Signal Ground VS Chassis Ground - Should they be coupled? - Page 1
Forum section: electronics / beginners

author: innkeeper
author_rank: Unknown
timestamp: January 24, 2020, 07:03:16 pm
content: I'm assembling a sensitive op amp circuit that will be comparing both balanced and unbalanced inputs for low level measurements.I am wondering if there should be any coupling between Earth/Chassis ground and the signal ground(PSU ground)?I am concerned with any kind of external noise influence.This project will be placed inside a metal case. Also inside this metal case will be an R-core transformer and and EIC connector with its ground attached to the chassis for safety though i am not opposed to moving the PSU external if noise becomes a factor.My question is should there be any coupling between chassis ground and the power ground of the op amp circuit.i am of two minds on this..i like that the metal chassis forms a Faraday cage. however, i am concerned that the chassis itself could also introduce noise if the coupling is not handled the right way.Eventually i can experiment with this once i go from a design to actual implementation, but, id like to get some input on what the traps are here and best practices.i know that for certain sensitive measurements the psu is totally replaced by batteries and the entire thing is enclosed in a metal box. I'm not sure if i am exactly at that point but, pretty close. I am taking care to make the power supply extra clean for example.I look forward to everyone thoughts on the topic.

author: jmelson
author_rank: Super Contributor
timestamp: January 24, 2020, 07:32:41 pm
content: Wow, you've opened Pandora's box! There are so many things to know before answering. But, in the most general terms, you don't want the electronics to be floating with respect to thechassis/shield. So, the op amp power ground should be tied to the chassis at ONE POINT only. Instrumentation amps need to have the signals within the common mode range of theop-amps, so the sensor that is the source of the signal needs to be referenced to that ground in some manner.Jon

author: innkeeper
author_rank: Unknown
timestamp: January 25, 2020, 12:16:37 am
content: i am thinking out loud here... if the signal ground is tied to chassis ground, and i connect it to another mains grounded system, that can case a long loop between grounds that can have voltage difference both in noise and mains. this would make for a noise source... or am i thinking completely wrong? if this is the case, then id think keeping the signal ground isolated from mains/chassis might make sense?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: January 25, 2020, 03:48:27 am
content: There's three sets of lectures on learning.oreilly.com that cover this. You can get a week long trial. The lectures are by Howard Johnson. He's considered the best signal engineer on the planet.

author: teecelly2020
author_rank: Newbie
timestamp: January 25, 2020, 05:02:17 am
content: I use a pair of diodes in parallel, but in opposite polarity and a small cap to break low voltage ground loops in audio equipment.You can learn more about it here:https://circuitcellar.com/resources/ee-tips/find-and-eliminate-ground-loops/Good luck!

author: bob91343
author_rank: Super Contributor
timestamp: January 25, 2020, 05:58:27 am
content: Here's the story. Signal ground is merely a reference point; the important issue is the current path. In other words, the signal needs a return path independent of any other currents.Chassis ground should only be used to prevent the chassis from wandering around, and its main purpose is shielding. No current should flow in the chassis.If you don't follow these rules you are setting yourself up for noise, hum, oscillation, and worse.You can connect signal and chassis grounds together as long as you avoid uncontrolled current flows.I realize this isn't the simple answer you seek but it's not simple stuff.

author: mansaxel
author_rank: Super Contributor
timestamp: January 25, 2020, 10:52:28 am
content: Quote from: bob91343 on January 25, 2020, 05:58:27 am(The few things I know in this field are all about analog audio systems, so the following is to be read with that in mind. Other fields of EE might need different treatment. Caveat emptor. )It is indeed complicated. Neil Muncy wrote a paper in Journal of the Audio Engineering Society, June 1995, "Noise Susceptibility in Analogand Digital Signal Processing Systems”, which is a very good description of the problem and pointers towards remedies. Tony Waldron, Technical Director at Cadac, wrote a series of articles on "Practical interference-free audio systems" that start with how rebar, piping, conduit and electrical grounds are interconnected in theatres and TV studios, and go all the way up to PCB design.The meat of it all is already mentioned, "one interconnection only" -- per device, that is. The screen of an incoming signal connection shall be well terminated to chassis at the entry point, and only the signal wires shall continue to the PCB. Signal ground on the PCB shall be interconnected with DC PSU 0V and at one place connected to chassis, and power ground. That is the basic rule, and it can be bent, but at best knowingly so. Between boxes, no screen shall be interrupted, unless you're building an AM antenna.ThereISa place for galvanic, i.e. transformer or optical isolation, but it is rarely needed as much as people believe.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2020, 11:48:06 am
content: Yes. Sometimes. Maybe. Depends.(Congratulations, you've won my laziest post of the day?..)Suffice it to say, it's complicated. You cannot look at one small part of a system in isolation, you must consider the whole system to determine which is best.Tim

