Hello,

I am working on a project in lab, and need to connect two devices.

One produces a ttl-compatible waveform square wave (0, 5 volts). The device to receive it is cmos, and cannot take 5 volts. the HIGH it wants must be at least 3.3 volts.

Inside of the receiving device is a pull up 3.3kOhm resistor to 3.3 volts. What must I do to connect these devices successfully?

Thank you!

 

Your question is not clear. "The high must be at least 3.3 volts"..."Can not take 5 volts".

I never met a CMOS that couldn't survive 5 volts. Can you send a drawing of the circuit?

 

Sorry for being vague. I do not have a schematic, but I do have the documentation I got this from. The bit about the pull up was on page 1, 3rd paragraph. About not going to 5 volts was in page 2, second paragraph. All this is talking about the "TS" connector, or pin 10.

Thanks

 

Your file did not come through. Please try again.

 

Gotta go. Backin an hour or so.

 

They give a clue right there in the documentation - use an open-collector device.

You could either use an actual open-collector TTL device, or wire a transistor to be an open-collector driver to the 3.3 volt device (use an NPN transistor with the emitter to gnd, the collector to the 3.3v logic line, then have the TTL drive the base through a current-limiting resistor of around 1k-10k.)

 

If only I could see the documentation!

 

Here you go Bychon:
http://forum.allaboutcircuits.com/attachment.php?attachmentid=20131&d=1275690764

 

Sadly, I don't know what to do with that, either. I expect I'm supposed to search on AAC, but the "search" button at the top of this page takes me to a search box that returns "no matches" for 1275690764

Can I get a lesson from you?

 

When I click on an002.pdf, I get "attachment.php"
When I search for the post 1275690764, I get my own post, "Sadly, I don't know..."

 

Bychon,
I can view the attachment with no problem; it's an Adobe .pdf document.

If you are using Microsoft Internet Explorer, your MSIE cache directory structure might be damaged (I've had that happen before.)

Use Disk Cleanup to ditch much of the junk.
CCleaner does a more thorough job.

Then do a chkdsk on your drive. (Explorer -> C: -> right-click, Properties, then Tools tab, then [Check now], and schedule for next reboot.)

Then move your cache directory using the Tools options in MSIE. Doesn't matter much where you move it to; just as long as you move it. I suggest C:\Temp. Make a note of where the directory was located.

You'll have to re-boot to get it to move the files. If you don't use Disk Cleanup first, it will take a lot longer.

Then using Explorer, go back in where the Temporary Internet Files directory was, and delete everything in there.

 

Bychon, Can you typically view attachments on posts?

If you click on the attachment on this post, do you see the larger picture?

Sorry couldnt resist


If you are still having trouble, there may be an option for handling attachments or downloads.

You may have set your browser to save rather than open attachments, or you may have .pdf not linked to open with Adobe Acrobat Viewer.

visit:
http://www.adobe.com

and click on:

when you get there.

That should re-install Acrobat and re-register it with your browser.

 

Good morning.

I am using XP with Firefox and I have PDF's from this site in rows on my desktop...and they work. I am sincerely puzzled about why this one file turns into a .php when it downloads. However...it's morning now and I can poke into this today. I will be checking your suggestions for a while.

Thank you!

 

Whenever I have a problem, I throw vodka at it until it goes away.

try that.

 

I'll probably find that PDF's from other threads work for me and never figure this out. After all, I've been downloading PDF's for years and never had a problem besides this one. I'll just become more cautious about declaring, "the download doesn't work" now that I know it might be my problem.

ps, it's morning. Vodka doesn't start until after noon.

 

ps, it's morning. Vodka doesn't start until after noon.

Its noon somewhere.

 

You're right. I've noticed a lot of Englanders on this site, and it's after noon for them. Yay, Englanders!

 

See bychon, if you think about it long enough, you can solve any problem.

..or start to solve it....with vodka.

 

I solved the download problem, too. I uninstalled, "Free Download Manager" and that fixed it.

The oddest part is that Free Download Manager correctly downloaded PDF's from Electro-tech, but not here. Go figure!

On second thought, don't go figure. It's fixed. That's good enough for me.

 

Sorry, was away for weekend. About the problem, I called the company, and apparently, the voltage restriction in the documentation is not specifically about that pin, but they still recommended a lower voltage than 5V, as they were unsure about tolerances.

To reiterate, I have a square wave , 5V , TTL - compatible output that I need to input into a CMOS 3.3 volt logic device.

My thoughts are that I could just use a voltage divider. The picture is attached (including the pull up resistor from inside the device). I derived a formula that gives the resistances that I need, given the output voltage that I want. Eg, if I want to go from 0, 5V to .5, 3.5V , I need resistances of R1 = 2012.2, R2 = 833.333 (see pic for defintions of R1, R2).

R2 = (5000. Vlow)/(Vhigh - 1. Vlow),
R1 = -((165000. Vlow)/(-165. + 33. Vhigh + 17. Vlow))

My question is, what range should I have? I understand that I need the upper to be >=3.3 , but what is the usual convention for the Vlow?

Thanks,

antonmaz