Yes, John is right about that. TTL inputs tend to float high. You can see that if you take a 74141 and power it up and look at each of the four inputs using a high impedance DVM. They will all be at some voltage just over the threshold that makes them high.
That floating high business is really not to be used in a circuit, but for quick tests, leaving a TTL input open usually results in it being interpreted as being high. Chuck > > >---- Original Message ---- >From: [email protected] >To: [email protected] >Subject: Re: [neonixie-l] Newbie 74141 help >Date: Tue, 25 Sep 2012 10:00:47 -0700 (PDT) > >> >> >>> In case you are wondering why this is so, it's because TTL logic >is >>> based on NPN transistors. They are good at pulling a signal down >to 0V, >>> but not good at pulling a signal up to 5V. So a resistor does the >>> pulling up, and the transistor does the pulling down. >>> >> >>You can see this in the >schematics<http://www.tayloredge.com/reference/Electronics/Logic/74xx >00.pdf>: >>The inputs are emitter followers that are pulled up to VCC-Vbe and >likewise >>the outputs are also emitter followers that can't go above VCC-Vbe >at best >>and 2 Vbe for the original TTL. >> >>Bottom line: Unconnected inputs are logic-1. >> >>-- >>You received this message because you are subscribed to the Google >Groups "neonixie-l" group. >>To post to this group, send an email to [email protected]. >>To unsubscribe from this group, send email to >[email protected]. >>To view this discussion on the web, visit >https://groups.google.com/d/msg/neonixie-l/-/k6WP08KBh7IJ. >>For more options, visit https://groups.google.com/groups/opt_out. >> >> >> $4.95/mo. National Dialup, Anti-Spam, Anti-Virus, 5mb personal web space. 5x faster dialup for only $9.95/mo. No contracts, No fees, No Kidding! See http://www.All2Easy.net for more details! -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit https://groups.google.com/groups/opt_out.
