update TCP client example and USAGE docs for on_get callbacks, see #51, contributes to #31

Author: brainelectronics

docs/USAGE.md

@@ -231,8 +231,9 @@ be `percent`.
 ###### Optional callbacks
 
 The optional keys `on_set_cb` and `on_get_cb` can be used to register a
-callback function on client side which is executed after a new value has been
-set or after a register value has been requested.
+callback function on client side which is executed **after** a new value has
+been set or **before** the response of a requested register value has been
+sent.
 
 ```{note}
 Getter callbacks can be registered for all registers with the `on_get_cb`

examples/tcp_client_example.py

@@ -99,9 +99,18 @@ def my_discrete_inputs_register_get_cb(reg_type, address, val):
 
 
 def my_inputs_register_get_cb(reg_type, address, val):
+    # usage of global isn't great, but okay for an example
+    global client
+
     print('Custom callback, called on getting {} at {}, currently: {}'.
           format(reg_type, address, val))
 
+    # any operation should be as short as possible to avoid response timeouts
+    # It would be also possible to read the leatest pin state at this time
+    val += 1
+    client.set_ist(address=address, value=val)
+    print('Incremented current value by +1 before sending response')
+
 
 def reset_data_registers_cb(reg_type, address, val):
     # usage of global isn't great, but okay for an example