Peter M. Groen / libmodbus

Commit 98c4aca38412f6b86ffeb3fd01c68ee9f1fd04eb

Authored by Stéphane Raimbault
1 parent 130cf4b2

Export MODBUS_MAX_ADU_LENGTH and documentation (ref #241)

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Showing 2 changed files with 25 additions and 6 deletions
doc/modbus_receive_confirmation.txt
  View file @98c4aca
... ... @@ -14,11 +14,18 @@ SYNOPSIS
14 14  
15 15 DESCRIPTION
16 16 -----------
17   -The *modbus_receive_confirmation(*_ function shall receive a request via the
  17 +The *modbus_receive_confirmation()* function shall receive a request via the
18 18 socket of the context _ctx_. This function must be used for debugging purposes
19 19 because the received response isn't checked against the initial request. This
20 20 function can be used to receive request not handled by the library.
21 21  
  22 +The maximum size of the response depends on the used backend, in RTU the _rsp_
  23 +array must be _MODBUS_RTU_MAX_ADU_LENGTH_ bytes and in TCP it must be
  24 +_MODBUS_TCP_MAX_ADU_LENGTH_ bytes. If you want to write code compatible with
  25 +both, you can use the constant _MODBUS_MAX_ADU_LENGTH_ (maximum value of all
  26 +libmodbus backends). Take care to allocate enough memory to store responses to
  27 +avoid crashes of your server.
  28 +
22 29  
23 30 RETURN VALUE
24 31 ------------
... ... @@ -27,6 +34,13 @@ response length if sucessful. The returned request length can be zero if the
27 34 indication request is ignored (eg. a query for another slave in RTU
28 35 mode). Otherwise it shall return -1 and set errno.
29 36  
  37 +EXAMPLE
  38 +-------
  39 +[source,c]
  40 +-------------------
  41 +uint8_t rsp[MODBUS_MAX_ADU_LENGTH];
  42 +rc = modbus_receive_confirmation(ctx, rsp);
  43 +-------------------
30 44  
31 45 SEE ALSO
32 46 --------
... ...
src/modbus.h
  View file @98c4aca
... ... @@ -97,14 +97,19 @@ MODBUS_BEGIN_DECLS
97 97 * the first MODBUS implementation on Serial Line network (max. RS485 ADU = 256
98 98 * bytes). Therefore, MODBUS PDU for serial line communication = 256 - Server
99 99 * address (1 byte) - CRC (2 bytes) = 253 bytes.
100   - *
101   - * Consequently:
102   - * - RS232 / RS485 ADU = 253 bytes + Server address (1 byte) + CRC (2 bytes) =
103   - * 256 bytes.
104   - * - TCP MODBUS ADU = 253 bytes + MBAP (7 bytes) = 260 bytes.
105 100 */
106 101 #define MODBUS_MAX_PDU_LENGTH 253
107 102  
  103 +/* Consequently:
  104 + * - RTU MODBUS ADU = 253 bytes + Server address (1 byte) + CRC (2 bytes) = 256
  105 + * bytes.
  106 + * - TCP MODBUS ADU = 253 bytes + MBAP (7 bytes) = 260 bytes.
  107 + * so the maximum of both backend in 260 bytes. This size can used to allocate
  108 + * an array of bytes to store responses and it will be compatible with the two
  109 + * backends.
  110 + */
  111 +#define MODBUS_MAX_ADU_LENGTH 260
  112 +
108 113 /* Random number to avoid errno conflicts */
109 114 #define MODBUS_ENOBASE 112345678
110 115  
... ...