HPCS6500-py/PROTOCOL.md

HPCS 6500 USB Protocol Reference

Reverse-engineered from USB packet captures of the vendor software.

Device Overview

The HPCS 6500 is a spectrophotometer / integrating sphere system for LED and light source testing. It includes a built-in programmable AC/DC power supply.

Capabilities:

• Spectral power distribution (380-1050 nm, 350 points)
• Photometric values (lumen, luminous efficacy, CCT, CRI Ra/R1-R15)
• Chromaticity coordinates (CIE xy, uv, u'v')
• Radiometric flux (total, UV, blue, yellow, red, far-red, IR)
• CIE 1931 tristimulus (X, Y, Z), TLCI-2012, SDCM
• Electrical parameters (voltage, current, power, power factor, frequency)
• Harmonics analysis (50 harmonics voltage/current %, UThd, AThd, waveforms)
• Built-in power supply: AC 100-240 V / 50-60 Hz, DC 1-60 V / 0-5 A

Connection:

• USB interface via STM32 Virtual COM Port (VID 0483, PID 5741)
• Appears as a serial port (e.g. COM42 on Windows)
• Baud rate: 115200

Frame Format

Every command and response starts with the sync byte 0x8C followed by a command ID byte. The device echoes the command ID in the response. All multi-byte numeric values are little-endian.

TX: 8C <cmd> [payload...]
RX: 8C <cmd> [data...]

Write commands receive a 2-byte ACK (8C <cmd>). Read commands return variable-length data after the 2-byte header.

Command Table

Cmd	Direction	TX len	RX len	Description
00	read	2	16	Identify device
01	write	6	2	Set integration time (uint32 LE, µs)
03	read	2	9	Poll instrument state
05	read	2	7	Status check
0E	write	3	2	Start (01) / Stop (02) measuring
13	read	2	4 hdr + 3904	Read measurement block
25	write	2	2	Reset instrument
26	read	2	196	Read instrument ranges
2A	read	2	122	Read device configuration
72	write	3	2	PSU output on (00) / off (01)
73	write	7	2	Set DC parameter (sub + float32)
77	read	2	4 hdr + 1584	Read electrical / harmonics block
78	write	7	2	Set AC parameter (sub + float32)
79	read	2	20	Read power supply settings
7A	write	3	2	Set output mode (00=AC, 01=DC)
7B	read	2	2	Read test config
2B	write	var	2	Write test config (see below)

Measurement Sequences

Single-Shot Measurement

The vendor software uses this flow for one-off readings. No explicit start command is needed — the stop command (8C 0E 02) triggers a single reading.

 1. TX 8C 00           -> Identify (expect "HPCS6500" in response)
 2. TX 8C 2A           -> Read device config
 3. TX 8C 01 <uint32>  -> Set integration time (µs), 0 = auto
 4. TX 8C 0E 02        -> Stop / trigger single reading
 5. TX 8C 03           -> Poll state until byte[2] = 0x01 (data ready)
 6. TX 8C 13           -> Read measurement data (3904 bytes)
 7. TX 8C 77           -> Read electrical data (1584 bytes)
 8. TX 8C 25           -> Reset instrument

Continuous Measurement

 1. TX 8C 00           -> Identify (expect "HPCS6500" in response)
 2. TX 8C 79           -> Read current PSU settings
 3. TX 8C 7A <mode>    -> Set output mode (00=AC, 01=DC)
 4. TX 8C 78/73 ...    -> Configure voltage/frequency/current as needed
 5. TX 8C 01 <uint32>  -> Set integration time (µs), or skip for auto
 6. TX 8C 72 00        -> Turn PSU output ON
 7. TX 8C 0E 01        -> Start continuous measurement
 8. TX 8C 03           -> Poll state until byte[2] = 0x01 (data ready)
 9. TX 8C 13           -> Read measurement data (3904 bytes)
10. TX 8C 77           -> Read electrical data (1584 bytes)
    (repeat 8-10 for continuous readings, ~550 ms per cycle)
11. TX 8C 0E 02        -> Stop measurement
12. TX 8C 72 01        -> Turn PSU output OFF
13. TX 8C 25           -> Reset instrument

