Actually a 1K resistor means that you will need about 5ma to pull the pin t= o ground (if the positive rail is +5V). I am guessing that some CMOS outputs would be hard pressed to sink that much current. As is nearly everything in life: It is a tradeoff. Another possibility is to make a 'shorting plug' to tie the input hard (or via a low resistance) to the +ve rail. Remove the shorting plug, connect up= , and off you go. -- Bob Ammerman RAm Systems -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Dwayne Reid Sent: Tuesday, April 15, 2014 11:23 AM To: Microcontroller discussion list - Public. Subject: Re: [EE] noise-proofing open serial interface lines At 06:51 AM 4/15/2014, KPL wrote: >It was not THAT huge issue, I tried both pull up and down with a 10K=20 >resistor, but got different reaction upon connecting of different=20 >usb-serial, that's why i asked for better options. Currently it's left=20 >with pull-up, which seemed more logical. A pull-up resistor is the proper way if its a TTL-level signal. If you hav= e having noise pickup issues, reduce the value of the pull-up resistor. 1k i= s about as low as you should go, though. dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/chang= e your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .