Fluke PM-3380B Conversion of trace data, Relation between screen position and trace value

USING THE COMBISCOPE INSTRUMENTS 3 - 31

3.4.3Conversion of trace data

The trace data is sent as a block of binary codes. Trace samples can be formatted to consist of 8 bits (1 byte) or 16 bits (2 bytes) codes, which can be selected by the FORMat command. Refer to section 3.10.1 "Trace formatting" for a further explanation of this command. After *RST the samples are sent as 2 byte codes. When samples are formatted as two bytes, the most significant byte (msb) is sent first, followed by the least significant byte (lsb). The sample values that are sent in the block, are coded according to the two’s complement notation. The relation between the screen positions, the values of the trace samples and the decimal value of the corresponding binary codes, is shown in the figure below.

	Screen	Trace sample			Decimal value
position (Ps)		value (Ts)			of byte code
		32767	(127)	32767		(127)
	top	25600	(100)	25600		(100)
		1	(1)	1		(1)
screen	mid	0	(0)	0		(0)	trace
range		-1	(-1)	65535		(255)	range
		-25600	(-100)	39936		(156)
	bottom
		-32768	(-128)	32768		(128)		ST7187

Note: Numbers between parenthesis apply to single byte format.

Figure 3.11 Relation between screen position and trace value

The value of the trace points relate to the vertical position of the corresponding sample on the screen of the CombiScope instrument. As the figure above shows, the sample with value 25600 corresponds with the top position of the screen. Similarly, the samples with values -25600 and 0 correspond to the bottom and mid-position respectively. This applies to trace samples that are formatted to consist of 16 bits (2 bytes). The values that apply to the 8 bit (1 byte) format are placed between parenthesis.

The ADC allows trace acquisitions that are somewhat outside the vertical screen boundaries. Trace acquisitions use the full dynamic range of the ADC. This results in a dynamic trace range of 65535 points, whereas the screen range is limited to 51200 points.