# 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 |

R*n* 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%.  H*n* 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%.  H*n* 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).