This is a description of the Bluetooth interface and data stream protocol of the
Digitech QM1578 Digital Multimeter
2022-06-25 mikem@airspayce.com

BLUETOOTH:

I used the excellent nRF Connect app from Nordic on IOS to find the relevant
Bluetooth LE details of this DMM to make a connection. The details found were:

Advertised Device: Name: QM1578_DMM, Address: f4:5e:ab:72:32:02, serviceUUID: 0000fff0-0000-1000-8000-00805f9b34fb, txPower: 0

Service: unknown FFF0
Attribute: Unknown characteristic FFF2

(There are other Bluetooth characteristics with unknown uses,
but FFF2 can notify with a stream of data records,
one for each measurement. Its the one we want)

Each data record is 15 bytes long, the last byte being 0x0D
The first 4 bytes of each packet on mine are always the same 0xD5 0xF0 0x00 0x0A
Dont know if this is a packet header or a device type or serial number.

The remaining bytes encode the current meter settings and last reading.
Packets are emitted one for each reading. The packet rate depends on the
measurement. Voltaves and currents aout 3 per second.
Continuity is much faster, capacitance and temperature is slower.

I have written a sketch QM1578BluetoothClient.ino (incluided in theis directory)
for ESP32 on Arduino, that looks for such QM1578 Bluetooth devices,
connects to the first one it finds and relays the stream of measurement data back
to the host serial port in raw binary.
It is therefore a Bluetooth-Serial converter for this meter.

PROTOCOL:

Each record is 15 bytes long.
The last byte is always 0x0d
The first 4 bytes (on my meter at least) are: 0xd5 0xf0 0x00 0x0a

Byte offset 	  Meaning
0    		  Always 0xd5. Ignore
1		  Always 0xf0. Ignore
2		  Always 0x00. Ignore
3		  Always 0x0a. Ignore

4		  Meter range switch setting:
		  0x01: ACV
		  0x02:	DCV
		  0x04:	Ohms
		  0x05: Capacitance
		  0x06: Temperature
		  0x07: DCA
		  0x08: DCmA
		  0x09: DCuA
		  0x0c: ACA 
		  0x0d: ACmA
		  0x0e: ACuA
		  0x0f: Diode test
		  0x10: Hz/%
		  0x20: Continuity

5		  Value of digit 4, least significant digit, 0x00 to 0x09. 0x0f means invalid. 0x0b means OverLoad
6		  Value of digit 3, 0x00 to 0x09. 0x0f means invalid
7		  Value of digit 2, 0x00 to 0x09. 0x0f means invalid
8		  Value of digit 1, most significant digit 0x00 to 0x09. 0x0f means invalid
		  Note: for Overload, digits 4 to 1 are: 0x0b 0x0a 0x00 0x0b

9		  Number of decimal places in value digits 0x00 to 0x04

10		  Units:
		  0x01: V
		  0x02: A
		  0x03: Ohms
		  0x04: Hz
		  0x05: F
		  0x06: Ohms continuity test
		  0x07: V diode test
		  0x08: DegC
		  0x09: DegF
		  0x10: %

11		  Multiplier:
		  0x00: no multiplier
		  0x01: k
		  0x02: M
		  0x03: n
		  0x04: u
		  0x05: m   (for Amps)
		  0x06: m   (for Volts)
		  
12		  Flags1:
		  1xxx xxxx	Negative
		  x1xx xxxx	Hold
		  xx1x xxxx	Low Z
		  
13		  Flags2:
		  1xxx xxxx	AC
		  x1xx xxxx	DC
		  xx1x xxxx	Relative
		  xxx1 xxxx	Auto scale
		  xxxx 11xx	Average
		  xxxx 10xx 	Minimum
		  xxxx 01xx	Maximum
		  xxxx xxx1	Peak
		  
14		  Always 0x0d. Record ends

Corrections or updates to this information are welcomed by the author