voltage. (the floating state is also known as the tri-state. ) The load presented by other devices on the network affects this floating so the resistor values may need to be changed depending on the number of devices installed and the values of the pull up/down resistors they are using. (You can imagine how tricky it is going to be to resolve this). If a device floats out of the specified range then to other devices it will look like the floating devices isn’t floating at all. The other devices will think that it is transmitting or receiving and thus blocking the line.

The simplest way of knowing if this is a factor – Does the device work properly when it is the only device on the network ?. When you install it in the full network other devices or this device stops working properly. This device and/or the pull up/down resistors of other devices are candidates for investigation.
A number of vendors have a range of pull up/down resistors installed and allow you to change the selection using software or jumpers.

• Line Drive On / Off

To use two wires for RS485 each devices transmitter and receiver must be set to an idle state to release the line for others use. When a device wants to send it must grab the line. When it has finished sending it must release the line. You can see there are potential problems here. What happens if one device waits too long after sending its last bit before releasing the line – its possible that the other devices will miss some bits of data.


Advice # 10

Modbus on RS485

Modbus on RS485 Advice 1 - 3

Advice #4


 

Take care where you run your cables. It seems obvious not to wind your cable around other cables or sources of electricity / magnetism. People are often surprised to find that the worst source of induced noise are switching DC loads. Another big culprit are Variable Frequency drives.

Advice #5



Cable selection does make a difference.
All cables offer impedance (resistance). Some cables are designed so that the impedance is relatively independent of distance. You want one of these cables. A clue to knowing if you selected one is to look at the cable’s Nominal Impedance. If they quote a number such a 100Ohms you have a good cable. If they quote an impedance per meter/foot you have chosen the wrong kind. Wrong in the sense – to determine the value of terminating resistors now requires measurements and calculations. Choose low capacitance cables. 

 
Can you use Cat5 cable? Yes. Use one pair for Tx,Rx and a conductor from another pair for the ground reference signal. 

 
We recommend these two cables. 

 
Belden 3106A

Multi-Conductor – EIA Industrial RS-485 PLTC/CM 22 AWG stranded (7x30) tinned copper conductors, Datalene® insulation, twisted pairs, overall Beldfoil® shield (100% coverage) plus a tinned copper braid (90% coverage), drain wire, UV resistant PVC jacket. 

 
Belden 3107A

Multi-Conductor - EIA Industrial RS-485 PLTC/CM 22 AWG stranded (7x30) tinned copper conductors, Datalene® insulation, twisted pairs, overall Beldfoil® shield (100% coverage) plus a tinned copper braid (90% coverage), drain wire, UV resistant PVC jacket.

Modbus RS232, RS485 and TCP/IP

There are 3 main physical layers for Modbus.
RS232 : One master and one slave. Typically a cable with 3 conductors with max length of approx a couple of hundred feet. Usually easy. Sometimes some jumpers are required at one end to defeat handshaking.
RS485: One master and up to 128 slaves but take care to read more if you plan on more than 32. There are two wiring systems – so called 2-wore and so called 4-wire. They can be incompatible but usually 4-wire devices can be made to work on 2-wire systems. Each device must have a unique address and all devices must be set to the same baud rate, data bits, stop bits and parity. Usually easy to implement. The RS485 physical layer allows up to 128 devices to be installed on a single network with a max physical length of 4000ft and speeds up to 115k baud. Using repeaters allows the length to be increased. Compare to Ethernet where the spec allows a max of 100 meters (330ft) on a single unrepeated segment.
TCP/IP: All devices are essentially peers. A single device can be a master and a server. Routers can be used to connect sub-nets together. Broadcasts are almost ever used so are not an issue.

Modbus: Old device – slow processors – limited capability

Many older devices have old microprocessors that can't do too much work at once. Often this microprocessor is used to run the device and handle the Modbus communication.

It is not uncommon to see device with the following limitations.

• You can only read one data point per message. I.e length must be 1.
• You must have a delay between sending messages.

Modbus: What about errors / exceptions

Modbus & Gateways
 
Modbus has a limited way of reporting errors. A server / slave device can respond to a message in a way that reports an error.

These are called exception messages.

If you are looking at a message byte stream, exceptions are easy to identify.

Code     Name / Meaning
1           ILLEGAL FUNCTION

The function code received in the query is not an allowable action for the slave. If a Poll Program Complete command was issued, this code indicates that no program function preceded it.

2           ILLEGAL DATA ADDRESS

The data address received in the query is not an allowable address for the slave.

3           ILLEGAL DATA VALUE

A value contained in the query data field is not an allowable value for the slave.

4           SLAVE DEVICE FAILURE

An unrecoverable error occurred while the slave was attempting to perform the requested action.

5           ACKNOWLEDGE

The slave has accepted the request and is processing it, but a long duration of time will be required to do so. This response is returned to prevent a timeout error from occurring in the master. The master can next issue a Poll Program Complete message to determine if processing is completed.

6           SLAVE DEVICE BUSY

The slave is engaged in processing a long–duration program command. The master should retransmit the message later when the slave is free.

Floating Point Numbers in Modbus

Modbus was not designed to transport floating point numbers. After the protocol was released and in use – some people came up with a scheme to using two consecutive 16 bit registers to transport one floating point number. The scheme is essentially a set of rules for interpreting the bits in the 2x registers as the elements of a floating point number(like a mini protocol). Other people came up with other schemes.

One of these schemes has come to dominate. It is called standard IEEE754.

Some devices (servers) do not support floating point numbers.

Many clients (masters) do not support floating point numbers.

A master and a server must use the same floating point scheme to work together.
Read more in Appendix.

Scaling in Modbus

MODBUS - INTRODUCTION

Because it is so commonly used, because it is so limited, because some vendors went to a lot of trouble and because some vendors hired bad programmers, Modbus, as simple as it seems, can offer lots of complications. Modbus was invented to transfer data as well as to program/configure PLC’s. For the purposes of this article, we are only interested in the data transfer functions.

There are 4 types of data

Holding Registers

An area of 16 bit words. Intended as read / write. Originally used as programmer scratch pad area and for analog outputs in old Modicon PLC’s. Also known as 4xxxx registers (xxxx is the place holder for the specific holding register’s point number).

Input Registers

Think Analog inputs. 16 bit words.
Also known as 3xxxx registers (xxxx is the place holder for the specific input register’s point number).

Inputs

Think Binary inputs.
Also known as Inputs.
Also known as 1xxxx inputs (xxxx is the place holder for the specific input’s point number).

Coils
Think Binary outputs. Named coils after the coil in a relay which is activated to energize a circuit. The original PLC’s were relay replacement machines.
Also known as Outputs.
Also known as 0xxxx inputs (xxxx is the place holder for the specific input’s point number).

5 Digit VS 6 Digit Addressing

Helix Meters

These types of meters are made up of two radically pitched helical rotors which results in an axial liquid displacement from one side of the chamber to the other side. Both the rotors are geared together and there is a very small clearance between the rotors and the casing. 


Nutating-disk Meters

Oval-gear Meters

These types of meters consist of two rotating, oval-shaped gears constructed with synchronized, close fitting teeth. In an oval gear meter, the rotation of gear shafts causes a fixed amount of liquid to pass through the meter. By monitoring the number of shaft rotations, one can calculate liquid flow rate. These types of meters prove to be very accurate when slippage between the housing and the gears is set very small. Turndown ratio of an oval gear meter gets influenced by the lubricating properties of the process fluid. 

Reciprocating Piston Meters