Need to know the * fan out for the pulse chip output * on the PDB

allie

· Hi, every one.
·· Could someone please let me know what the fan out for 74ls IC's would be, from the PDB's pushbuttons. I have 2 -74ls157's, 1 -74ls170 read, 1 -74ls670 read, 1 - 74ls541.
·If this is not worded right, the question I'm asking is : can the PDB's pulse out line handle this.

·Thanks in advance
·allie
·

Beau Schwabe

allie,

I think I understand your question... The PDB's pulse out line should be able to drive all 5 inputs, since under a worse case scenario the maximum current would be about 3mA.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

roadrunner3g

fan-out for 74lsxx is 10 gates. I would use a couple of inverters (74ls04) off of the stamps pin to keep the fan-out in the same IC family. also the smaller the fan-out the better.

allie

· Yes Beau, you did understand the question. The inputs to the 157's and 541·IC's are the ENABLES and the inputs to the 170 and 670 are the read ENABLES.··································································································· I don't want to overload the pullup resistors for the pushbuttons, how much current can they handle.

·I also thought about the·7404,·I already have one on the board with two inverters left over. Maybe this is the best way to go, if I don't need them later for something else as the board is full and I can't add anymore IC's.

Regards allie

Post Edited (allie) : 3/22/2009 3:59:03 PM GMT

allie

·Sorry Beau,

·I did have the question worded wrong, but your answer, answered one of my questions. I had two questions mixed into one.

·Q#1; The fanout from the PDB's pulse line to 74lsxx IC's.

·Q#2; The fanout from the PDB's pushbuttons to 74lsxx IC's. Here I have the 5 inputs,all are ENABLES.

·New Question** The 74ls170's outputs are open collector,I have them wired to the inputs of the 74ls157. Will I need pullup resistors and will 10k·be the best choice.

·Regards
·allie
·