I bought a new Sparkfun Cellular Shield for SM5100B which I want to connect to a Mega 2560. Not very experienced with HW, I made a very simple test setup to avoid blowing up my Cellular Shield, using a Uno to simulate the Cellular Shield, connected as shown in attachment Att_1, A. The PC run the TeraTerm terminal program. The trivial sketch on the Uno sent back the characters received. This setup worked well, no problems. Characters typed on the PC were echoed by the Uno.
Next I swapped the Uno with the Cellular Shield, see Att_1, B. No reaction from the Cellular Shield when 5V was connected, except that the on-diode lit up. No reactions to AT commands either. The testing was repeated several times, restarting the PC etc. (No PIN in SIM card, which has worked well before.)
Most thankful if anybody can tell me where I have gone wrong!