February 2009 Newsletter

New! BACnet Capable Virtual Thermostat

This thermostat can send commands to a controller using BACnet Ethernet/IP/MSTP. Easy to install, configure and use. The buttons, date and other display elements are configurable. You can brand the display with your corporate logo or stream images/forecasts from a web server.

The thermostat can be used without input as a temperature controller or it can be used as a fully functional thermostat as it accepts inputs from other BACnet capable devices or from a PC connected sensor.

When not in use it minimizes to an editable icon in the system tray with configurable mouse-over display.

IN THIS ISSUE

Virtual Thermostat
Modbus RTU Data
New Drivers
Bacnet BBMD

 

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How Real and 32-bit Data is Encoded in Modbus RTU

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 OrderModbus itself does not define a floating point data type but it is widely accepted that it implements 32-bit floating point data using the IEEE-754 standard. However, the IEEE standard has no clear cut definition of byte order of the data payload. Therefore the most important consideration when dealing with 32-bit data is that data is addressed in the proper order.

For example, the number 123456.00 as defined in the IEEE 754 standard for single-precision 32-bit floating point numbers appears as follows:

The affects of various byte orderings are significant. For example, ordering the 4 bytes of data that represent 123456.00 in a “B A D C” sequence in known as a “byte swap”. When interpreted as an IEEE 744 floating point data type, the result is quite different:

Ordering the same bytes in a “C D A B” sequence is known as a “word swap”. Again, the results differ drastically from the original value of 123456.00:

Furthermore, both a “byte swap” and a “word swap” would essentially reverse the sequence of the bytes altogether to produce yet another result:

 

Clearly, when using network protocols such as Modbus, strict attention must be paid to how bytes of memory are ordered when they are transmitted, also known as the ‘byte order’.

Determining Byte OrderThe Modbus protocol itself is declared as a ‘big-Endian’ protocol, as per the Modbus Application Protocol Specification, V1.1.b:

“Modbus uses a “big-Endian” representation for addresses and data items. This means that when a numerical quantity larger than a single byte is transmitted, the most significant byte is sent first.”

Big-Endian is the most commonly used format for network protocols – so common, in fact, that it is also referred to as ‘network order’.

Given that the Modbus RTU message protocol is big-Endian, in order to successfully exchange a 32-bit datatype via a Modbus RTU message, the endianness of both the master and the slave must considered. Many RTU master and slave devices allow specific selection of byte order particularly in the case of software-simulated units. One must merely insure that both all units are set to the same byte order.

As a rule of thumb, the family of a device’s microprocessor determines its endianness. Typically, the big-Endian style (the high-order byte is stored first, followed by the low-order byte) is generally found in CPUs designed with a Motorola processor. The little-Endian style (the low-order byte is stored first, followed by the high-order byte) is generally found in CPUs using the Intel architecture. It is a matter of personal perspective as to which style is considered ‘backwards’.

If, however, byte order and endianness is not a configurable option, you will have to determine the how to interpret the byte. This can be done requesting a known floating-point value from the slave. If an impossible value is returned, i.e. a number with a double-digit exponent or such, the byte ordering will most likely need modification. 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 swap [ 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
0x2000 0x47F1 2.i16-1.float 123456.00 1.float-2.i16 0x2000 0x47F1
0xF147 0x0020 2.i16-1.float-s 123456.00 1.float-2.i16-s 0xF147 0X0020
0x0020 0xF147 2.i16-1.float-sb 123456.00 1.float-2.i16-sb 0x0020 0xF147
0x47F1 0x2000 2.i16-1.float-sw 123456.00 1.float-2.i16-sw 0x47F1 0x2000

 

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. An utility for this can be found 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.


New Drivers

TOA VS900 Driver

The TOA VS-900 Serial Driver allows the FieldServer to poll remote stations for log data, remote dial and perform other communication channel commands. The Log data can be used to determine the current status of a station. The driver supports a state lookup table so that the VS900 states can be mapped onto a different set of states.

Harshaw Trane XML Schedule Driver

The XML Schedule driver will use the HTTP protocol over TCP/IP to poll for XML Schedule data from a configurable URL. The data will be parsed according to fixed filter criteria and stored in Data Array locations that are configurable for each zone. This data may then be served or written to a remote device/system by using another FieldServer driver such as BACnet/IP.The Driver will process an XML file looking for <Schedule> objects. It will identify the zone. For each zone the driver will compare the Start time against the FieldServer current time and find the object for that zone that is currently active or that will be active next. The start date and time will be extracted from the object and stored in FieldServer Data array configured for that zone. Each element of the date and time will be stored in a different data array location.Since FieldServer allows any Data Array locations to be mapped onto another protocol it will be possible to serve or write this data to another device using another protocol such as Bacnet/IP.

For example – in this object:

<Schedule>
  <Zone>MRC Zone 1</Zone>
  <Start>2008-10-07T11:30:00</Start>
  <End>2008-10-07T14:30:00</End>
</Schedule>

The driver allows for a remote device to periodically synch the time of the FieldServer.


Bacnet BBMD

The BACnet discovery uses two services – called ‘Who-Is’ and ‘I-Am’. They rely on the use of broadcasts.

Routers join IP networks together so messages from one network can be sent to another. Most routers do not forward broadcast messages and this means discovery cant discover devices on another network.

To solve this problem BACnet provides a technology called BBMD – BACnet/IP Broadcast Management Device.

In overview the technology is simple. You install a BBMD (might be a physical device or just a software application on a computer) on each network. You can configure the BBMD by specifying the IP Address and mask of the each BBMD. This makes both BBMD configs identical. When the one BBM receives a broadcast, it forwards the messages to the other BBMD which in re-broadcasts on the other network. They are configured by BDT files and these may be modified on the fly using selected Bacnet services.

The technology also provides for foreign device registration. This allows a device on one network to communicate with a device on another network by using the BBMD to forward and route the messages.


Free CAS BACnet Explorer

Add $795* value to your FieldServer for free. Simply purchase a FieldServer with a BACnet protocol from CAS and we will provide you with a free (and transferable) license to our famous BACnet Explorer. Use the explorer to test and commision the installation and then use it on your next job or pass it on to your customer.
*795 – Our competitors price

This is a full license and includes the USB key which allows you to transfer your license from laptop to laptop with no hassles. You get all updates to the product for a period of 1 year.

You can try a free demo of the CAS BACnet Explorer.

2009 Chipkin Automation Systems – bacnet@chipkin.com
BACnet is a registered trademark of ASHRAE.

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