SIM7600A gives an error when executing commands, but works after executing it again

Question

I am currently using a WaveShare cellular modem hat for my Raspberry Pi. The module has an onboard SIM7600A.

I've been reading the documentation and although all the commands work as they are supposed to, some commands fail when I try to execute them for the first time but succeed on a subsequent call.

An example of this would be:

AT+CGPS=1 #Enable GPS
ERROR 
AT+CGPS=1 
OK

Another example of this happening is when I try to get the GPS lock position (although it happens to most commands usually when attempting to execute them for the first time.)

AT+CGPSINFO #Return GPS fixed position 
ERROR
AT+CGPSINFO
+CGPSINFO: ,,,,,,,,
OK

This is my first time working with a cellular modem so I don't know if this behavior is expected, or if something is wrong with my board.

The following steps I tried to do to resolve this issue were increase my BAUD rate from 9600 to 115200. The reason for the change of BAUD was because I read somewhere that it times out when it doesn't receive the carriage return fast enough.

Another step I set AT+CMEE (Report mobile equipment error) to 2 in attempts to return verbose errors to track the issue. It does work when executing commands with purposely incorrect variables such as attempting to insert a wrong SIM pin. But with the errors mentioned above they are just reported as "ERROR".

I've looked all over google and can't find much on the subject so I was hoping someone with expertise can point me in the correct direction.

Answer 1

RS232 might be a bit unreliable if used with long cables or crummy cables, but the fact is that across the "distance" of a couple of centimeter from the Pi to the "hat" there should be no problem.

I work with GSM modems in my day job, and never had a problem with RS232. It is more likely you are doing something wrong than that there are communication errors - especially given that they are so reproducible.

• Check that you are sending correct line ends. Many programs will send a carriage return and a line feed at the end of a line of text. Some only send a carriage return or only a line feed. For most modems, you want to send only carriage returns. An unexpected line feed will cause some modems to return an "ERROR" with no further explanation.
• Check that the power to the GSM modem is stable. If the power supply for your Pi is a little weak, the power to the modem may "wobble" when it tries to execute commands. That can cause errors.
• Make sure that you are sending complete, correct commands. For example, your GPS command (AT+CGPS=1) takes some optional parameters on some modems (AT+CGPS=1,3 mean start the GPS session in UE-assisted mode for some modems.) If your modem tries to use a default mode that doesn't work, that may cause problems. In other words, use a more specific command (AT+CGPS=1,1 to start standard standalone GPS.)

Given your description, I tend towards the idea that you are sending mixed carriage returns and line feeds.