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HEy we have designed our own 8051 board using 4066 to convert single serial port to 4 port but gives lots of garbage once we power up
What will keep the input to the MAX232 at a logic high, or logic low, level when the 4066 disconnects the processor output pin (with the internal pull-up) from the MAX232?
8051 ---------+ + | | | +-+ | | | | 4066 MAX232 | | | +-------+ +------+ +-+ | | | | | | | | / | | | TXD -+---|--+--|--o o-|----|------|--o RS232 TXD | | | | | | | | | | | | | | +-------+ +------+ ---------+ | | 4066 MAX232 | +-------+ +------+ | | | | | | | | | | +--|-o--o--|----|------|--o RS232 TXD | | | | | | | | +-------+ +------+
You have one TX output from the processor.
You have multiple MAX232.
Only one external serial port will be connected at a time to the processor TX signal.
That means that the TXD pin on your processor can only maintain a correct logic level on one (1) MAX232 input. What about the other MAX232 inputs? When the 4066 disconnects, these signals will be floating unless you have added a resistor for each multiplexed MAX232 logic-level input.
A solution with digital multiplexing means that the multiplexer chip can drive multiple outputs even if only one of the ouputs are connected to an input.
The 4066 has bilateral (dual-direction) switches - one side will see pull-up/pull-down from the other side when the switch is closed. But will not see any reference when the switch is open.
hey I have tried by placing pullup of 1kohms for TXD and RXD line then I have faced problem downloading not happening and even not able to communicate with any serial devices.
downloading not happening
What do you mean by that?
after adding pullups,I am trying download new .hex file then it is not happening.......