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The protocol executes a slaver implements a server (slave) communication with arbitrary devices that support standard the MODBUS RTU standard either in versions of a serial communication and version or a MODBUS over TCP/IP variant.
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- Line category: Serial (serial communication), SerialOverUDP Device Redundant (serial communication).
- Line category TCP/IP-TCP (MODBUS over TCP/IP). Use a symbolic address ALL or *, in order for the KOM process listens to listen on a selected TCP port on all existing network interfaces. TCP port 502 is commonly used, but any of the ports can be used.
Line number - set the value for example 1.
Note: KOM process works as a multitasking TCP server and that is why it is able to operate more handle multiple clients at the same time.
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| Parameter | Meaning | Unit | Default value | ||||||
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| Delay which is entered between A delay before the start of transmission of each data packet. | ms | 50 | ||||||
| If timeout passes and no valid request comes, all stations on the line will go to a communication error. However, the values of output I/O tags will not be invalidated (as this is a server protocol). | mi:ss | 1:00 | ||||||
| If the value of the parameter is set on to YES, the KOM process replies by error SLAVE_DEVICE_FAILURE error to each request which is sent to a non-existent station (a station with an unknown address). If the value is NO, the KOM process ignores this request and does not answer send a reply to it. | YES/NO | YES | ||||||
| Switching time of Moxa redundant devices in case of communication error or some problems. As this is a server protocol which that waits for requests from external devices, a failure to receive a communication request for a longer time than the value of this parameter is considered to be an error. It is effective only for SerialOverUDP Device Redundant line. | sec | 10 sec |
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It is possible to configure more stations with different addresses on one line, the KOM process will reply on behalf of every configured station. See also the line protocol parameters Single Server.
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Every address space is independent, providing 2-byte addressing, i.e. addresses from 0 up to 65535.
The I/O tag with an address starting with %IGNORE will be ignored.
Address format of I/O tag:
Address format is [I|U|L|Ll|S|Sl|f|F|C|D]Fn.Address[.BitNr] in which:
- The first optional character defines the type of I/O tag:
- I - Integer 16 bit
- U - Unsigned 16 bit (default)
- 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)
- f - Float 32 bit (two registers) in big-endian format (bytes B4, B3, B2, B1 will be sent, B4 is highest byte and B1 is the lowest byte of float)
- F - Float 32 bit (two registers) in little-endian format (bytes B2, B1, B4, B3 will be sent, B4 is highest byte and B1 is the lowest byte of float)
- C - Request counter up (16 bit unsigned, which is incremented by every read request). Works only for Fn=3 or Fn=4
- D - Request counter down (16 bit unsigned, which is decremented by every read request). Works only for Fn=3 or Fn=4
Note: Request counter up/down can be used to configure a "watchdog" I/O tag to monitor the status and speed of Modbus communication.
- Parameter Fn is a function of Modbus protocol for data reading, which inserts I/O tag into proper address space:
- 1 - Coils: binary statuses
- 2 - Discrete Inputs: binary inputs
- 3 - Holding Registers: status registers
- 4 - Input Registers: input registers
- Parameter Address is a 2-bytes address of a register in the range of 0 up to 65535.
- Parameter BitNr optionally specifies a bit of register in the range of 0 up to 15.
Note: coexistence of an I/O tag without BitNr parameter and multiple I/O tags with BitNr parameter having the same Address is possible.
Implementation of protocol supports the following functions (commands of MODBUS Client for a D2000 KOM process):
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Note: This is a server type of protocol that is primarily intended for sending the values out of the D2000 system. Therefore the I/O tags should be configured as an output (Ao, Co, Do) because of the manipulation of their values directly or by control objects. If the I/O tag is configured as input (Ai, Ci, Di), the KOM process is unable to send a valid value in a reply to reading by functions 1 - 4 until the value is written by function 5, 15, 6, or 16 from outside.
If the KOM process does not have the valid value of I/O tag or request to read a nonexistent I/O tag is received, an implicit value False or 0 is sent as a response to a read request (MODBUS protocol does not support the transfer of value quality).
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You can read blogs about about the Modbus protocol (for now, in Slovak language only): |
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