Commit a38e976bb2788890bf12f6a528ba87f422be3f77
1 parent
3540cd74
Reduce the length defined for the shortest query and add better comment.
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10 additions
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2 deletions
modbus/modbus.h
| ... | ... | @@ -39,8 +39,16 @@ extern "C" { |
| 39 | 39 | #define CHECKSUM_LENGTH_RTU 2 |
| 40 | 40 | #define CHECKSUM_LENGTH_TCP 0 |
| 41 | 41 | |
| 42 | -/* 8 + HEADER_LENGTH_TCP */ | |
| 43 | -#define MIN_QUERY_LENGTH 14 | |
| 42 | +/* It's not really the minimal length (the real one is report slave ID | |
| 43 | + * in RTU (4 bytes)) but it's a convenient size to use in RTU or TCP | |
| 44 | + * communications to read many values or write a single one. | |
| 45 | + * Maximum between : | |
| 46 | + * - HEADER_LENGTH_TCP (6) + slave (1) + function (1) + address (2) + | |
| 47 | + * number (2) | |
| 48 | + * - HEADER_LENGTH_RTU (0) + slave (1) + function (1) + address (2) + | |
| 49 | + * number (2) + CRC (2) | |
| 50 | +*/ | |
| 51 | +#define MIN_QUERY_LENGTH 12 | |
| 44 | 52 | |
| 45 | 53 | /* Page 102, Application Notes of PI–MBUS–300: |
| 46 | 54 | * The maximum length of the entire message must not exceed 256 | ... | ... |