data cable have max232 circuit within the port. and it get power from DB-9 PC.
i dont know what voltage and from which pins of PC's DB-9 the data cable get the power.
You have to supply these voltage to DB-9 connector
pin 7 DB9 (RTS) -9 Volt
pin 4 DB9 (DTR) +9 Volt
pin 5 DB9 (GND)
But be careful that the following pins
pin 2 DB9 (RXD)
pin 3 DB9 (TXD)
has a RS232 Voltage level, that's >+5 Volt as logic level '0' and <-5 Volt as logic level '1', you can't connect this pin to micro controller's TXD pin and RXD pin directly, it may burn out that pins!
once i connected direct C55 's TX,RX & grd with CELL phone the at commands works but other day it doesn't work
Without the above voltage supplies, the data cable doesn't works!
some body told me that u must use MAX232 is to interface and use data cable.
Yes, its true!
Another connection is like this: