|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 - first the lower word is received, then the higher word (higher byte within the word is always first)
- 3412 - first the higher word is received, then the lower word (lower byte within the word is always first)
- 1234 - bytes are received from lowest to highest (direct opposite of big-endian)
|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:
Little endian = the lowest bytes are sent first
Big endian = the highest bytes are sent first
OFF = variable mode is switched off
Note 1: Variable and byte modes are incompatible and only one of them can be enabled.
Note 2: Emerson FloBoss 103 device: text strings and time stamps of 6-byte are sent always from the lowest byte.
Note 3: Variable mode is implemented only for Protocol Mode=RTU.
Note 4: A data encoding big-endian is used automatically, according to the default parameter values Byte mod=NO and Variable mode=OFF (i.e. according to MODBUS protocol specification).
|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.
|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 data Model
|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 over 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).
Bitmask 01 means that only the lowest bit is considered. If the address of the I/O tag specifies the use of only the lower/upper byte of the register, the lowest/second lowest byte of the mask is applied.
|-||FF FF FF FF|
|This parameter is used to avoid reading the values from addresses that are not configured.|
Description and example:
The requests for data, which are limited by protocol parameter "Max. Registers" or "Max. Bytes", are sent as standard. If I/O tags with addresses "Holding Registers" 1, 2, and 5 have been configured, one request reading 5 registers starting with address 1 is sent although the I/O tags with addresses 3 and 4 are not configured. It is more efficient to obtain the required data by one request than by two ones even if the unnecessary data are also read.
If the parameter "Skip Unconfigured" is set on YES, two requests are sent, the first one reads two registers from address 1 and the second one reads one register from address 5.
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).
|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:|
This extra check is reasonable on high-latency and variable-latency lines - e.g. GPRS networks - to detect and avoid the situation when read request (#1) is repeated due to timeouts and then two responses are received, the second of which could be considered to be an answer to another read request (#2), thus causing wrong values being assigned to I/O tags addressed by this read request #2.
- if Byte mode is on (Byte mode=YES), the length of received data must be equal to the number of registers
- if both Byte mode and variable mode are off, the length of received data must be equal to double the number of registers
- if the variable mode is on (Variable mode=little-endian or big-endian), check has not been implemented yet
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.
This mode can be used for special devices that send all data (with variable size registers - 2, 4, 8 bytes - in a single response).
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.
Write function for writing the password (TCP Write Password) on the TCP/IP-TCP line.
If the number of bytes of the password is greater than 2, function 16 (Write Multiple Registers) is used even if function 6 (Write Single Register) is specified.