State Polling (8C 03)

9-byte response: 8C 03 <b2> <b3> <b4> <b5> <b6> <b7> <b8>

Byte	Meaning
b2	Data flag: 00 = no new data, 01 = data available
b3	Mirrors b2
b4	Always 00
b5	State: 04 = idle, 01 = measuring
b6	Always 00
b7	Always 00
b8	Always 01

Measurement Block (8C 13 — 3904 bytes)

The response has a 4-byte transport header (8C 13 0F 40) followed by a 3904-byte payload. All floats are IEEE 754 single-precision, little-endian.

Device Header (offset 0-35)

Offset	Size	Type	Field
0	10	ASCII	Device ID ("HPCS6500")
10	26	—	Internal calibration data

Photometric (offset 36-75)

Offset	Type	Field	Unit	Example
36	float32	Luminous flux	lm	479.57
40	float32	Luminous efficacy	lm/W	57.05
44	float32	Correlated color temp	K	5653
48	float32	Duv	—	0.00553
52	float32	CIE x	—	0.3289
56	float32	CIE y	—	0.3489
60	float32	CIE u	—	0.2015
64	float32	CIE v	—	0.3206
68	float32	CIE u'	—	0.2015
72	float32	CIE v'	—	0.4809

Color Rendering (offset 76-143)

Offset	Type	Field	Example
76	float32	SDCM	4.71
80	float32	Ra	83.0
84-140	float32	R1-R15 (4 bytes each)	R1=82 .. R15=76

Rn is at offset 80 + n × 4 (R1 at 84, R2 at 88, ..., R15 at 140).

Radiometric (offset 144-175)

Offset	Type	Field	Unit	Example
144	float32	Total radiant flux	mW	1491.256
148	float32	UV flux	mW	0.000
152	float32	Blue flux	mW	469.836
156	float32	Yellow flux	mW	679.454
160	float32	Red flux	mW	330.864
164	float32	Far-red flux	mW	11.462
168	float32	IR flux	mW	0.000
172	float32	Total (duplicate)	mW	1491.256

CIE Tristimulus & Sensor (offset 224-255)

Offset	Type	Field	Example
224	float32	CIE 1931 X	661.9
228	float32	CIE 1931 Y	702.15
232	float32	CIE 1931 Z	648.535
236	float32	TLCI-2012	68
244	float32	Peak Signal	53088
248	float32	Dark Signal	2267
252	float32	Compensate lvl	2834

Timestamps (offset 272-291)

Offset	Size	Type	Field	Example
272	11	ASCII	Test date	2026-02-04\0
283	9	ASCII	Test time	16:04:17\0

Spectral Data (offset 432-1831)

Offset	Type	Field
430-431	uint16	Spectrum header (zero)
432-1831	float32 × 350	Spectral irradiance (µW/cm²/nm)

The 350 floats cover 380 nm to 1050 nm at ~1.92 nm steps ((1050-380) / (350-1) = 1.917 nm/step). Values are absolute spectral irradiance; the vendor software normalizes by dividing by the peak value.

Unmapped Regions

Offsets 176-223, 256-271, and 1832-3903 contain additional data not yet fully mapped. They likely hold extended parameters visible in the vendor software's advanced views.

Electrical / Harmonics Block (8C 77 — 1584 bytes)

The response has a 4-byte transport header (8C 77 06 30) followed by a 1584-byte payload.

Basic Electrical (always present)

Offset	Type	Field	Example
0-7	—	Reserved (zero)	—
8	float32	Voltage	230.30 V
12	float32	Current	0.065 A
16	float32	Active power	8.406 W
20	float32	Frequency	50.02 Hz
24	float32	Power factor	0.558

Waveforms (when harmonics enabled, offset 28-543)

Offset	Type	Count	Field
30-285	int16 LE	128	Voltage waveform
286-541	int16 LE	128	Current waveform

Each waveform contains 128 signed 16-bit samples representing one complete AC cycle.