author: innkeeper
author_rank: Unknown
timestamp: January 25, 2020, 09:14:07 pm
content: @JustMeHere - is oriley safari worth the 399 a year?@teecelly2020 - that is a very interesting approach to connecting chassis ground. The more i think about it the more i like the concept, though, not sure how it will behave int he wild. What i might do is take my pcb and add some pads to connect components from the chassis ground points to the signal gorund. this way i can try this and other ideas. or not use it.@bob91343 - sounds like the way to control current flow in the chassis would be to only allow 1 connection point to the chassis?@mansaxel - in my case, i will have a device under test, with a single ended signal going into both the device under test and my device and then the single ended output of the device under test also going into my device. then yet another output from my device to other equipment for measurement purposes, all sharing common grounds. I could break the two grounds to the DUT by potentially making the input to my device differential and detecting the difference from the incoming ground and the signal line and therefor not grounding the returning line to the device however the measurement equipment wold still share the same ground as the input to the device under test. sorry if thatch a bit convoluted.

author: bob91343
author_rank: Super Contributor
timestamp: January 29, 2020, 12:55:05 am
content: That is correct, only one point on chassis should connect. Otherwise there will be currents flowing in the metal with resultant voltage drops which can interfere in the signal path.If you have a PCB with a signal ground, it should be one point only. And the input signal from the jack, mounted on chassis, should be insulated and its ground carried to the PCB signal ground (NOT touching chassis!). Never have two signal grounds! Never rely on the chassis to have zero impedance!A while back a friend had a signal mixer to use in an audio system. It had some hum and he sent it to the factory for some standard hum-removing modification which didn't do the job. I got the unit and discovered the cause of the hum. If you guessed chassis current, you get the prize. I had to redo the whole circuit philosophy to fix it. Later he came over and showed me how he used it. I looked at him and said, no hum? He did a double take and said you are right, no hum! How did you do that?

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: January 29, 2020, 02:22:40 am
content: The circuit ground need not be connected to the chassis ground. For e.g. in case of USB powered devices, the USB cable has both a shield and a GND wire. The best practice is to have the GND (black wire) connected to the circuit ground. The shield wire (metal braid) gets connected to the chassis. It is also a good practice to add a small capacitor (few 100pF) from the circuit GND to the Shield for the high frequency grounding. This helps to minimize EMI.As a general rule, the chassis should be at the earth potential to avoid any safety hazards. The circuit ground can be at any potential w.r.t. chassis.

author: mbless
author_rank: Regular Contributor
timestamp: January 29, 2020, 03:01:55 am
content: Quote from: bob91343 on January 29, 2020, 12:55:05 amCan you clarify the "insulated" part of your statement? From what I've read elsewhere, I'm surmising that a shielded cable has the shield connected to the chassis (typically through the connector), and the signal and signal ground go directly to the PCB. Or are you talking about shielding the signal in the interior of the chassis from the connector to the PCB?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2020, 03:50:31 am
content: A lot of nonsense happens when designers don't know about RF versus galvanic or audio grounding. Most of the solutions seen for one type, are horrid when considered by the other, and vice versa.Single point star grounding, case in point. What an electromagnetic disaster. There is, however, an RF version which is general, covering both cases; the trouble is, few audio designers know it.The system is the biggest problem in audio. You can't fix a badly wired system even with the best possible amplifier. Like I said before, you can't analyze a component in a vacuum, it's the system that matters!In RF, you don't much care about ground loop currents, because they are rejected out-of-band, and isolation is easy (some coupling capacitors or a little isolation transformer). Differential signaling may also be used. You can make up for some sins this way.In audio, the fault arises because ground currents are allowed to flow in signal conductors. Namely, signal cables are one pair and that's it: signal and "ground". Except it's not really "ground", or it shouldn't be: it's signal reference, negative. It's forced to serve both purposes, and the system is fragile as a consequence.Oscilloscope probes suffer the same fate: they are both wideband, unbalanced, and sensitive to common mode (= ground loop) interference.Commercial signaling systems avoid this by separating the signal reference and ground, and often using differential. Examples:Shielded XLR, a direct analog example, differentialUSB, a digital example, differential with poor CM range making it highly vulnerable, if not for its shieldADSL, a hybrid example, differential, symbols are transmitted as phased AC with up to 32768 statesADSL must be isolated? And Ethernet is transformer isolated (it uses differential signaling and relatively few states, e.g. PAM-3 for 100BASE-T), with the bonus that their signals are high frequency, so avoid low frequency noise problems doubly so.XLR is sometimes transformer isolated, but this should only be needed in the absolute worst case, where ground loop voltage is so high that the receiver's CM range is exceeded (probably around 15V in most cases?).The trick for "pro-sumer" audio is to use RCA jacks differentially. The shield must be RF grounded with capacitors. The shield can additionally be semi-grounded to the circuit or chassis through a resistance, but this should be nonlinear for protection only -- i.e., a large resistor to bleed down static, in parallel with a TVS to shunt fault currents from cross-wired circuits. The differential voltage is then sensed with a diffamp, and you don't have to worry about ground loop currents on board at all. Mind to include an RF filter there, to clean up remaining trash, especially if you're using slower or older or bipolar circuits that love to rectify RF.Such a method won't solve CM currents already on the cables, or between other equipment in the system, of course.Tim

