ABB Relion 620 Series Protocol Manual page 22

Section 3
Vendor-specific implementation
16
RTU frame in the RTU mode. This parameter can be given with the accuracy of one
character. The default setting is three characters but the user can increase or decrease
the value.
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.
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.
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.
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."
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.
1MRS757645 B
620 series
Communication Protocol Manual