5.
Check the receive and send delay settings in the relay. If link frames are not
received properly there might be character timing problems.
6.
Check for receive errors, checksum errors or several retransmissions. If these are
found, the line may be noisy.
7.
If Modbus link messages are received, check that the response messages are sent
to the master.
8.
Check the serial driver transmitted character counter. If it is running, then the
relay is transmitting. If the master receives nothing, then check the cable (Tx
line).
3.2.1.4
Character framing in different serial link modes
According to the Modbus standard, the character length in the Modbus RTU mode
should be 11 bits and in Modbus ASCII mode 10 bits. It is possible to freely define the
character parity: even, odd or no parity. No parity means that the bit length of the serial
character is reduced by one. Thus, the character is compensated with an additional
stop bit.
Table 3:
RTU characters
Coding system 8-bit binary
Bits per
character
1 start bit
8 data bits, the least significant bit is sent first
1 bit for even/odd parity; no bit if parity is not used
1 stop bit if parity is used; 2 stop bits if parity is not used
Table 4:
ASCII characters
Coding system Two ASCII characters representing a hexadecimal number
Bits per
character
1 start bit
7 data bits, the least significant bit is sent first
1 bit for even/odd parity; no bit if parity is not used
1 stop bit if parity is used; 2 stop bits if parity is not used
3.2.2
TCP/IP link
The IED operates as a Modbus TCP/IP server. A Modbus TCP/IP client can establish
a connection to the IED through the standardized TCP socket port 502.
The Modbus TCP/IP interface of the IED can be configured to accept up to five
simultaneous Modbus client connections. It is possible to grant connections only to
the predefined TCP/IP clients. The write authority of the Modbus TCP/IP client is
configurable.
Modbus TCP usually shares the Ethernet connection with the other
Ethernet based protocols of the IED. The number of Ethernet based
clients that can be simultaneously connected to the IED is restricted.
Section 3
1MRS757645 B
Vendor-specific implementation
18
620 series
Communication Protocol Manual
