Section 3
1MAC052634-MB D
Vendor-specific implementation
10
615/620 series ANSI
Communication Protocol Manual
Start delay
The intraframe delay on serial Modbus RTU link is defined as a silent interval of 3.5
characters. The delay is essential for Modbus devices to recognize the beginning and end
of each RTU frame. If the end delay is decreased in this IED, the response messages may
be transmitted too fast according to the link standard especially true with slower baud
rates. The start delay parameter adds idle characters before the transmission, thus
increasing the silent interval between the Modbus RTU link frames. The start delay default
setting is four idle (silent) characters.
Serial port
It is possible to define which serial port is used for separate Modbus serial instances:
COM1 or COM2. The serial communication instance is not active if this parameter is set
to “Not in use.”
3.1.1.2
Modbus serial diagnostic counters
Modbus Serial diagnostic counters can be viewed via the LHMI path
Monitoring >
Communication > Modbus > Serial
.
Counters related to the possible Modbus serial instances 1...2 have the suffixes 1…2 (N).
The counters show complete Modbus protocol link frames and Modbus errors. The serial
communication drivers (COM1, COM2) maintain their own counters for lower level serial
communication diagnostics.
In a multidrop RS-485 Modbus network the unit may detect and receive
response messages from other slave units. Thus, consider the minimum
silent time between the response frame and the beginning of master’s next
request frame when setting the end delay in Modbus RTU mode.
This parameter has no meaning in the Modbus ASCII link mode.
To set the timing properly, consider also how the other slave units in a
multidrop RS-485 network detect the Modbus traffic between the master
and this IED.
If this protocol does not operate as expected, make sure that other serial
protocols are not using the COM port as well.
Baud rate is defined on the serial driver side and are therefore located via
the LHMI paths
Configuration > Communication > COM1
and
Configuration > Communication > COM2
.