Voltage Harmonics (when harmonics enabled, offset 544-747)

Offset	Type	Count	Field
544-740	float32	50	H1..H50 voltage (% of H1)
744	float32	1	UThd (%)

H1 = 100.0%. Hn is at offset 544 + (n-1) * 4.

Current Harmonics (when harmonics enabled, offset 800-1003)

Offset	Type	Count	Field
800-996	float32	50	H1..H50 current (% of H1)
1000	float32	1	AThd (%)

H1 = 100.0%. Hn is at offset 800 + (n-1) * 4.

Harmonics Detection

When harmonics is disabled, offsets 28+ are all zero. To detect whether harmonics data is present, check if the float32 at offset 544 equals 100.0 (H1 voltage fundamental = 100%).

Power Supply Control

Output On/Off (8C 72)

TX: 8C 72 00        -> Output ON
TX: 8C 72 01        -> Output OFF
RX: 8C 72           -> ACK

The PSU output must be turned on before taking measurements.

Output Mode (8C 7A)

TX: 8C 7A 00        -> AC mode
TX: 8C 7A 01        -> DC mode
RX: 8C 7A           -> ACK

AC Parameters (8C 78)

TX: 8C 78 00 <float32 LE>    -> Set voltage (V), range 100-240
TX: 8C 78 01 <float32 LE>    -> Set frequency (Hz), 50 or 60
RX: 8C 78                    -> ACK

DC Parameters (8C 73)

TX: 8C 73 00 <float32 LE>    -> Set current limit (A), range 0-5
TX: 8C 73 01 <float32 LE>    -> Set voltage (V), range 1-60
RX: 8C 73                    -> ACK

Read Settings (8C 79)

20-byte response: 8C 79 + 18 bytes payload.

Offset	Type	Field
0	float32	AC voltage (V)
4	float32	AC frequency (Hz)
8	float32	DC voltage (V)
12	float32	DC current (A)
16	uint8	Mode (0=AC, 1=DC)
17	uint8	Marker (0xFF)

Integration Time (8C 01)

TX: 8C 01 <uint32 LE>    -> Integration time in microseconds
RX: 8C 01                -> ACK

Common values: 200000 (200 ms), 500000 (500 ms), 1000000 (1000 ms).

Identify (8C 00)

16-byte response: 8C 00 + "HPCS6500\0\0" + 4 bytes device metadata.

Test Configuration (8C 2B)

The vendor software sends this before each measurement to configure test parameters. It consists of two sub-commands (sub 00 and sub 01).

Sub 00 — Test Parameters

TX: 8C 2B 00 <~58 bytes>    -> Write test parameters
RX: 8C 2B                   -> ACK

Known fields within the sub 00 payload (offsets from start of payload after sub byte):

Offset	Type	Field	Example
15	float32	Frequency?	50.0
35-55	float32	Five x 1.0 values	1.0 each
55	float32	Scale factor?	0.5

Sub 01 — Auto-PSU Flag

TX: 8C 2B 01 <~44 bytes>    -> Write test config
RX: 8C 2B                   -> ACK

Offset	Type	Field
37	uint8	Auto-PSU: 01 = on with test, 00 = off

When auto-PSU is enabled (01), the device automatically turns the PSU on before measuring and off after. When disabled (00), the PSU must be controlled manually via 8C 72.

Read Configuration (8C 7B)

TX: 8C 7B           -> Read current test config
RX: 8C 7B           -> ACK (config is returned via subsequent 8C 2B reads?)

Notes

• The instrument computes all photometric, colorimetric, and radiometric values internally. The host software only reads and displays results.
• During continuous measurement, the host polls 8C 03 and reads 8C 13
  • 8C 77 on each cycle (~550 ms per reading).
• All float values are IEEE 754 single-precision (4 bytes), little-endian.
• All integer values are unsigned little-endian unless noted (waveform samples are signed int16).