Having some problems communicating with a Simcom SIM300Z GPRS modem hooked up to a mega640 (low voltage logic levels, not RS232) and just wanted to double check with like-minded people that I've not got thing mixed up.
I can issue AT commands etc. quite happily when not attempting to do any flow control, so the modem is happy to talk it would seem. However, the modem's CTS line spends almost all of its time high (deasserted) no matter what I do with the RTS line so I cannot get hardware flow control to work properly.
When we're talking about logic level modem control signals, RTS low means the uC has data to transmit and CTS low means the modem is ready to receive. This is correct, yes?
When the modem is initialising it transmits the unsolicited response "Call Ready" to the uC. When it does this, CTS goes low briefly. Does this sound as if the modem's CTS line might be non-inverted (i.e. active high instead of active low)?
The datasheet for the modem inconsistently refers to the CTS line as CTS and /CTS and says nothing about the exact operation of this line, which is really helpful! :evil:
Anyway, if someone could confirm that my thoughts are correct or not, I'd be grateful.