QModbusDataUnit startAddress offset

[Mark81]

Referring to the manufacturer modbus' register table, I noticed I have to pass to the startAddress parameter a value with offset -1. Example:

QModbusDataUnit readRequest()
{
    return QModbusDataUnit(QModbusDataUnit::HoldingRegisters, 49, 2); // <--- actually is 50
}

void read()
{
    if (auto *reply = _device->sendReadRequest(readRequest(), 1))
    {
        if (!reply->isFinished()) QObject::connect(reply, &QModbusReply::finished, [reply]()
        {
        // .....
}

In that table the actual address is 50. If I use, say, modpoll I pass 50 as start address. But with QModbusDataUnit I have to pass 49!

Is this offset documented anywhere?

[J.Hilk]

@Mark81 seems like modpoll is starting its counting by 1 instead of 0.
Which is the incorrect way, since register 0 is defined as a valid register in the standard

[Mark81]

@J-Hilk got it, that would mean also the manufacturer modbus table is wrong, because the addresses in the table match with the modpoll "convention".