Issue printing to LCD through SDA/SCL

Hi, I'm new to these forums and I am currently working on a project where I am converting an old rotary telephone to a mobile phone through the use of a GSM shield. I would like to interface an LCD screen to the Arduino UNO to print the number that is being called. My issue is the GSM shield uses the SCL and SDA ports already and although both components work as intended separately, the GSM shield interferes when I try to use the LCD screen at the same time. Does anyone have a suggestion for a workaround?

I have two Arduino UNO boards available to my disposal. What I was thinking was using each UNO for a separate function (One to print the number and the other to send the call via GSM). This seems overly complicated though and I was wondering if you guys had any thoughts about how I could get this LCD to work in conjunction with the GSM shield.

Any help is appreciated :) I'll slowly become more active as today is the first time I have even browsed through this forum. My apologies if this isn't the correct section to post such a question in (in that case please redirect me :D)

Cheers!

Edit: The LCD is a 1602A V2.0 with a control board already soldered to it. The outputs on the control board are simply GND VCC SDA and SCL.

Link to GSM shield : http://www.geeetech.com/wiki/index.php/Arduino_GPRS_Shield

My issue is the GSM shield uses the SCL and SDA ports already and although both components work as intended separately, the GSM shield interferes when I try to use the LCD screen at the same time.

Why don't you provide us with the code for the two programs that work as well as the one that doesn't?

Don

Sorry I fixed this issue sometime yesterday and didn’t update :slight_smile: The reason they didnt work was because the GSM shield used SCL/SDA so I had to simply manually wire 12 pins on my LCD screen to avoid using SCL/SDA on the LCD through the control board that was provided.

Well, that is one solution ...

:astonished:

I'd really like to know what went wrong when he had both devices on the I2C bus.

Don

floresta: I'd really like to know what went wrong when he had both devices on the I2C bus.

Don

My first guess would be missing pullups. While it can usually work without pullups with 1 device, adding additional devices may cause things to tip over.

--- bill

My first guess would be missing pullups.

Or it could be just the opposite, too many pullups. If each board has relatively low resistance pullups already installed the then putting two sets on the line could cause the failure.

Don

floresta: Or it could be just the opposite, too many pullups. If each board has relatively low resistance pullups already installed the then putting two sets on the line could cause the failure.

Don

It could be but the devices I've seen that include pullups used 4.7k pullups and since down to 1k is is required by the spec, it would take quite a few (more than two) before there would be an issue.

--- bill