Do I need pull resistors

Comments

• Carl Hayes Posts: 841

  2009-03-28 19:01 edited 2009-03-28 19:01

  allie said...
  Hi all
  ·I'm interfacing a 74ls170 4 x 4 register file for memory of 4 words of 4 bits each to a 74ls157 data selector.

  Just how are you going to do that?· You have four input words to select from, but the 74LS157 will select only between two, not among four.
  
  One way to do it would be to use three 74LS157 devices, of course.· If you do that, you won't need pullups on the 74LS157 that is fed by the other two 74LS157s, but you will·need pullups on the two 74LS157s that are fed by the 74LS170.
  
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  · -- Carl, nn5i@arrl.net
• Phil Pilgrim (PhiPi) Posts: 23,514

  2009-03-28 19:01 edited 2009-03-28 19:01

  I was going to give the same answer as Carl (orginally), but hesitated on the word "need". If I were designing the circuit, I'd include pullups. That said, however, driving a TTL input using an open-collector driver might well succeed without pullups. The reason is that TTL inputs source their own current, requiring only a pull-down to switch. Here's a schematic of the TTL input structure (from Wikipedia):
  
  
  
  Still, I'd use the pullups. You might decide later to switch to 74HCxxx parts.
  
  -Phil
  
  P.S. Like Carl, I also wonder about the logic of your proposed circuit.
  
  Post Edited (Phil Pilgrim (PhiPi)) : 3/28/2009 7:08:57 PM GMT
• Carl Hayes Posts: 841

  2009-03-28 19:17 edited 2009-03-28 19:17

  Phil, that schematic applies to nearly all 74xx devices, but it doesn't apply to most 74LSxx devices.· Some Fairchild 74LSxx devices, for example, used multiple-emitter inputs like the one you show, but National Semiconductor 74LSxx devices did not.· When Fairchild and NSC merged, mostly they continued the NSC forms thereafter.· I have no information on the circuits used by 74LSxx devices from other manufacturers.
  
  Even among the 74LSxx devices from NSC, there are differences in the input circuits from one 74LSxx device to another.· For example, 74LS00 used a different input from that used by 74LS54.
  
  So the Wikipedia article is not quite correct.
  
  Some 74LSxx devices will work from open-collector outputs without external pullups, but some will not -- and even those that will work will suffer a loss in switching speed.
  
  Use the pullups.· 1500 ohms ought to work fine, and be fast and reliable.
  
  
  
  Edit:· added the word some at the start of second sentence of first paragraph.
  
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  · -- Carl, nn5i@arrl.net
  
  Post Edited (Carl Hayes) : 3/28/2009 7:33:55 PM GMT
• Phil Pilgrim (PhiPi) Posts: 23,514

  2009-03-28 19:42 edited 2009-03-28 19:42

  Thanks, Carl, for the the thorough explanation. And you're certainly right about the switching speed. Without the pullup, the rise time would be prolonged significantly.
  
  -Phil
• allie Posts: 109

  2009-03-31 05:00 edited 2009-03-31 05:00

  Thanks for the replies on the pull-ups.
  
  ·I am using 2 74ls157's which are switched between busses A and B by a 74ls76, in toggle mode. I'm using the 74ls76 because I'm combining a 74ls170 and a 74ls670 and a 74ls90 into two 74ls157's and outputting these into a74ls48 for readout and also into a 74ls541 for an eight led readout of the binary form. For now the read addresses I'm doing it manually, but am planning to do it auto later.
  
  when I get time I'll send·a schmatic. I'm new to the schmatic drawing tools,so it will take me some time to get it drawn,but I will try.
  
  allie