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- Byte mode - allows working with devices that implement the registers as 1-byte variables (in contrast with Modbus standard in which the register value is 2 bytes).
- Variable mode - allows working with devices that implement the registers with different sizes than standard 2 bytes. It was implemented because of support of the flowmeter FloBoss 103 made by Fisher Controls International (at this time a part of Emerson Process Management): 1-byte variables, 4-byte unsigned/signed integers, text strings of length 10,12,20,40 characters, a 6-byte time stamp, and other.
- Passive (scanning) mode allowing to work in eavesdropping mode. This applies especially to serial communication when the communication port of the device is already used to communicate with another Master device. Due to the nature of the Modbus protocol, it is necessary to receive both requests and responses in this mode.
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- Line category Serial (serial communication)
- Line category SerialOverUDP Device Redundant (serial communication).
- Line category RFC2217 Client (serial communication).
- Line category TCP/IP-TCP and TCP/IP-TCP Redundant (MODBUS over TCP/IP). Reserved TCP port 502 is commonly used, but it is possible to use any other one according to the setting of the device. The line number is not used, set the value e.g. to 1.
Note: For redundant systems, it is possible to enter multiple names/addresses separated by commas.
Note: In the case of WAGO 750-8100 type PLC and communication via MODBUS TCP, it was necessary to set a small polling period (e.g. 1 second) in the time parameters of the station. In the case of a longer period (5 seconds), the connection was closed quite often by the PLC.
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A dialog window of communication line configuration - Protocol parameters tab.
They influence some optional protocol parameters.
The line protocol contains the following parameters:
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Forced disconnection: If all stations on the TCP/IP-TCP or TCP/IP-TCP Redundant line are in the simulation mode or the communication is stopped for them, the line will be disconnected (the communication socket will be closed). If the simulation is disabled for at least one station and the communication is not stopped for it (the Parameters tab of Station type object), the line will be connected again.
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A dialog window of communication line configuration - Protocol parameters tab.
They influence some optional protocol parameters.
The line protocol contains the following parameters:
Parameter | Meaning | Unit | Default value | ||||||||||||||||||
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| line categories. The parameter activates the disconnection of the TCP connection after the execution of each read cycle, or after the value is written. The parameter was implemented due to problems with connection stability on mobile GPRS networks.YES/NO | NO | |||||||||||||||||||
Kotva | psm | psm | Passive ModeThis parameter activates the eavesdropping mode. In this mode, requests are not sent and writing does not work. Only received packets are parsed. The nature of the Modbus protocol implies that it is necessary to eavesdrop on both the requests and responses of existing communication. | YES/NO | NO | ||||||||||||||||
Kotva | tnd | tnd | Tcp No DelaySetting Tcp No Delay parameter to YES causes low-level socket option TCP_NODELAY to be set, thus turning off the default packet coalesce feature.The parameter is implemented only for TCP/IP-TCP and TCP/IP-TCP Redundant line categories. | YES/NO | NO |
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The parameter activates the disconnection of the TCP connection after the execution of each read cycle, or after the value is written. The parameter was implemented due to problems with connection stability on mobile GPRS networks. | YES/NO | NO | |||||||
| This parameter activates the passive (scanning) mode. In this mode, requests are not sent and writing does not work. Only received packets are parsed. The nature of the Modbus protocol implies that it is necessary to receive both the requests and responses of existing communication. | YES/NO | NO | ||||||
| Setting Tcp No Delay parameter to YES causes low-level socket option TCP_NODELAY to be set, thus turning off the default packet coalesce feature. The parameter is implemented only for TCP/IP-TCP and TCP/IP-TCP Redundant line categories. | YES/NO | NO |
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- Communication protocol "Modbus Client".
- The station address is a decimal number mostly in the range of 1 up to 247. Address 0 is reserved as broadcast.
Station protocol parameters
Configuration dialog box - tab Parameter.
They influence some optional parameters of the protocol. The following station protocol parameters can be set:
Table 1
Parameter | Meaning | Unit | Default value | ||||||
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| Maximum count of request retries. If no response returns after a request had been sent, the station's status will change to a communication error. | - | 2 | ||||||
| Timeout before resending a request if no response has been received |
- Communication protocol "Modbus Client".
- The station address is a decimal number mostly in the range of 1 up to 247. Address 0 is reserved as broadcast.
Station protocol parameters
Configuration dialog box - tab Parameter.
