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MB_DIRd() is called when a Modbus master sends a Function Code 2 command. MB_DIRd() read the value of a single discrete input. MB_DIRd() should only be called by µC/Modbus.

Prototype

CPU_BOOLEAN MB_DIRd (CPU_INT16U di,

CPU_INT16U * perr)  

Arguments

di

Is the discrete input number that you want to read and can be a number between 0 and 65535 (depending on your product). It is up to you to decide which discrete input is assigned to what variable in your product.

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

if the discrete input number you specified is a valid discrete input and you are able to have code access the value of this discrete input.

MODBUS_ERR_RANGE

if the discrete input number passed as an argument is not a valid discrete input number for your product.

Returned Value

MB_DIRd() returns the current value of the specified discrete input (TRUE or FALSE). If an invalid discrete input number is specified, you should return FALSE.

Notes / Warnings

Code is enabled when MODBUS_CFG_FC02_EN is set to DEF_ENABLED in your product’s mb_cfg.h file.

Called By:

MBS_FC02_DIRd() in mbs_core.c

Example

In this example, our product has 19 discrete inputs. 16 of these discrete are placed in AppDITbl[] by your application. The other three discrete inputs actually represent the status of three switches that your application reads and places the status into the following variables: AppSwStart, AppSwStop and AppSwReset. A pressed switch is indicated by a TRUE and a released switch is represented by a FALSE.

Your systems Engineer decided to assign Modbus discrete input numbers 100, 101 and 102 to the three switches and the other discrete inputs to 103 through 118.

CPU_BOOLEAN  AppDITbl[16];
CPU_BOOLEAN  AppSwStart;
CPU_BOOLEAN  AppSwStop;
CPU_BOOLEAN  AppSwReset;
 
CPU_BOOLEAN  MB_DIRd (CPU_INT16U di, CPU_INT16U *perr)
{
    *perr = MODBUS_ERR_NONE;
    switch (di) {
        case 100:
   return (AppSwStart);
 
        case 101:
   return (AppSwStop);
 
        case 102:
   return (AppSwReset);
 
        case 103:
        case 104:
        case 105:
        case 106:
        case 107:
        case 108:
        case 109:
        case 110:
        case 111:
        case 112:
        case 113:
        case 114:
        case 115:
        case 116:
        case 117:
        case 118:
   return (AppDITbl[di – 103]);
 
        default:
   *perr = MODBUS_ERR_RANGE;
   return (FALSE);
    }
}