How to enter negative speed with ESF-MEAS

[FSet89]

Describe the bug

• Device Model/Generation: u-blox F9R
• pyubx2 version: 1.2.22

I can't send a negative speed using the ESF-MEAS message:

import pyubx2
pyubx2.UBXMessage("ESF", "ESF-MEAS", pyubx2.SET, timeTag=1000, numMeas=1, dataField_01=-10, dataType_01=11)

Traceback (most recent call last):
File "/usr/lib/python3.7/site-packages/pyubx2/ubxmessage.py", line 107, in _do_attributes
offset, pdict, key, index, **kwargs
File "/usr/lib/python3.7/site-packages/pyubx2/ubxmessage.py", line 165, in _set_attribute
att, offset, index, **kwargs
File "/usr/lib/python3.7/site-packages/pyubx2/ubxmessage.py", line 219, in _set_attribute_group
offset, attd, key1, index, **kwargs
File "/usr/lib/python3.7/site-packages/pyubx2/ubxmessage.py", line 157, in _set_attribute
att, offset, index, **kwargs
File "/usr/lib/python3.7/site-packages/pyubx2/ubxmessage.py", line 333, in _set_attribute_bitfield
self._payload += bitfield.to_bytes(bfs, "little")
OverflowError: can't convert negative int to unsigned

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "", line 1, in
File "/usr/lib/python3.7/site-packages/pyubx2/ubxmessage.py", line 82, in init
self._do_attributes(**kwargs)
File "/usr/lib/python3.7/site-packages/pyubx2/ubxmessage.py", line 131, in _do_attributes
) from err
pyubx2.exceptions.UBXTypeError: Overflow error for attribute 'group' in SET message class ESF-MEAS

[semuadmin]

Hi @FSet89

According to the F9R Integration Manual, the ESF-MEAS dataField attribute is an unsigned 24-bit integer (U24), so this is expected behaviour.

I recommend having another look at the F9R Integration Manual for clarification of how to use the dataField and dataType attributes.

[FSet89]

Hi, you're right, however according to the application notes the speed is signed (section 6.1.2): https://content.u-blox.com/sites/default/files/documents/ZED-F9R-GettingStarted_AppNote_UBX-22035176.pdf

[semuadmin]

Hi, you're right, however according to the application notes the speed is signed (section 6.1.2): https://content.u-blox.com/sites/default/files/documents/ZED-F9R-GettingStarted_AppNote_UBX-22035176.pdf

OK so clearly a discrepancy between different u-blox documentation sources!

In point of fact, different ESF-MEAS dataTypes have different attribute types (some are signed, some are unsigned, some use individual bitflags), so the dataField attribute is actually implemented in pyubx2 as generic X24 (24-bit binary data). This means that, to enter a -ve number, you have to use what's known as the "two's complement" binary representation of the number, e.g. -10 in 24-bit two's complement format is 0b111111111111111111110110, derived as follows:

1. start with the +ve number: 10 in 24-bit binary = 0b000000000000000000001010
2. invert all the bits (1 becomes 0, 0 becomes 1), which gives you 0b111111111111111111110101
3. add 1, which gives you 0b111111111111111111110110

In Python terms, this is equivalent to -10 & 0b111111111111111111111111.

So the following amended code snippet should work:

from pyubx2 import UBXMessage, SET

value = -10
value_as_twoscomp = value & 0b111111111111111111111111
print(f"{value_as_twoscomp:024b}")

msg = UBXMessage(
    "ESF",
    "ESF-MEAS",
    SET,
    timeTag=1000,
    numMeas=1,
    dataField_01=value_as_twoscomp,
    dataType_01=11,
)

print(msg)

111111111111111111110110
<UBX(ESF-MEAS, timeTag=1000, timeMarkSent=0, timeMarkEdge=0, calibTtagValid=0, numMeas=1, id=0, dataField_01=16777206, dataType_01=11)>

FYI https://portal.u-blox.com/s/question/0D5Oj00000XUqA9KAL/ubx-esfmeas-how-to-enter-ve-speed-value-in-datafield

[semuadmin]

@FSet89 FYI #168