Hello all,

I am working on a digital tape measure project which requires an option for the leading zeros to be blanked. It is to go from 0-999, so I am using 3 4510 BCD counters and they are feeding 3 4543 decoders. I have it counting up and down depending on the input phase, but am having difficulty with blanking the leading zeros. Any help would be greatly appreciated.

 

Pin 7 of the CD4543 is the blanking pin, active high, can you post your circuit?

 

I can post it when I get home. I saw the blanking pin/active high, but was unsure as to how to only blank the leading zero and not every zero.

 

I have CD4511s. I have found if I write a 10 to the chip then I get a blank display. But I am using a Pic. I have been meaning to check what is actually going on. I try to investigate and post back.

 

I can post it when I get home. I saw the blanking pin/active high, but was unsure as to how to only blank the leading zero and not every zero.

You will need to add one additional logic to blank the middle zero if the Most significant digit is binary input is 0000 the middle digit is 0000. That same logic can be used to check for 0000 on the most significant digit.

A dual four input OR gate will only be low when the value is zero. 7421 chip. Add an AND gate for the middle digit.

If you are using a Microcontroller, do it in software.

 

Assumong that you don't want a completely blank display for a value of all zeros, for the third and second digits combine the four bits in a 4-in NOR. This will blank the digit on zero. But you have to AND the output of the two NOR gates into the second digit so it stays lit if it is zero but the third digit is not.

ak

 

I find it amazing that people use 6 expensive (obsolete) logic chips and a mass of wiring instead of just using one microcontroller.

By the time you fix the leading zero issue you might be up to 9 logic chips, for a very simple task.

 

I agree with RB. Had you used a 7447 seven-segment decoder driver the solution would have been trivial.

A microcontroller chip would have provided a single chip solution.

In the current situation, blanking leading zeros is a luxury and I would simply scrap the idea.

 

Maybe I'm reading too much into the original post, but someone who has that task and is not considering a uC probably has ruled it out, or had it ruled out for him, for unstated reasons, such as he or the company having no uC development infrastructure or experience, or an bad firmware adventure in the past, or or or.

If you've never used a PIC before, or don't have a bunch of code blocks from past projects to draw from, 6 chips on a proto socket is a couple of hours while your first ever firmware experience could be a month.

OTOH, a CPLD....

ak

 

I would assume that this is a school project.
No one in there right mind would specify 4510/4543 chips in a modern commercial design.

As a school project, getting the digital tape measure to work is the meat, sufficient accomplishment.

Implementing blanking leading zeros is gravy.

 

I would assume that this is a school project.
No one in there right mind would specify 4510/4543 chips in a modern commercial design.

That is pretty harsh. I just built a project with 4511s. The 7 segment displays I had on hand just were not bright enough to stand up to multiplexing. Perhaps I could have tried a bit more to get tit to work but I had the 4511s and the displays and it just seemed the way to go. Pehaps the OP is in the same situation?

 

It is a design project for school. I would have loved to use a PIC but that, and PLD's, are prohibited for the design. The 4510's replaced 4516's that were in the original design. I cannot use 7447's because the output is going to an LCD, and that would require a ton of gates to invert the plane.