Pull-up and pull-down on TTL output - Page 1
Forum section: electronics / beginners

author: bitbanger
author_rank: Regular Contributor
timestamp: May 21, 2019, 02:44:48 am
content: 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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2019, 04:01:10 am
content: 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

author: bitbanger
author_rank: Regular Contributor
timestamp: May 21, 2019, 07:48:13 am
content: 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'.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!

author: bitbanger
author_rank: Regular Contributor
timestamp: May 21, 2019, 07:50:22 am
content: PS this is from their manual. Unfortunately they make no mention of when the jumper is to be used/removed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2019, 04:00:25 pm
content: Oh, it enables thegroundto the cable!Better left in then, it seems.Quote from: bitbanger on May 21, 2019, 07:48:13 amOh 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

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2019, 10:11:19 pm
content: Quote from: T3sl4co1l on May 21, 2019, 04:01:10 amThat is exactly what the jumper is for, to enable or disable Thevinin termination in a transmission line environment. A Thevinin parallel termination is used with TTL because it draws less power and compensates for the asymmetrical current output of the driver. It can also be selected to provide a valid TTL logic level when the bus is not driven.

author: bitbanger
author_rank: Regular Contributor
timestamp: May 22, 2019, 01:51:50 am
content: Spot on you guys - see scope captures below. This was w/100 Ohm termination and scope grounded to the (-) ref line, at the very end of the ribbon.Part of the problem (slightly in my defense) is I had a breakout board on the end of that ribbon, and I wasn't 100% clear on the pinout. So when I measured the 20MHz "triangle" at 10Vpp I was actually looking at the (-) line referenced to single-point-ground. On the (+) I was seeing a very very small amplitude signal, heavily distorted of course. Anyway the 10Vpp was a red herring - wasn't even the wire I should have been measuring on.

author: bitbanger
author_rank: Regular Contributor
timestamp: May 22, 2019, 01:52:48 am
content: Of course once I figured out how to drop the clock frequency down (to 5kHz) it cleaned right up.

author: bitbanger
author_rank: Regular Contributor
timestamp: May 22, 2019, 02:00:25 am
content: Quote from: David Hess on May 21, 2019, 10:11:19 pmSo this is what's referred to as a back-match termination? This termination shown is *not* for the receiver, rather the output of that transmitter?One thing that never made sense is why they specify the source current as a negative - when I first saw this I thought it was due to open-collector (my logic was that for an inverting OC output driver logic "high" was technically sinking, so they spec as negative value to indicate this). What's the deal here?Many thanks to both.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2019, 03:57:57 am
content: Source current is current flowing out of a pin. Conventionally, positive current flows into a pin. That's all.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: May 22, 2019, 06:41:54 am
content: Quote from: bitbanger on May 22, 2019, 01:52:48 amThat is, of course, deceptive. Any termination problems would still be there, since they occur during transitions.Any voltage spikes could still be (slowly) damaging the receivers, and whether or not the digital signals would be received correctly depends on the details of how the receiver interprets the analogue voltages as digital signals.