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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)

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).

Note: In the case of "MODBUS over TCP/IP", the parameter value is ignored and Protocol Mode=RTU is used.

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.

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 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.