CEM DT-8852

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CEM DT-8852
CEM DT-8852.png
Status supported
Source code cem-dt-885x
Connectivity USB
Frequency range 31.5Hz - 8kHz
Measurement range (A) 30dB - 130dB
Resolution 0.1dB
Accuracy (94dB@1kHz) 1.4dB
Frequency weighting A, C
Time weighting F, S
Standards IEC 61672-1 Class 2
Website cem-instruments.com

The CEM DT-8852 is an IEC 61672-1 class 2 compliant sound level meter with USB connectivity.

See CEM DT-8852/Info for more details (such as lsusb -vvv output) about the device.

Some random facts:

  • The device only starts logging to USB when the SETUP key is pressed on the keypad. This also disables auto power-off mode.
  • Power consumption is 9.5mA at 9V, regardless of USB or recording status; 15mA when the backlight is on.
  • The battery low warning starts when the battery delivers less than 7V.
  • Sound pressure level measurements are sent to the host at a rate of 20Hz.

Hardware

Photos

Protocol

The device communicates at 9600 baud, no parity, 8 data bits, and 1 stop bit (9600/8n1).

Commands

None of the following commands are needed to receive live measurements from the device; see the next section.

The commands are always 1 byte, take no parameters, and receive no acknowledgement or reply. Figuring out whether the command was successful or not must be done by watching the continual stream of settings. This is not optional: the device is very unresponsive, and commands nearly always have to be resent.

The following commands are supported:

Token Description
0x33 Power down the device.
0x55 Toggle measurement recording on/off.
0x11 Toggle Max/Min/normal mode.
0x77 Toggle Fast/Slow time weighting
0x88 Toggle measurement range.
0x99 Toggle dBA/dBC frequency weighting.
0xac Request transfer of stored measurement records.

Measurements and settings

Current settings and measurements are continually streamed over the serial bus, without any prompting from the PC. Data is encapsulated in packets of between two and five bytes. The packets are structured as follows:

1 2 3 4 5
0xa5 Token Data

The number of data bytes depend on the token. These are the tokens:

Token Data Description
0x02 0 Time weighting Fast
0x03 0 Time weighting Slow
0x04 0 Max hold mode
0x05 0 Min hold mode
0x06 3 Current time in BCD; first nibble unknown
0x07 0 Current measurement is over measurement range high threshold
0x08 0 Current measurement is under measurement range low threshold
0x09 0 Memory store is full
0x0a 0 Device is currently recording to memory
0x0b 1 Last measurement sent with 0x0d was shown on display readout
0x0c 0 Last measurement sent with 0x0d was shown in bargraph on display
0x0d 2 Current measurement, multiplied by 10, in BCD
0x0e 0 Live measurements mode (not max/min hold)
0x0f 0 Battery is low
0x11 0 Current measurement is within the current measurement range
0x19 0 Memory store is not full
0x1a 0 Device is not recording to memory
0x1b 1 Frequency weighting dBA
0x1c 1 Frequency weighting dBC
0x1f 0 Battery is OK
0x30 0 Measurement range 30-80 dB
0x40 0 Measurement range 30-130 dB (auto)
0x4b 0 Measurement range 50-100 dB
0x4c 0 Measurement range 80-130 dB

Recorded measurements

After the host has requested the recorded measurement log, a packet will be inserted into the regular output stream. Instead of 0xa5, this packet begins with 0xbb:

1 2 3
0xbb Length

The length field is big-endian encoded, with an offset of 100. A length field of 0x0064 thus indicates no stored samples.

This is followed by a series of records, where the first byte (token) indicates the content to follow:

Token Description
0xaa Measurements in this record use dBA frequency weighting. Metadata follows.
0xcc Measurements in this record use dBC frequency weighting. Metadata follows.
0xac Measurements follow, until the next token. Encoding is BCD times 10, as in the live stream.
0xdd End of recorded measurement dump.

The metadata following 0xaa and 0xcc denotes the date and time this record was started, and the sampling rate. It is encoded in 7 bytes:

Byte Description
0 Last two digits of year (BCD)
1 Month (BCD)
2 Day of month (BCD)
3 Hour (BCD)
4 Minutes (BCD)
5 Seconds (BCD)
6 Samplerate, as seconds between measurements (1-59, BCD)

Bugs

  • A packet indicating memory is empty is not supposed to have any sub-packets: only the 0xbb token, length, and 0xdd to indicate end of transfer. However this always seems to include a 0xaa or 0xcc byte as well:
1 2 3 4 5
0xbb 0x00 0x64 0xaa 0xdd
  • The length of the memory transfer (as sent after 0xbb) refers to all the data sent afterwards, excepting only 0xac and 0xdd, which are protocol tokens. However:
    • The count is always off by one; one byte less is sent than the length indicates.
    • The last record's measurement data's last byte is in fact a random extra byte, half of a BCD-encoded SPL value. This is another off-by-one: the byte has to be discarded.

The length is thus actually off by two.

Resources

Partial protocol documentation