They influence some optional parameters of the protocol. The following station protocol parameters can be set:
Table 1
Parameter | Meaning | Unit | Default value | ||||||||||||||||||||
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Kotva | rc | rc | Retry CountMaximum count of request retries. If no response returns after a request had been sent, the station's status will change to a communication error. | - | 2 | ||||||||||||||||||
Kotva | rt | rt | Retry TimeoutTimeout before resending a request if no response has been received. | s | 0.1 | ||||||||||||||||||
Kotva | wft | wft | Wait First TimeoutThe delay after sending the request and before reading the response. | s | 0.1 | Kotva | | wt | wt | Wait TimeoutThe delay between the response readings.s | 0.1 | ||||||||||||
| The delay after sending the request and before reading the response. | s | 0.1 | ||||||||||||||||||||
| The delay between the response readings. | s | 0.1 | ||||||||||||||||||||
| The The maximum number of retries of the response reading. | - | 20 | ||||||||||||||||||||
| "Start silent interval" before the beginning of the transmission in RTU mode. | ms | 50 | ||||||||||||||||||||
| "Stop silent interval" after ending of the transmission in RTU mode. | ms | 50 | ||||||||||||||||||||
| lem
| lem |
| After writing to the I/O tag, the reading immediately follows. By setting the parameter to the NO value, it is possible to reduce the load on communications (especially serial ones) with a large number of writes. | YES/NO | YES | |||||||||||||||||
| Byte order in Little-endian mode for 4 | Little Endian ModeByte order in Little-endian mode for 4-byte variables. The individual options indicate in which bytes (1-lowest, 4-highest) the individual bytes from the communication will go:
| - | 2143 | |||||||||||||||||||
| Special byte mode of transmission in which the values of registers have a length of 1 byte and not 2 bytes as it is defined in Modbus protocol specification. | YES/NO | NO | ||||||||||||||||||||
| Special variable mode of transmission in which the values of registers have variable lengths. The setting of Variable mode: Note 1: Variable and byte modes are incompatible and only one of them can be enabled. | OFF Little endian Big endian | OFF | ||||||||||||||||||||
| Logging of detailed debug information about communication in the line log. | YES/NO | NO | ||||||||||||||||||||
| Protocol mode: RTU or ASCII. Note: In the case of "MODBUS over TCP/IP", the parameter value is ignored and Protocol Mode=RTU is used. | RTU ASCII | RTU | ||||||||||||||||||||
| Sets an address model of MODBUS protocol: MODBUS PDU data are addressed from 0 up to 65535. MODBUS data Model data are addressed from 1 up to 65536. Note: MODBUS PDU is a default value. If the MODBUS data Model is set, the object with the address X is addressed as X-1 in MODBUS PDU. After you change this parameter, a restart of the respective communication process was required in the past (KOM binaries older than May 27, 2021). | MODBUS PDU MODBUS data Model | MODBUS PDU | ||||||||||||||||||||
| Select a variant of the protocol in case of TCP/IP communication: "MODBUS TCP" is a variant of communication without control checksum. Safeguarding is done by the underlying TCP protocol. "MODBUS over TCP" is a variant where a payload is MODBUS RTU data containing a checksum. | "MODBUS TCP" "MODBUS over TCP" | "MODBUS TCP" | ||||||||||||||||||||
| Maximum count of registers that are read by one request. | - | 100 | ||||||||||||||||||||
| Maximum count of bytes that are required by one request (only in "Byte mode"). | - | 100 | ||||||||||||||||||||
| If a value of the integer type (Holding Registers, Input Registers) is assigned to an I/O tag of the Di or Dout type, this is done by comparing the read value with zero. If the value is zero, the value of the I/O tag is False, otherwise True. The Bool Mask parameter allows specific bits to be filtered out before the comparison is made, based on a bitmask specified as a hexadecimal number (the leftmost byte is the highest). The bitmask FF FF FF FF means that all bits are considered (for 1- and 2-register integer addresses). | - | FF FF FF FF | ||||||||||||||||||||
| To read This parameter is used to avoid reading the values from addresses that are not configured is not allowed. Description and example: | YES/NO | Some Modbus servers respond by exception to reading of a range of registers that contains "unknown" registers (which e.g. they don't have mapped to internal memory). | YES/NO | NONO | ||||||||||||||||||
| If this parameter is set to YES, then an extra check is performed when receiving a response to a read request: the length of received data is checked whether it matches the number of registers in a read request:
| YES/NO | request #2. | YES/NO | NO | ||||||||||||||||||
| If the parameter is set to YES, then a single request (which contains the total number of registers) is used. It is necessary that the I/O tags are defined for all addresses 0..N. | YES/NO | NO | ||||||||||||||||||||
| On the TCP/IP-TCP line, immediately after the connection is established, it is possible to write a specific value to the selected address (TCP Password Address) using the selected function (TCP Password Function). In this way, the ComAp control unit enables the authorized Modbus client to be authenticated. The value is entered in hexadecimal in the order of the bytes as they will be transmitted (e.g. "01 0A BC D0". If an odd number of bytes is entered, a byte with the value 0 will be added at the end. Setting the parameter to an empty value causes the write to not be performed after the connection is established. | - | |||||||||||||||||||||
| Address for writing the password (TCP Write Password) on the TCP/IP-TCP line. If the password is longer than 2 bytes, it is the address of the first register. If the password has e.g. 6 bytes, written as 3 registers from the specified address. | - | 0 | ||||||||||||||||||||
| Write function for writing the password (TCP Write Password) on the TCP/IP-TCP line. | 6 16 | 6NO |
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Independent addressing with the address size of 2 bytes, i.e. addresses from 0 up to 65535 (so-called MODBUS PDU addressing model), is in an address space of each type of register. Some devices work with address space starting with 1 (so-called MODBUS Data Model). In this case, it is necessary to deduct 1 in the address at configuration I/O tags in the D2000 system or change the setting of the parameter Addressing model to the MODBUS data Model.
