“Modbus Slave” Tab

“Settings” Panel – “Modbus Slave” Tab
Parameter	Value	Description
General Settings
Unit ID	1 … 247	Specifies the Modbus unit ID (slave ID)
Behavior on PLC Stop	Behavior of the Modbus slave when the control system has halted (control system in “STOP” state):
	Keep last value	Data exchange possible For Modbus read requests, the last frozen values are returned. For write requests, the values are carried over unchanged to the Modbus process image without passing them to the controller.
	No data exchange	No data exchange possible Modbus requests will always be answered by the exception response “ILLEGAL FUNCTION” (0x81).
	Use substitute value	Data exchange possible For Modbus read requests, substitute values (0) are returned. For write requests, the values are carried over unchanged to the local Modbus process image without passing them to the controller.
Behavior on fieldbus error	Response of the Modbus slave in the event of detected fieldbus errors (interruption of communication).
	Keep last value	Data exchange possible For PLC read functions, the last frozen values from the Modbus process image are returned. For write accesses, the values are carried over unchanged to the Modbus process image without passing them to the Modbus master.
	No data exchange	No data exchange possible
	Use substitute value	Data exchange possible For PLC read functions, the substitute values (0) from the Modbus process image are returned. For write accesses, the values are carried over unchanged to the Modbus process image without passing them to the Modbus master.
Response Delay [ms]	0 … 32	Transmission of the response to a Modbus request is delayed from the time of processing (read and/or write register values) by the time according to the setting. 32 ms = maximum delay 0 ms = no delay The actual duration is always longer due to parallel requests/system utilization. Changes take effect immediately for each subsequent request.
Update Variables	Always update	Variables are always updated, even if they do not appear in the program, but only used in the Visu Task.
Update Variables	Only when used in the PLC task	Variables are only updated when the appear in the program.
Watchdog Settings
Function Mode	Advanced Mode	The watchdog must be controlled explicitly via commands (see registry 0xFA00 [Watchdog Command]).
Function Mode	Simple Mode	The watchdog is activated directly with a timeout > 0 s in registry 0xFA01 (Watchdog Timeout). Each trigger restarts both the running watchdog as well as the expired one.
Reset Timeout [ms]	0 … 65535	Specifies the watchdog reset timeout.
Explicit Trigger	On	Only writing the value 0x5555 (WATCHDOG_START) to registry 0xFA00 (Watchdog Command) is considered a watchdog trigger (exception: registry 0xFA02 [Watchdog Status]).
Explicit Trigger	Off	All valid Modbus requests are considered watchdog triggers (exception: registry 0xFA02 [Watchdog Status]).
Trigger on Status	On	Reading the watchdog status triggers the watchdog.
Trigger on Status	Off	The watchdog trigger is disabled. Watchdog status read-out is not considered a watchdog trigger.
Resetting of the TCP Connections	On	All Modbus TCP connections are disconnected when the watchdog expires.
Resetting of the TCP Connections	Off	Existing Modbus TCP connections remain open.
TCP Settings
TCP Port	1 … 65535	Port number for the TCP link 1 = minimum port number 502 = Modbus default port 65535 = maximum port number
Timeout [10 ms]	1 … 65535	Timeout for a TCP link The connection is closed when no more packets are received within the timeout period. Example: Entering “1” yields 10 ms. Entering “600” yields 6 s. Entering “65535” yields 10 min 55 s 350 ms.
Type of Service (IP)
Low Delay	On	Sets the “Low Delay” attribute in the IP header
Low Delay	Off	The “Low Delay” attribute is not set.
High Throughput	On	Sets the “High Throughput” attribute in the IP header
High Throughput	Off	The “High Throughput” attribute is not set.
High Reliability	On	Sets the “High Reliability” attribute in the IP header
High Reliability	Off	The “High Reliability” attribute is not set.
Keepalive (TCP)
Enabled	On	Enables “KeepAlive” for TCP connections
Enabled	Off	Disables “KeepAlive” for TCP connections
KeepAliveTime [s]	0 … 65535	The time span in seconds between the last data packet and the first “KeepAlive” packet (probe)
KeepAliveInterval [s]	0 … 65535	Time between two successive connection attempts if “KeepAlive” packet fails
KeepAliveProbes [Number]	0 … 65535	Number of unanswered attempts until a connection is considered lost
UDP Settings
UDP Port	1 … 65535	Port number for the UDP link 1 = minimum port number 502 = Modbus default port 65535 = maximum port number
Type of Service (IP)
Low Delay	On	Sets the “Low Delay” attribute in the IP header
Low Delay	Off	The “Low Delay” attribute is not set.
High Throughput	On	Sets the “High Throughput” attribute in the IP header
High Throughput	Off	The “High Throughput” attribute is not set.
High Reliability	On	Sets the “High Reliability” attribute in the IP header
High Reliability	Off	The “High Reliability” attribute is not set.
Multicast Settings
Multicast Group 1 … 5	xxx.xxx.xxx. xxx	Specifies the IP address of multicast groups 1 to 5