Thank you for asking.
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Please refer to? "AX.25 Link Access Protocol for Amateur Packet Radio Version 2.2 Revision: July 1998" which can be found here: ?? ?
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Section 3 describes the AX.25 frame structures. ? Information (I) frames contain a Protocol ID (PID) field.
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Figure 3.2 contains the possible values for the PID field.? 0 is not listed as a valid value in the table.? direwolf is warning that an invalid value is being used.
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It's apparently not causing a problem so you can just ignore it.
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If you would like to invest some extra effort, to arrive at the truth, add the "-dakn" debug option to the direwolf command line.? This will display to communication between direwolf and WinLink.? We will be able to see whether WinLink is generating an improper PID value or if it is a problem with direwolf.
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73,
John WB2OSZ
