I realise I’m replying to my own old post here but I came up with solutions and figured they might be useful for someone.
Firstly, the development kit for the SL8090 modem really means it when it specifies a 4V @ 2A supply. The 4V supply I was using came in at around 4.3V and that was enough for the modem to refuse to start full functionality (AT+CFUN=1 created an ERROR). Typing:
AT!PCINFO?
Showed the “Volt” parameter as 1, indicating an issue. Changing the supply to be bang on 4V brought everything to life.
I had a real problem with the UART going to sleep with no activity after a few seconds. The UART was woken again by typing, however several characters would get “absorbed” before the system returned fully to life. I found this did the trick:
AT!ENTERCND=“A710”
AT+WHCNF=6,1,0,0,0,0,1
Then reset the modem for UART sleep to be disabled.
The Echo issue was resolved by resetting to factory defaults.
Hope that helps someone!