512 is max length for variable length, not fixed

adafruit_ble/characteristics/__init__.py

@@ -51,7 +51,7 @@ class Characteristic:
     :param int write_perm: Specifies whether the characteristic can be written by a client,
        and if so, which security mode is required. Values allowed are the same as ``read_perm``.
     :param int max_length: Maximum length in bytes of the characteristic value. The maximum allowed
-       is 512, or possibly 510 if ``fixed_length`` is False. The default, 20, is the maximum
+       is 512, or on nRF, 510 if ``fixed_length`` is True. The default, 20, is the maximum
        number of data bytes that fit in a single BLE 4.x ATT packet.
     :param bool fixed_length: True if the characteristic value is of fixed length.
     :param buf initial_value: The initial value for this characteristic. If not given, will be

adafruit_ble/characteristics/string.py

@@ -51,7 +51,7 @@ class StringCharacteristic(Characteristic):
             properties=properties,
             read_perm=read_perm,
             write_perm=write_perm,
-            max_length=510,  # shorter than 512 due to fixed_length==False
+            max_length=512,
             fixed_length=False,
             initial_value=initial_value,
         )