MB_FileRd() is called when a Modbus master sends a Function Code 20 command. MB_FileRd() reads a single integer value from a file. As mentionned in the Modbus specifications, a file is an organization of records. Each file can contain up to 10,000 records (addressed from 0 to 9999). You must 'map' the File/Record/Ix to the actual application's corresponding data. MB_FileRd() should only be called by µC/Modbus.
Prototype
CPU_INT16U MB_FileRd (CPU_INT16U file_nbr,
CPU_INT16U record_nbr,
CPU_INT16U ix,
CPU_INT08U record_len,
CPU_INT16U * perr)
Arguments
file_nbr
Is the number of the desired file.
record_nbr
Is the desired record within the file, a number between 0 and 9999.
ix
Is the desired entry in the specified record.
record_len
Is the total length of the record.
perr
Is a pointer to a variable that will contain an error code based on the outcome of the call. Your code thus needs to return one of the following error codes:
MODBUS_ERR_NONE
the specified file/record/entry is valid and your code is returning its current value.
MODBUS_ERR_FILE
if the specified file_nbr is not a valid file number in your product.
MODBUS_ERR_RECORD
if the specified record_nbr is not a valid record number in the specified file.
MODBUS_ERR_IX
if the specified ix is not a valid index into the specified record.
Returned Value
MB_FileRd() returns the current value of the element in the file as an unsigned value. Of course, you can also return ‘signed’ values but those need to be cast to CPU_INT16U. You should note that the value will not be changed if you cast a signed variable to CPU_INT16U. The Modbus master will receive the proper value and it’s up to the Modbus master to properly retrieve the signed data . If an error is detected, you should return 0.
Notes / Warnings
Code is enabled when MODBUS_CFG_FC20_EN is set to DEF_ENABLED in your product’s mb_cfg.h file.
Called By:
MBS_FC20_FileRd() in mbs_core.c
Example
In this example, we have two ‘files’ that we implemented as an array of 16-bit integers.