Hello, I'm trying to make a logic analyzer using my parallel port but I don't want to load down the target circuit, what would be the best way to do this?
My first thought would be to use a MOSFET since they have a high input resistance.<p>Any ideas?<p>RonH
Creating a high impedence probe
Re: Creating a high impedence probe
Ron:
How about starting with a 10x scope probe and its ~10 meg isolation. I don't experience any problems except on the feeblest....millivolt level.....probings in even RF circuitry.<p>73’s de Edd
eddw35@attbi (Interstellar ~~~~Warp~~~~Speed)
[email protected] (Firewalled-Spam*Cookies*Crumbs)<p>[ February 12, 2003: Message edited by: Edd Whatley ]</p>
How about starting with a 10x scope probe and its ~10 meg isolation. I don't experience any problems except on the feeblest....millivolt level.....probings in even RF circuitry.<p>73’s de Edd
eddw35@attbi (Interstellar ~~~~Warp~~~~Speed)
[email protected] (Firewalled-Spam*Cookies*Crumbs)<p>[ February 12, 2003: Message edited by: Edd Whatley ]</p>
Re: Creating a high impedence probe
Since you are using the parallel port, you must be looking at logic signals. The solution is to use CMOS logic buffer such as 74AC244 or 74HC244.
Re: Creating a high impedence probe
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Russ Kincaid:
Since you are using the parallel port, you must be looking at logic signals. The solution is to use CMOS logic buffer such as 74AC244 or 74HC244.<hr></blockquote><p>I actually did think about using that, I have a couple 74HC244's here but I wasn't sure if it was the right way to go or not, I guess I'd have to use resistors on the inputs to limit the current draw?<p>Scope probes would work but since I'm making an 8 input logic analyzer I didn't think that would be cheap enough for me, I'm acutally making this to see what is happening in another circuit that I'm making( neccesity is the mother of invention ), a PS/2 keyboard emulator.
Since you are using the parallel port, you must be looking at logic signals. The solution is to use CMOS logic buffer such as 74AC244 or 74HC244.<hr></blockquote><p>I actually did think about using that, I have a couple 74HC244's here but I wasn't sure if it was the right way to go or not, I guess I'd have to use resistors on the inputs to limit the current draw?<p>Scope probes would work but since I'm making an 8 input logic analyzer I didn't think that would be cheap enough for me, I'm acutally making this to see what is happening in another circuit that I'm making( neccesity is the mother of invention ), a PS/2 keyboard emulator.
-
- Posts: 26
- Joined: Fri Jun 21, 2002 1:01 am
- Location: Nevada
- Contact:
Re: Creating a high impedence probe
Also, consider that your parrallel port will be extremely slow (real time) in comparison to logic analyzers which have local memory then transfer results in non-realtime. I have played with the parrallel port with a pentium class (mmx 200) and was able to handle about 100KHz aggregate (meaning that 8 channels would be approx 12 KHz). If this is fast enough then a logic level buffer (the HCT is my favorite for input capacitance and impedence). A resistor won't hurt, but may not buy you anything, since the input bias current is so low on these things anyway. Have fun!
Rick Capps
Special Projects
Nevada Automotive Test Center
Special Projects
Nevada Automotive Test Center
Re: Creating a high impedence probe
Thanks for all of your help<p>I used a 74LS244 and it's working like a charm, the probram that I', using was written in ASM and is fairly fast, I'm not too concerened about realtime being in windows I never expected that, I just wanted to see the relationship between the lines. I'm running a P3/733 and it's doing a good job, I can get 10us without a problem, that's fast enough for me, now I just wish I didn't have a stupid 65K barrier!<p>Now I can work on my original project a keyboard emulator, if anyone has had any luck breaking the PS/2 protocol I'd love to hear from you.<p>Thanks
-
- Posts: 1263
- Joined: Wed Dec 05, 2001 1:01 am
- Location: Harviell, MO (Poplar Bluff area)
- Contact:
Re: Creating a high impedence probe
Resistance is rarely the culprit when it comes to loading of circuits by things like scope probes .... it's the capacitance. While an X10 attenuator probe for a scope presents a 10M ohm load to the circuit, the capacitance can be as low as 227 ohms of Xc at 100MHz for a 7pF capacitive load. That's why active probes are so popular for reducing the load. Yes, their resistance is high, but their capacitance is usually less than a picofarad.<p>Don't forget that the use of an attenuator probe also means a loss of sensitivity!<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).
R.I.P.
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).
R.I.P.
Who is online
Users browsing this forum: No registered users and 17 guests