Pymodbus: how do I read the individual bits of this input register?

Question

I have a solar power project and real-time monitor which uses an EpSolar Tracer charge controller, and a Raspberry Pi to read the energy data via MODBUS.

I'm reading the input registers easily when they are simple integers, like this -

# Request the range of registers that hold the solar/battery realtime data (3100 - 3105)
result = client.read_input_registers(0x3100,6,unit=1)

# Solar voltage is register 3100, divide by 100
sV = float(result.registers[0] / 100.0)

..but I'm having trouble understanding how I read an input register in which each of the individual bits stand for something different. In particular, I want to read a 'charging status' register which holds 12 different pieces of information -

When I read this register (0x3201) all I get back is the number 7. How would I go about reading each of the 12 different pieces of data in this register? I thought perhaps 7 is the decimal representation of 0000000000000111 - but I don't think this can be right.

Any help on this would be appreciated!

Matt

Answer 1

You're on the right track. And it's easy to get those bits out. Here's how:

16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
                                           1  1  1

The top line is the bit position and the bottom line is the value 7...let's say you're interest in bit 2, the third from the right. Let's make a bit mask for that bit by taking a 1 and shifting it over n times, where n is the bit position. So we want to shift it 2 times:

int mask = (1 << 2);

This will yield:

16 15 14 13 12 11 10 09 09 07 06 05 04 03 02 01 00
                                           1  0  0

Now all we do to determine if bit two is set is to AND the value and the mask

16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
                                           1  1  1   value
                                           1  0  0   mask
--------------------------------------------------
                                           1  0  0   ANDed result

So, in C:

int value = 7;  // or any 16-bit value
int mask = (1 << 2);  // == 4

if ((value & (1 << 2)) == mask)
    // bit is set
else
    // bit is not set

Now, sometimes you want to extract more than one bit. Here's an example of how to extract a two-bit combination. First let's stipulate that two bits 11 is the value 3 (one 1 and one 2). So now w're going to do a similar process to extract two bits from the same value (7).

16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
 0  0  0  0  0  0  0  0  0  0  0  0  0  0  1  1  1

Now we mask with a 3 instead of a 1:

int mask = (3 << 2);

This will yield:

16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
 0  0  0  0  0  0  0  0  0  0  0  0  0  1  1  0  0

Now all we do to determine if bit two is set is to AND the value and the mask

16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
 0  0  0  0  0  0  0  0  0  0  0  0  0  0  1  1  1   value
 0  0  0  0  0  0  0  0  0  0  0  0  0  1  1  0  0   mask
--------------------------------------------------
                                        0  1  0  0   ANDed result

But the documentation might say something like bits:3-2 is "status x." values -

00 - a
01 - b
02 - c
03 - d

So now we have to take our final value, 0100 from the ANDed result above, and shift it two to the right to get these possible values:

result = (old_result >> 2);
0100 >> 2 == 01 

Since the result is 01, decimal 1, hexadecimal 1, then the result is b from above.

EDIT:

Now the final calculations in python:

>>> value = 0x0007
>>> mask = (3 << 2)
>>> result = value & mask
>>> print (result)
4
>>> shifted_result = result >> 2
>>> print (shifted_result)
1

Same result, bits 3-2 are equal to 1, which means (based on your datasheet) that the diagnostic result is:

bit 0: "running" bit 1: "fault" bit 3-2: "float"