Update param protocol with PARAM_ERROR and improve #644
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hamishwillee
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Oct 29, 2025
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| 4. If the targeted component does not have any parameters then it should ignore the `PARAM_REQUEST_LIST` request. | ||
| The GCS may resend the request on timeout if no `PARAM_VALUE` is received, but should cease after resends. |
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Should we create an error for this case so that no need for a retry on PARAM_REQUEST_LIST?
hamishwillee
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Oct 29, 2025
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| In some cases the drone may not be able write a value, for example because it is a read-only parameter, the specified parameter id or index does not exist, or the id exists but the encoding specified does not match the stored values. | ||
| If the parameter is known (i.e. the id or index is valid) the drone should acknowledge the `PARAM_SET` by broadcasting a `PARAM_VALUE` (in the case of error, with the current/unchanged parameter value). | ||
| 1. GCS should update the [parameter cache](#parameter_caching) (if used) with the new value. | ||
| 1. The GCS may restart the sequence if the expected `PARAM_VALUE` is not received within the timeout, or if the write operation fails (the value returned in `PARAM_VALUE` does not match the value set). | ||
| In older versions of the protocol, if the parameter is unknown the `PARAM_SET` would be silently ignored. | ||
| In newer versions, if [PARAM_ERROR](#PARAM_ERROR) message is supported, it should be sent for any error (alongside `PARAM_VALUE` if its value can be known). | ||
| The GCS may restart the sequence if the expected `PARAM_VALUE` and/or `PARAM_ERROR` is not received within the timeout. |
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This is PARAM VALUE/ERROR handling for a write. Note that it states that you return the PARAM_VALUE if there is a value that can be returend.
hamishwillee
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Oct 29, 2025
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| In some cases the drone may not be able read a value, such as if an invalid parameter id or index was specified. | ||
| In older versions of the protocol the drone would silently ignore the message. | ||
| In newer versions the drone may return a [PARAM_ERROR](#PARAM_ERROR) message. | ||
| The GCS may restart the sequence if a `PARAM_VALUE` or `PARAM_ERROR` acknowledgment is not received within the timeout. |
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This is PARAM VALUE/ERROR handling for a read.
hamishwillee
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Oct 29, 2025
| - C-style encoding of parameters is supported. | ||
| Note however that ArduPilot does not set `MAV_PROTOCOL_CAPABILITY_PARAM_ENCODE_C_CAST`. | ||
| C-style encoding of parameters is used and `MAV_PROTOCOL_CAPABILITY_PARAM_ENCODE_C_CAST` is set on versions since circ a 2022. |
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If you know a version or date that might be handy? I haven't modified the following text for off spec behaviour. To be confirmed how it works.
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This updates the param protocol with information about PARAM_ERROR. Designed to match mavlink/mavlink#2365 - in progress.
Some questions for you @peterbarker - we should iterate this in parallel with the link above.
FYI @DonLakeFlyer