2input serial port with 8051

Hi, As both devices are not used simultaneously, you can think of below implementation:

Put 2 number of MAX232 Line Receiver/Drivers (choose an IC with Enable option). Short the both Rx's of MAX232 to Rx of controller and similarly for Tx.
1) Keep MAX232 of Scanner Enabled as default and LCD off.
2) When ever a scan is completed:
a) Disable scanner MAX232, Enable MAX232 of LCD
b) Transmit all the data
3) Diable LCD MAX232 and Enable the Scanner MAX232
4) Repeat the cycle.....
Assumptions are: LCD and scanner use same baudrate. Data Trasmited to LCD is only few bytes, so that Scanner will be ready fast for next scan.

Hi, As both devices are not used simultaneously, you can think of below implementation:

Put 2 number of MAX232 Line Receiver/Drivers (choose an IC with Enable option). Short the both Rx's of MAX232 to Rx of controller and similarly for Tx.
1) Keep MAX232 of Scanner Enabled as default and LCD off.
2) When ever a scan is completed:
a) Disable scanner MAX232, Enable MAX232 of LCD
b) Transmit all the data
3) Diable LCD MAX232 and Enable the Scanner MAX232
4) Repeat the cycle.....
Assumptions are: LCD and scanner use same baudrate. Data Trasmited to LCD is only few bytes, so that Scanner will be ready fast for next scan.

I have a problem only on Rx (bar code reader and RF resivier).
At TX we have only RF transmitter

Thanks

I guess you still need to use 2 MAX232 devices, as RS232 is a point to point type not a multi drop!

Put 2 number of MAX232 Line Receiver/Drivers (choose an IC with Enable option). Short the both Rx's of MAX232 will the smoke be blue or grey? to Rx of controller and similarly for Tx.

I know of no RS232 transciever that does not have push-pull output of the Rx. If you are going to short the RX outputs, you better find one with open collector OP.

Erik

Thanks Eric for your comments! Devices like LTC1387 will do the job. When a channel in high impedance on RS232 side, shoritng RX (at TTL) may not give you chance for smoke. Else it is simple to build a switch to isoalte TTL side, with old style MAX232 device.

Thanks Eric for your comments! Devices like LTC1387 will do the job ... as far as I can see there is no disable of the (TTL level) Rx output and it IS push-pull. Thus parallelling two WILL give smoke.

Erik

continuing:
why not just use an OR gate?

Erik

Thanks for every body who try to help me.

yes, I was thinking to use an OR gate...

thanks a lot

So now you're discounting all the reasons that have been repeatedly explained to you why trying to bodge this onto a single UART is a Really Bad Idea!

Don't do it!

You will compromise the functionality of your project, you will make your design & development harder, and you are unlikely to save any cost on the hardware - but may well make it more expensive!

Why on earth would you do that?!

Thank you Andy

unfotunitly I searched about 8051 with 2 serial port, but the company didn't deliverd the part the UAE and other companies the product is out of stock....

So what can I do????????????/
I should look for alternative!!!!

digi-key http://www.digikey.com/ ship all over the world
and I believe, so does Mouser and others

Erik