feat(inputs.modbus): add config option to pause after connect (#11983)

etc/telegraf.conf

@@ -5970,6 +5970,8 @@
 #
 #   ## Enable workarounds required by some devices to work correctly
 #   # [inputs.modbus.workarounds]
+#     ## Pause after connect delays the first request by the specified time. This might be necessary for (slow) devices.
+#     # pause_after_connect = "0ms"
 #     ## Pause between read requests sent to the device. This might be necessary for (slow) serial devices.
 #     # pause_between_requests = "0ms"
 #     ## Close the connection after every gather cycle. Usually the plugin closes the connection after a certain

etc/telegraf_windows.conf

@@ -5764,6 +5764,8 @@
 #
 #   ## Enable workarounds required by some devices to work correctly
 #   # [inputs.modbus.workarounds]
+#     ## Pause after connect delays the first request by the specified time. This might be necessary for (slow) devices.
+#     # pause_after_connect = "0ms"
 #     ## Pause between read requests sent to the device. This might be necessary for (slow) serial devices.
 #     # pause_between_requests = "0ms"
 #     ## Close the connection after every gather cycle. Usually the plugin closes the connection after a certain

plugins/inputs/modbus/README.md

@@ -196,6 +196,8 @@ Registers via Modbus TCP or Modbus RTU/ASCII.
 
   ## Enable workarounds required by some devices to work correctly
   # [inputs.modbus.workarounds]
+    ## Pause after connect delays the first request by the specified time. This might be necessary for (slow) devices.
+    # pause_after_connect = "0ms"
     ## Pause between read requests sent to the device. This might be necessary for (slow) serial devices.
     # pause_between_requests = "0ms"
     ## Close the connection after every gather cycle. Usually the plugin closes the connection after a certain
@@ -211,9 +213,9 @@ those debug messages, Telegraf has to be started with debugging enabled
 (i.e. with the `--debug` option). Please be aware that connection tracing will
 produce a lot of messages and should __NOT__ be used in production environments.
 
-Please use `pause_between_requests` with care. Ensure the total gather time,
-including the pause(s), does not exceed the configured collection interval. Note
-that pauses add up if multiple requests are sent!
+Please use `pause_after_connect` / `pause_between_requests` with care. Ensure
+the total gather time, including the pause(s), does not exceed the configured
+collection interval. Note that pauses add up if multiple requests are sent!
 
 ## Configuration styles
 

plugins/inputs/modbus/modbus.go

@@ -24,8 +24,9 @@ var sampleConfigStart string
 var sampleConfigEnd string
 
 type ModbusWorkarounds struct {
-	PollPause        config.Duration `toml:"pause_between_requests"`
-	CloseAfterGather bool            `toml:"close_connection_after_gather"`
+	AfterConnectPause config.Duration `toml:"pause_after_connect"`
+	PollPause         config.Duration `toml:"pause_between_requests"`
+	CloseAfterGather  bool            `toml:"close_connection_after_gather"`
 }
 
 // Modbus holds all data relevant to the plugin
@@ -265,6 +266,10 @@ func (m *Modbus) initClient() error {
 func (m *Modbus) connect() error {
 	err := m.handler.Connect()
 	m.isConnected = err == nil
+	if m.isConnected && m.Workarounds.AfterConnectPause != 0 {
+		nextRequest := time.Now().Add(time.Duration(m.Workarounds.AfterConnectPause))
+		time.Sleep(time.Until(nextRequest))
+	}
 	return err
 }
 

plugins/inputs/modbus/sample.conf

@@ -188,6 +188,8 @@
 
   ## Enable workarounds required by some devices to work correctly
   # [inputs.modbus.workarounds]
+    ## Pause after connect delays the first request by the specified time. This might be necessary for (slow) devices.
+    # pause_after_connect = "0ms"
     ## Pause between read requests sent to the device. This might be necessary for (slow) serial devices.
     # pause_between_requests = "0ms"
     ## Close the connection after every gather cycle. Usually the plugin closes the connection after a certain

plugins/inputs/modbus/sample_general_end.conf

@@ -1,5 +1,7 @@
   ## Enable workarounds required by some devices to work correctly
   # [inputs.modbus.workarounds]
+    ## Pause after connect delays the first request by the specified time. This might be necessary for (slow) devices.
+    # pause_after_connect = "0ms"
     ## Pause between read requests sent to the device. This might be necessary for (slow) serial devices.
     # pause_between_requests = "0ms"
     ## Close the connection after every gather cycle. Usually the plugin closes the connection after a certain