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[SOLVED] How to connect GSM UART with two different devices

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Wajiduddaim

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Hi
i m using GSM Module SIM 808, and i want to connect it's UART with microcontroller and PC but one at a time (PC OR microcontroller), i m designing PCB, and i want some kind of diversion or blocker that when i use microcontroller it will not damage max232 IC

Please see the picture to understand

gsm.png
 

The signal is at logic levels so you can either do it with discrete logic gates or by using a multiplexer, for example a 74LS157 but be careful with the supply voltages if the SIM808 can't handle 5V levels.
It would be wise to use a MAX3232 instead of a MAX232 if the supply is lower then 5V as it will work down to lower supply voltage.

Brian.
 
Hi,

Tx --> multiple Rx is no problem. Just connect them.

Multiple Tx --> single Rx may be combined with AND logic.
This works as long as not two Tx try to send at the same time. In this case nothing will be damaged, but communication will become confused.

For sure a multiplexer is a good approach, too. But it needs an additional control line.
Who/what controls this line? Either PC or microcontroller?

Klaus
 
The best approach is to add UART bridge with two circular FIFO inside to ensure that no data was lost. Simple Atmel Mcu can do the job for you, if there are not enough UARTs inside - make software UARTs
 

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