MODBUS Server communication protocol

Supported device types and versions
Communication line configuration
Communication station configuration
Line protocol parameters
I/O tag configuration
Literature
Changes and modifications
Document revisions

Supported device types and versions


The protocol executes a slaver communication with arbitrary devices that support standard MODBUS RTU in versions of serial communication and MODBUS over TCP/IP.

Communication line configuration


Line protocol parameters


Configuration dialog box - table Parameters.
They influence some optional parameters of the protocol. Following line protocol parameters can be set:

Table 1

ParameterMeaningUnitDefault value
Silent Interval
Delay which is entered between the start of transmission of each data packet.ms50
No Request Timeout
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:ss1:00
Single Server
If the value of the parameter is set on YES, the KOM process replies by error SLAVE_DEVICE_FAILURE to each request which is sent to a non-existent station (a station with unknown address). If the value is NO, the KOM process ignores this request and does not answer reply to it.YES/NOYES
Moxa Timeout
Switching time of Moxa redundant devices in case of communication error or some problems. As this is a server protocol which waits for requests from external devices, a failure to receive a communication request for longer time than the value of this parameter is considered to be an error. It is effective only for SerialOverUDP Device Redundant line.sec10 sec

Communication station configuration


It is possible to configure more stations with different addresses on one line, KOM process will reply on behalf of every configured station. See also the line protocol parameters Single Server.

Station protocol parameters

Configuration dialog box - tab Parameter.
They influence some optional parameters of the protocol. Following station protocol parameters can be set:

Table 2

ParameterMeaningUnitDefault value
Addressing model
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 a change of this parameter, the KOM process must be restarted.
MODBUS PDU
MODBUS data Model
MODBUS PDU

I/O tag configuration


Possible value types: Ai, Ao, Di, Do, Ci, Co.

I/O tag address:

In the MODBUS protocol, the basic address space is divided into registers of Coils type (reading/writing), Discrete Inputs (reading), Holding Registers (reading/writing), and Input Registers (reading).

Every address space is independent, providing 2-byte addressing, i.e. addresses from 0 up to 65535.


Address format of I/O tag:
Address format is [I|U|L|Ll|S|Sl|f|F|C|D]Fn.Address[.BitNr] in which:

Implementation of protocol supports the following functions (commands of MODBUS Client for a D2000 KOM process):

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).

Literature



You can read blogs about  Modbus protocol:


Changes and modifications


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Document revisions


Communication protocols