need compatibility solution over rs232 and lvttl 3.3v

hello friends,

i'm trying to interface two devices with my microcontroller atmaga8l UART. one of the device support LVTTL signal(3.3v), so i can interface my controller directly with that device through 3.5 mm audio jack. now the second device support rs232 interface, so to interface with this device i have used max232. I want to connect both this device on same 3.5mm jack to achieve compatibility. To achieve this i'm using analog mux(sn74lv4052a) to route uart signal coming from uc to 3.3v supported device1 and rs232 level supported device2 through max232.
now the problem is that when i will connect device2 which support rs232 level(on the same 3.5mm audio jack to which 3.3v o/p signal coming from mux o/p1 is connected)there is quiet possibility to damage analog mux pin since mux pin can handle peak voltage of 5.5v. so i need solution over that such that when i connect device2 and send rs232 signals it should not damage my analog mux o/p1. for better understanding below i'm attaching image

where is image?

sorry,
here is the image file

waiting for your help

If I understand the problem, your concern is that the MAX232 outputs are at 5V TTL levels but the circuit it connects to used 3.3V and you are worried the extra voltage will cause damage. The solution is simple, use a MAX3232 instead of a MAX232. The 3232 is the same as the 232 but works on 3.3V supply and has LVTTL logic inputs and outputs.

Brian.

yes , your point is correct but I'm worrying about o/p voltage of max3232 which will give o/p swing of +/-12v. this voltage will be delivered to device2 connected to 3.5mm audio jack but at the same time this voltage will also damage o/p1 of analog mux since this o/p is also connected to 3.5 mm audio jack. I want to protect it from this +/-12v swing since mux pin can carry peak voltage of +5.5v only

I hope you understand my problem and thank you for your help

Using the same input for RS232 and an analog signal is very bad practise. However, if you really want to do it, add a resistor in series with the analog signal to the mux and then from the mux input connect two small Shottcky diodes, one to ground and one to the analog supply with both normally not conducting. In most cases this would be anodes toward ground and cathodes toward supply. The diodes will only conduct if the voltage goes below 0V or above supply by more than about 0.3V which should keep it within safe limits for the mux. Type BAT85 or 1N6263 would be suitable.

Brian.

thank you for your reply

is it possible to use transistor as series pass transistor element in 3.3v o/p line such that i can switch on it whenever i want to connect 3.3v device and when i want to use rs232 device i will simply switch off it so that my mux will remains protective from rs232 voltage. Can you also suggest me which line transistor will be suitable?

please help me

A transistor switch that isolates as much as +12/-12V swing is not impossible but probably more trouble than it's worth. As change-over speed is not important, would it be feasible for you to use a small relay instead?

Brian.

thank you for your reply

now I'm trying to use transistor as series pass switch on serial line(o/p 1 of mux) and lets see what will happen.

i think my problem will be solved if i use microcontroller which is capable to invert its UART signal, since rs232 uses inverted data. by this way i can use non inverted uart for device 1 and inverted one for device.

can anyone tell me avr microcontroller which having capability of inverting uart signal(either TX or RX or both at the same time)

waiting for reply

Data is inverted in code. For device 2 connect NOT gate. (Inverter chip).