We have interfaced this SPL meter to a TP-Link MR3020 running OpenWrt OS. The SL-814 had a few jumpers added to enable it to (1) always stay on and (2) be powered from the 5V on the USB connector.
1. Short C3 - causes it to turn on automatically and stay on whenever power is applied.
2. jumper USB pin 5 (pin 5 == top most pin closest to top of circuit board where top == LCD display side of PCB) across to 9V power trace (the fat one that runs past the USB connector)
3. Jumper C12+ to u3 pins 2+3 shorting +9 to +5 (note there are TWO 5V regulators on the board, one for digital +5 and one for Analog +5, a good design practice isolating the two)
4. short R8 making "low battery" and associated not helpful auto power off go away
1. My SL-814 from amazon did not come with a cable
2. the "USB" port on the SL-814 is a USB port in physical form only. What they are doing is using a USB connector to output serial TTL I/O from the Atmel's UART. it is in no way, shape or form USB and the claims on Amazon etc that it has USB are flat out false.
3. Use a PL2303 to TTL cable and a USB type B connector donor cable.
4. Cut and strip the USB donor cable's B connector side and connect
i. black - black (GND) ii. red - red (+5) iii. green - white (Data) iv. white - green (Data)
Plug into your PC, assuming you have PL2303 driver should show up as a COM port or /dev/ttyUSBx in the case of OpenWrt/Linux. From there follow other advice on this wiki regarding commands and responses.