Anlogue Output Voltage

knebb

Hi,

finally I was able to set (and read) the analogue output (AO1) on my Neuron M103 device through C with modbus_read_registers().

So I am using as documented regsiter 3000-3001 as floating point variable.

Works fine so far- I can set and verify the value by reading.

Now when attaching a voltmeter to measure the real physical AO voltage I am getting some strange numbers.

Here is a little table from what I observed:
Setting the register to value (upper line) yields in measuring a voltage of the lower line on the AO1:

Set to: 0 1.3 3.5 5.0 5.7 7.9 10.
Result: 0 1.1 2.9 4.0 4.8 6.5 8.2

Somehow I doubt it is mismeasured. The device usually is accurate.

So how does it come to these differences?

Thanks!

/KNEBB

Martin Kudláček

Hi @knebb,
can you please post the C code?

Thank you and have a nice day,
Martin

knebb

@Martin-Kudláček :

Of course. Here you are. I set the registers and am measuring the AO port during the sleep() cycle.

#include "header.h"
#include "vars.h"
void main(int argc, char* argv[])
{

modbus_t *modbusglob;
union {
       uint16_t word[2];
       uint32_t dword;
       float fl;
} counter,direct;
int n[9],mode;

/********************************* Function **********************************/
void fieldbus_free(modbus_t *modbus)
{
        if (modbus != NULL) {
        modbus_close(modbus);
        modbus_free(modbus);
    }
}

/********************************* Function **********************************/
modbus_t * fieldbus_new(void)
{
int err;
modbus_t *modbus;
char host[15];
int port;

    strcpy(host,"127.0.0.1");
    port   = 502;
    modbus = modbus_new_tcp(host, port);
    if (modbus == NULL )
    {
       fieldbus_free(modbus);
       exit (55);
    }
    err = modbus_connect(modbus);
    if (err  == -1)
    {
        fieldbus_free(modbus);
        modbus = NULL;
        exit (55);
    };
    err=modbus_set_slave(modbus, 0);
    if(  err == -1)
    {
        fieldbus_free(modbus);
        modbus = NULL;
        exit (60);
    } else {
        return modbus;
    }
}

/********************************* Function **********************************/
int setvoltage(float wish)
{
int errlvl;
union {
       uint16_t word[2];
       uint32_t dword;
       float fl;
} wert;
uint16_t ao;

        wert.fl=wish;
        errlvl=modbus_write_registers(modbusglob, 3000, 2, (&wert.word[0]));
        if (errlvl == -1) {printf("ERR1 modbus_write_register, %s\n",modbus_strerror(errno));}
        else { printf("Register set to %f\n",wert.fl);}
        return errlvl;
}
modbusglob=fieldbus_new();
mode=1;
setvoltage(1.3);
sleep(60);
setvoltage(3.5);
sleep(60);
setvoltage(5.7);
sleep(60);
setvoltage(7.9);
sleep(60);
setvoltage(10.0);
sleep(60);
setvoltage(0);
sleep(60);
setvoltage(5.0);

fieldbus_free(modbusglob);
exit (0);
}