Note the differences between a signed and unsigned value of the
same length and the difference between a normal and reversed representation of the
same value.
For example, the message representation FF FF FF FF is -1 if the specifier is int16
(signed) and 65535 if the specifier is uint16 (unsigned).
The Modbus protocol specifies sixteen bit values be placed in the Modbus message
frame with the high order byte first and the low order byte second. The Modbus
specification doesn’t define a storage element greater than sixteen bits. To
accommodate 32 bit integers and 32 bit IEEE floating point numbers across two
Modbus registers, this interface extends the Modbus high byte - low byte ordering
by ordering all four bytes from the highest to the lowest in the Modbus message.

请注意
相同的长度，以及标准和反向表示之间的差异
同样的价值。
例如，如果说明符为int16，则消息表示FF为-1
如果说明符为uint16（未签名），则为65535（已签名）。
Modbus协议规定在Modbus消息中放置16位值
首先是高阶字节，然后是低阶字节的帧。Modbus协议
规范没有定义大于16位的存储元素。到
跨两个域容纳32位整数和32位IEEE浮点数
Modbus寄存器，该接口扩展Modbus高字节-低字节顺序
通过将Modbus消息中的所有四个字节从高到低排序。