Diagnostic functions - Arc protection - Auto synchronization - Bay control and measurement - Merging unit - Transformer protection - 2 winding - Feeder protection - Voltage regulation - Capacitor bank protection - Petersen Coil control - Grid automation - Busbar differential protection (high impedance) - Back-up protection - Motor protection - Busbar protection (voltage and frequency) - Transformer protection - 3 winding - Interconnection protection - Generator protection - Power management/Load shedding - Modbus Communication Manual - REX640 Protection and control - PCL4 - IEC - ANSI

The diagnostic functions are only intended for serial communication. However, the serial diagnostic counters can be read, but not reset, via the Modbus TCP/IP interface. The serial line cannot be forced to the listen mode via the Modbus TCP/IP interface.

Table 1. Supported diagnostic subfunctions
Function code	Name	Description
00	Return query data	The data in the query data field is returned (looped back) in the response. The entire response is identical to the query.
01	Restart communication option	The server's peripheral port is initialized and restarted and the communication event counters are cleared. Before this, a normal response is sent provided that the port is not in the listen-only mode. If the port is in the listen only mode, no response will be sent.
04	Force listen only mode	The server is forced to enter the listen only mode for Modbus communication.
10	Clear counters and diagnostic register	All counters and the diagnostic register are cleared.
11	Return bus message count	The response returns the number of messages in the communication system detected by the server since its last restart, clear counters operation or power up.
12	Return bus communication error count	The response returns the number of CRC errors encountered by the server since its last restart, clear counters operation or power up.
13	Return bus exception error count	The response returns the number of Modbus exception responses sent by the server since its last restart, clear counters operation or power up.
14	Return server message count	The response returns the number of messages addressed to the server or broadcast which the server has processed since its last restart, clear counters operation or power up.
15	Return server no response count	The response returns the number of messages addressed to the server for which a response (a normal response or an exception response) has not been sent since its last restart, clear counters operation or power-up.
16	Return server NACK response count	The response returns the number of messages addressed to the server for which a negative acknowledgement response has been sent.
18	Return bus character overrun count	The response returns the number of messages addressed to the server for which it has not been able to send a response due to a character overrun since its last restart, clear counters operation or power up.