Serial link setting parameters can be accessed with Parameter Setting tool in PCM600, via the HMI path .
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 the protection relay, the response messages may be transmitted too fast for the link standard especially if slower baud rates are used. 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.
Note: To set the timing properly, consider also how the other server units in a multidrop RS-485 network detect the Modbus traffic between the slaves and this protection relay.
The end of message delay, or timeout, is used only in the Modbus RTU link mode. According to the Modbus standard, an idle period of 3.5 characters, that is the time it takes to transmit 3.5 characters with the used baud rate, defines the end of a Modbus 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.
Note: 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 next request frame when setting the end delay in Modbus RTU mode.
It is possible to define which serial port is used by separate in the Modbus Master instance by selecting either “COM1” or “COM2” for the Port parameter.
Note: If the protocol is not operating as expected, ensure that no other serial protocols are using the same COM port.
Note: The baud rate is defined on the serial driver side and are therefore located via the HMI paths and .