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The protocol implements client (master) communication with arbitrary devices which supports support MODBUS RTU or MODBUS ASCII standards (serial communication) as well as MODBUS over TCP/IP. Moreover, it supports two extensions:
- 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.
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Parameter | Meaning | Unit | Default value | ||||||
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| The parameter is implemented only for TCP/IP-TCP and TCP/IP-TCP Redundant 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 | ||||||
| Setting Tcp No Delay parameter to YES causes low-level socket option TCP_NODELAY being 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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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. | 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 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 | ||||||
| Byte 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 mode 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 a proper communication processof 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 | ||||||
| To read the values from addresses that are not configured is not allowed. Description and example: | YES/NO | NO | ||||||
| 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 | NO |
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Kotva | ||||
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Address format is [I|U|Uu|Ul|f|F|L|Ll|S|Sl|B|X|sn.|an.|An.][d|D][b][s]RdFn[-WrFn[d]].Address[.BitNr] in which:
- First 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 considered processed (1st in sequence)
- Ul - Unsigned16 - one register is read, unsigned, only the lower byte is considered 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 number of 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.
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You can read blogs about the Modbus protocol |
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