Two Unos over a pair of HC-05s so the one with the GSM modem reports both

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Gönderen Ekim 23, 2026: 10:06 am #1

Home project. Uno A in the shed reads soil moisture for the greenhouse beds and a door contact. Uno B in the house has a SIM800L GSM module (no broadband at the house yet, long story). B should forward A’s readings to Hermetry together with its own. The shed is about 15 m away so I’m trying two HC-05s, A as slave and B as master.

Configured in AT mode:

slave (A):   AT+ROLE=0   -> OK
             AT+ADDR?    -> +ADDR:98d3:31:fb1234
master (B):  AT+ROLE=1   -> OK
             AT+CMODE=0  -> OK
             AT+BIND=98d3:31:fb1234  -> ERROR:(1D)

The BIND fails, and when I tried CMODE=1 instead the master LED keeps blinking fast forever and never connects. What am I doing wrong?

Gönderen Ekim 28, 2026: 12:29 pm #2

Fast blink means not connected. Once they link you get two quick blinks every couple of seconds.

Gönderen Ekim 29, 2026: 4:47 pm #3

I did exactly this for a uni lab last term and got the same ERROR on BIND. The address comes out of AT+ADDR? with colons, but AT+BIND wants commas, in the order NAP,UAP,LAP:

AT+BIND=98d3,31,fb1234

Full sequence that worked for us, both modules entered with KEY/EN held high at power-up (AT mode is 38400 baud):

slave A:  AT+ORGL
          AT+ROLE=0
          AT+PSWD="4321"
          AT+UART=9600,0,0

master B: AT+ORGL
          AT+ROLE=1
          AT+CMODE=0
          AT+PSWD="4321"
          AT+BIND=98d3,31,fb1234
          AT+UART=9600,0,0

Same PSWD on both, then power-cycle both without KEY held. Master pairs within a few seconds. After that it’s a plain 9600 serial line between the Unos.

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