Read modbus holding register with scale

Projects Programming
1

Hi,
I use library for 8266 - works also on ESP32.
#include <ModbusIP_ESP8266.h>

how can I define types and compute correct value of a register defined like:
uint16_t = PowerAC;
uint16_t = pwrscale;

raw PowerAC value = 9850
raw pwrscale value = 0 => 0

read out row:
mb.readHreg(remote, REG, &PowerAC, 1, nullptr, UNIT_ID);
mb.readHreg(remote, REG, &pwrscale, 1, nullptr, UNIT_ID);

or

uint16_t = Frequency;
uint16_t = freqscale;

raw Frequency value = 4997
raw freqscale value = 65534 => -2

Serial.println(PowerAC value * pow(10, pwrscale value)); // frequency * scale doesn't work

read out row:
mb.readHreg(remote, REG, &Frequency, 1, nullptr, UNIT_ID);
mb.readHreg(remote, REG, &freqscale, 1, nullptr, UNIT_ID);

Thanks.

2

what is a "holding register"?

???

3

It looks like you want a signed 16 bit integer for for freqscale rather than an unsigned 16 bit int.

4

modbusRTU or mobusTCP communication protocol with e.g. energy meter or photovoltaic inverter

5

yes scale is send from modbus as unsigned integer, but should be converted into signed integer.
Frequency value shoud be computed as
Frequency * 10 ^ sint

Frequency computed is 5001 * 10 ^ -2 = 50.01 Hz

6

I found it out :0)

unsigned int x = 65529;
    int y = (short) x;      //  If short is a 16-bit integer.

    or alternatively:

    unsigned int x = 65529;
    int y = (int16_t) x;    //  This is defined in <stdint.h>
7

The just make it a signed integer.

uint16_t = Frequency;
uint16_t = freqscale;
mb.readHreg(remote, REG, &Frequency, 1, nullptr, UNIT_ID);
mb.readHreg(remote, REG, &freqscale, 1, nullptr, UNIT_ID);

int16_t scale = freqscale;

float freq = Frequency * pow(10, scale);

Serial.println(freq);
1 Like
8 
Serial.println(Frequency * pow(10, (short)freqscale)); // frequency * scale
9

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