Hello,
as a title I created a small circuit to interface a SIM800 module with an Arduino. (See the one used on the datasheet, 2 bypass on both rails 100nF and a 10K pullup for OE)
Unfortunately communication does not work. I tried to replace the GSM module with an FTDI adapter and tried to read a serial output from the arduino. I tried to put a 50hz and 1khz square wave on side A and checked if on the opposite side I got something but i got nothing.
To note after the failed attempt with the 2.8V of the SIM800 IO I decide to do some test on a more standard voltage: 3.3V on side A and 5V on side B.
The pin has an intermediate value between the supply of the side from which I am analyzing and the other.
Thinking I had blown with the soldering temperature when I replaced the chip, but the second and third gave a similar problem.
In one case I obtained random characters as if there were a GND problem. (Not the case due to the power supply that share the same ground on both 5V and 3.3V rails)
I tried to apply a pullup resistor to one side to see if the other side pin was miming the behaviour. But that pin was stuck on "HI" even if on side B i was settingthe pin back to GND.
I used the same chip back in time when I was working on a serial connection between the RPI and a arduino. In that case i was having similar problems but those was due to some fake ics I got from ebay...
the one i'm using right now are from farnell/element14.
I don't have lots of equipment to test this out. (I don't have even have a proper scope)
If someone have an idea on what is going wrong I would appreciate that.
I'm still learning so it may be a dummy mistake!
Thanks,
hitech95
Datasheet:
http://www.ti.com/lit/ds/symlink/txb0108.pdf