The I/O tag with an address starting with %IGNORE will be ignored.
I/O tag address can be in a basic or extended format (for a variable mode).
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- The first character defines a type of I/O tag:
- I - Integer16 (default) - one register is read, signed
- U - Unsigned16 - one register is read, unsigned
- Uu - Unsigned16 - one register is read, unsigned, only the upper byte is processed (1st in sequence)
- Ul - Unsigned16 - one register is read, unsigned, only the lower byte is processed (2nd in sequence)
- f - Float (4 bytes = 2 registers) - two registers with Address and Address+1 are read and transmitted as big-endian (see Note).
- F - Float (4 bytes = 2 registers) - two registers with Address and Address+1 are read and transmitted as little-endian (so-called Modicon format), (see Note)
- L - Unsigned long (4 bytes = 2 registers) - two registers with Address and Address+1 are read, unsigned, and transmitted as big-endian (see Note)
- Ll - Unsigned long (4 bytes = 2 registers) - two registers with Address and Address+1 are read and transmitted as little-endian, unsigned (see Note)
- S - Signed long (4 bytes = 2 registers) - two registers with Address and Address+1 are read, signed, and transmitted as big-endian (see Note)
- Sl - Signed long (4 bytes = 2 registers) - two registers with Address and Address+1 are read and transmitted as little-endian, signed (see Note)
- B - Byte unsigned, only the upper 8 bits of the register value
- X - Byte unsigned, only the lower 8 bits of the register value
- sn. - Text string with the length of n characters, one register is one character, n registers with Address up to Address+n-1 are read
- an. - Text string with the length of 2*n characters, one register is two ASCII characters, characters are transmitted in the same order as they appear in the string, n registers with Address up to Address+n-1 are read
- An. - Text string with the length of 2*n characters, one register is two ASCII characters, characters are transmitted in big-endian order (i.e. "1234" is transmitted as "2143"), n registers with Address up to Address+n-1 are read
- Modifier d indicates that a number is an 8-byte number (4 consecutive registers). It can be used for types L, Ll, S, Sl, F,f, and it is used for configuration of signed/unsigned 8-byte integer as well as an 8-byte float (big-endian <B8>..<B1> and little-endian <B1>..<B8> formats).
Modifier D indicates that a number is an 8-byte number (4 consecutive registers). It can be used for types Ll, Sl, F and it is used for configuration of signed/unsigned 8-byte integer as well as an 8-byte float (little-endian format <B2><B1><B4><B3><B6><B5><B8><B7>).
Note: when using a modifier d or D, the I/O tag must be of Analog type (Ai), because the Integer type (Ci) in D2000 is implemented as a 4-byte variable, and overflow might occur. Writing of Integer type (Co) as an 8-byte number is supported. - Modifier b indicates that figure is coded by BCD. It can be used for I/O tags of I, U, B, L, Ll types.
Modifier s indicates that a status register (Unsigned16) located on address Address is followed by a big-endian Float value located on address Address+1 .. Address+2. This indicator is used for type f and it is implemented for calorimeter Endress+Hauser RMS621. The following table shows the values of the status register and their mapping to D2000 attributes.
Status register D2000 attributes 0: Invalid value Weak 1: Measured value valid Valid 2: Overflow warning
3: Overflow error
4: Underflow warning
5: Underflow error
6: Saturated steam alarm
7: Error in differential pressure calculation
8: Wrong medium for DP calculation
9: Wrong value range - DP calculation inaccurate
10: Differential pressure - general error
11: Range overshoot (Tsat > 350 etc.) on
12: Change in state of aggregation
26 : Differential pressure --> general error
99: No measured value is assigned to the register in the setup of the ModBusWeak - Parameter RdFn is a function of the Modbus protocol for a data reading. The following functions are implemented:
- 1 - Read Coils: binary status reading
- 2 - Read Discrete Inputs: binary input reading
- 3 - Read Holding Registers: status register reading (Integer16/Unsigned16 and Float32 - reads two successive registers)
- 4 - Read Input Registers: input register reading (Integer16/Unsigned16 and Float32 - reads two successive registers)
- 0 - A value is not read, it is only written. The function for writing (WrFn) must be set.
- Parameter WrFn is the function of the Modbus protocol for data writing. The following functions are implemented:
- 5 - Write Single Coil: binary status writing (default for Read Coils)
- 6 - Write Single Register: status register writing (default for Read Holding Registers)
- 16 - Write Multiple registers: multiple registers writing, it must be used when 2-register type is written (e.g. Float, Unsigned long, etc.).
Note: function can be used to write more than two registers at once if a text string is used. Example:
if we have an I/O tag with address a3.0-16.#8A00 (i.e. text string covering 3 registers, having length of 6 characters) and we write a string '123456', then hexadecimal values 0x3132, 0x3334 and 0x3536 (ASCII code for '1' is 0x31, for '2' is 0x32 etc) will be written to registers 0x8A00, 0x8A01 and 0x8A02. - 22 - Mask Write Register: write affects only the value of the particular bit BitNr of the status register. It is usable only for Do value types with the address parameter BitNr.
- Parameter d activates the function "delayed write". Sending of the value is delayed until the request to write the value of the object without parameter d comes. All accumulated requests waiting to be written are sent. If the function WrFn is set to "Write Multiple Registers", the values are sent in one packet.
- Parameter Address is a 2-byte address of the register (0-65536). See also the protocol parameter Addressing model.
Note: address can be specified as a hexadecimal number using a number sign (#), e.g. #50CE - Parameter BitNr is a bit's position in a word. The values 0-7 are allowed to be used for binary statuses and inputs, values 0-15 are allowed to be used for reading of bit from 16-bit status or input registers.
Note: coexistence of an I/O tag without BitNr parameter and multiple I/O tags with BitNr parameter having the same Address is possible.
Note: coexistence of an I/O tag without BitNr parameter and multiple I/O tags with BitNr parameter having the same Address is possible.
Note on writing: if only a part of the register (lower/higher byte or selected bit) is written and the I/O tag also has RdFn (function for a data reading) configured, then when writing, the value of the entire register is read first, and then the relevant part of the register is modified and the entire register is written.
Note about the byte and register order Kotva pozn_1 pozn_1
1. MODBUS protocol uses the big-endian, i.e. the most significant byte (MSB) is transmitted as first. Examples:
Received bytes of MSB-LSB | I/O tag type | Value |
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0x00 0x01 | I, U | 1 |
0xFF 0xFE | I | -2 |
0xFF 0xFE | U | 65534 |
0x01 0x02 | B | 1 |
0x01 0x02 | X | 2 |
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Address format is [xN].[I|U|F|B|C|T][b]RdFn[-WrFn].Address[.BitNr] in which:
- xN indicates the number of bytes that read or write. Valid values for N are 1, 2, 4, 8 (in combination with I, U, F), 6 for T type, and an arbitrary number for C type.
- A letter defines the type of I/O tag. Besides standard I, U, F, B, two extra types have been added:
- C - text string of fixed length (e.g. x10.C3.1001 is a 10-character string on address 1001)
- T - time stamp with length of 6 bytes (ss:mi:hh dd:mm:yy)
- The meaning of other parameters is in compliance with the standard mode.
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Honeywell UDC1700 controllers (probably generally UDC1xxx):
The basic parameters and current data of these controllers are not normally read by means of functions 0x01 up to 0x04. It is necessary to use the function functions 0x14/0x15 Read/write configuration reference data. These controllers use "big-endian" byte order. Therefore, for proper functionality, it is not necessary to modify parameters that change byte mode and endianness.
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20.039 - 16-bit number from address 39(0x27) f20.040 - 32-bit real number from address 40(0x28)39(0x27) f20.040 - 32-bit real number from address 40(0x28)
Note: Honeywell-made products (UDC 2xxx/3xxx, HC900, DPR, Trendview) use the common Modbus function codes 1-4 in spite of the UDC manuals referring only to 0x14/0x15 Read/write configuration reference data register tables. For more detailed information, see this post.
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- Ver. 1.0 - November 27th, 2006 - document creating.
- Ver. 1.1 - November 21st, 2007 - document update.
- Ver. 1.2 - April 24th, 2009 - document update.
- Ver. 1.3 - November 3rd, 2010 - document update.
- Ver. 1.4 - December 6th, 2010 - document update.
- Ver. 1.5 - September 5th, 2022 - document update (support for 8-byte values in the extended format).
- Ver. 1.6 - September 7th, 2022 - document update (added parameter "Dummy Request Mode").
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