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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.
Note: this mode enables work with devices implementing the so-called Enron Modbus or Daniel Modbus. - Passive (scanning) mode allows 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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Parameter | Meaning | Unit | Default value | |||||||||
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| Maximum count of request retries. If no response returns after a request has 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 | |||||||||
| 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-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 the 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 | |||||||||
| This parameter is used to avoid reading the values from addresses that are not configured. Description and example: | YES/NO | NO | See an example of such communication: | 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:
Kotva | | crl | crl | Check Receive LengthIf 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 | ||||||
| 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 | |||||||||
| The write function for writing the password (TCP Write Password) on the TCP/IP-TCP line. | 6 16 | 6 |
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The attached ZIP contains the configuration of two lines and two stations with the Modbus MODBUS Client/Modbus MODBUS Server protocol that communicate via TCP (Modbus MODBUS Server listens on a TCP port 9999). Data written through one line is received through the other line. Four I/O tags are configured on each line (Float, with address 3.0, Signed with addresses 4.0 and 4.1, and Bool with address 1.0). Modbus MODBUS Server I/O tags values are controlled by the system second (Sec) and use linear conversion (Float is divided by 1000, Signed with address 4.0 is multiplied by 10). The value of the Bool I/O tag is controlled by the eval tag P.TrueFalse, which changes the value True/False every second. by 10). The value of the Bool I/O tag is controlled by the eval tag P.TrueFalse, which changes the value True/False every second.
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The attached ZIP contains the configuration of the line, station, and I/O tags for communication with an Emerson chromatograph that implements MODBUS DANIEL®.
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You can read blogs about the Modbus protocol |
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