Modbus: extend documentation

Clarify that default Modbus client implementation does not handle all
FunctionCodes and that this needs to be implemented in the custom
Modbus clients. Also explicitly mention that UnknownError can also mean
that the received function code is not implemented.

This commit amends 09927894825d97197ffa656576e6be780371dda0

Fixes: QTBUG-103879
Change-Id: Ifb70107cda2595293b9759504fcfc1ca1a3abfa7
Reviewed-by: Karsten Heimrich <karsten.heimrich@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
(cherry picked from commit 8360fcd497bcf6e52412629cd4e29f1ef05a93d6)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 13 insertions, 2 deletions

diff --git a/src/serialbus/qmodbusclient.cpp b/src/serialbus/qmodbusclient.cpp
index 72cc5a0..a91ba46 100644
--- a/src/serialbus/qmodbusclient.cpp
+++ b/src/serialbus/qmodbusclient.cpp
@@ -222,8 +222,12 @@ QModbusClient::QModbusClient(QModbusClientPrivate &dd, QObject *parent) :
 }
 
 /*!
-    Processes a Modbus server \a response and stores the decoded information in \a data. Returns
-    true on success; otherwise false.
+    Processes a Modbus server \a response and stores the decoded information in
+    \a data. Returns \c true on success; otherwise \c false.
+
+    \note The default implementation does not support all
+    \l {QModbusPdu::}{FunctionCode}s. Override this method in a custom Modbus
+    client implementations to handle the needed functions.
 */
 bool QModbusClient::processResponse(const QModbusResponse &response, QModbusDataUnit *data)
 {
diff --git a/src/serialbus/qmodbusdevice.cpp b/src/serialbus/qmodbusdevice.cpp
index 8036355..91a7652 100644
--- a/src/serialbus/qmodbusdevice.cpp
+++ b/src/serialbus/qmodbusdevice.cpp
@@ -204,6 +204,13 @@ void QModbusDevice::setConnectionParameter(int parameter, const QVariant &value)
     \value ReplyAbortedError    The reply was aborted due to a disconnection of
                                 the device.
     \value UnknownError         An unknown error occurred.
+
+    \note An UnknownError can also indicate that the received
+    \l {QModbusPdu::}{FunctionCode} is not supported in the current
+    implementation. In this case custom Modbus client implementations need to
+    override the \l {QModbusClient::}{processResponse()} and
+    \l {QModbusClient::}{processPrivateResponse()} methods to provide support
+    for needed functions.
 */
 
 /*!