Reply To: smbus2 write_i2c_block_data: what is the “cmd” byte for an ADS1115?

“cmd” is SMBus naming. In an SMBus block write the first byte after the address is called the command byte, and for register-based chips like the ADS1115 that byte is the register pointer. So your first line puts the pointer on the Config register (0x01) and writes 0xC3 then 0x83 into it. On the wire: start, 0x90, 0x01, 0xC3, 0x83, stop.

0xC383 means: start a single conversion, AIN0 against GND, ±4.096 V range, single-shot mode, 128 samples/s. The read sets the pointer to 0x00 (Conversion register) and reads two bytes.

Your bug is the byte order. The ADS1115 sends the most significant byte first, and the result is signed:

data = bus.read_i2c_block_data(0x48, 0x00, 2)
raw = (data[0] << 8) | data[1]
if raw > 0x7FFF:
    raw -= 0x10000
volts = raw * 4.096 / 32768

With the bytes swapped, a small change in the low byte becomes a huge jump, which is your 900 vs 52,000. For AIN1 change the MUX bits: [0xD3, 0x83]. And 10 ms is enough at 128 SPS (one conversion is about 8 ms), so the sleep is fine.