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Pull-up and pull-down on TTL output
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bitbanger:
Hi folks -

I'm looking at an output circuit on an older parallel data card (PCI), and the output drivers have a 150ohm pull-up to 5V, then a jumper selectable pull-down of 390ohm to ground. The drivers are 74abt(to verify) 244's with tristatable outputs. The output also has configurable receivers so I'm just not sure what the jumper is for (it's termed "TTL termination). At first I thought the pull-up was for an open collector output, but that doesn't seem to be the case. The '244 can source 32mA and sinc 64mA.

I'm trying to diagnose why the output seems to be swinging so high when clocking: at idle it's around 3.6V but when clocking (20 MHz) I seem to have a ground reference issue because the output is severely distorted (more like triange wave) and nearly 10Vp-p.

Anyway, just looking for some insight into why the output may be configured like that with the resistors. Any thoughts? Jumper to adhere to LVTTL? That wouldn't seem to hold though because when high the output should still be 5.0V...hmm
T3sl4co1l:
What scope and probing method are you measuring with?

150 || 390 = 108 ohms, a good termination for ribbon cable and twisted pair (often used for parallel port cables).  This is relevant when the port is open circuit (output disabled, presumably input mode).  The 74ABT family is more than capable of driving the piss out of such a termination, so it can be ignored in output mode.

The pull-up wouldn't seem to matter, or more importantly, that they should be disable-able along with the 390s to save current consumption.

150 ohms termination (alone) wouldn't be a bad termination either (only a 50% mismatch, not really enough to corrupt the logic levels), though the fact that the Thevenin source voltage is quite high (i.e., the full 5V) may be annoying for most TTL drivers and possibly for long cables (i.e., pulling up so much on an active-low output may violate the V_IL(max) at the receiver end, thus giving bad data).

Are you sure just the 390s are on the jumper, and not the other way around, or both?

Tim
bitbanger:
Well you hit the nail - the output is provided on an 80-pin twisted-pair ribbon cable. 9 ft of it in fact. The card is apparently also offered in an LVDS version as well, but I'm looking at the TTL version. Funny the documentation refers to this as 'differential TTL'.  :o

I was unable to get direct access to the pins in the PC so I was probing at the end of the ribbon cable, referenced to chassis ground. I suspect a grounding issue because the triangle was swinging slightly negative (-4 to +6 let's say), yet dead-on at 20MHz. Scope + probe were ~150MHz so system bandwidth may be contributing to mishapen waveform.

20MHz seems a little crazy at these lengths, even over twisted ribon. I mean sure SATA/66 can do faster but that's in a grounded chassis, maybe 1.5-2' in length?  Even though it's not differential, would some sort of AC termination be needed at the end of this cable?

Thanks for the input!
bitbanger:
PS this is from their manual. Unfortunately they make no mention of when the jumper is to be used/removed.
T3sl4co1l:
Oh, it enables the ground to the cable!

Better left in then, it seems.


--- Quote from: bitbanger on May 21, 2019, 07:48:13 am ---I was unable to get direct access to the pins in the PC so I was probing at the end of the ribbon cable, referenced to chassis ground. I suspect a grounding issue because the triangle was swinging slightly negative (-4 to +6 let's say), yet dead-on at 20MHz. Scope + probe were ~150MHz so system bandwidth may be contributing to mishapen waveform.

--- End quote ---

Oh well there's your problem, unterminated cable so you're getting bullshit at the end.  Also need to measure with respect to the same grounds at the same point in the cable itself, not against some distant ground.

Try probing between signal and ground, at the end of the cable, with a 100 ohm resistor terminating it. :)

Tim
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