author: bob91343
author_rank: Super Contributor
timestamp: January 29, 2020, 04:15:19 am
content: To mbless, yes the shield of the input cable and the low side of the input jack should be insulated from chassis. They need to go to the signal return in the amplifier.It's good to ground to chassis at some point. But only one point.Some systems use a shield ground and a signal ground at the input connector. Whatever it takes to eliminate the so very common ground loops.It's easy to get sloppy and just ground stuff. Sometimes it comes out okay but there are too many times when one gets oscillation, power supply hum, and other undesirable effects.A shield should shield from fields. Don't ask it to carry current unless you are very careful how you do it.Trace the current path of the signal and make sure it only goes where you want it to.

author: mansaxel
author_rank: Super Contributor
timestamp: January 31, 2020, 09:06:53 am
content: Quote from: T3sl4co1l on January 29, 2020, 03:50:31 amWhat was discovered, there in the middle 90s, was that one had to learn both; mostly because digital logic at then-modern CPU speeds acts more like RF than DC... And excepting luddites, most every audio has device a computer inside.As you write, star grounds are effective in the audio passband, but not further up. Their impedance at higher frequencies is simply too large. The conclusion by Muncy and Waldron among others was that one had to build a broadband grounding scheme. They ended up with a few things:As noted above, mixing interference with signal defiles the signalThe practice of interrupting screens that sometimes, at random, gives useful results in audio is an efficient antenna design paradigm at HF.The job of a screen is to siphon interference current off of the signal path.Therefore, the screen needs to be as complete as possible, in order to be able to carry interference currents.The screen needs to be not only continous in a DC ohms sense, but also for RF, which means that things like barrier strips were very problematic, since they can't be screened and present a high impedance bump at RF.Since we can't mix signal and screen, balanced audio is a requirement for any connection that will travel any length, especially between powered devices.These conclusions led to a few recommendations and products:A device design model that takes only signal positive and negative onto the PCB, leaving screen and 0V and phantom power ground etc to travel together with interference through the chassis instead, meeting DC 0V in the PSU. Not in the preamplifier where there's a lot of eager, non-discriminating, gain.New RF-shielded XLR connectors andXLR chassis connectorsthat short Pin 1 to ground as broad-band-ly as possibleA grounding paradigm that did away with all those crazy ideas of putting racks on wood plinths to isolate them, instead opting for as much grounding as possible in the audio system design, cross-bonding with much everything, frequently.This works. But, as also has been said upthread, the system needs to be analyzed as a whole.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2020, 09:36:41 am
content: Regarding single-end shields: it's very old advice indeed, old enough that I wonder if it's nothing but a holdover from the tube days.Namely, in vacuum tube circuits, circuit impedances are quite high (kohms in RF circuits, 100s kohms in audio), and being higher than Zo/2 (Zo = impedance of free space), magnetic fields don't matter very much. Or to put it another way, signal levels are high enough that magnetic field induction is almost negligible. Electric induction (i.e. capacitive coupling) instead is the major offender. So just putting a screen around everything sensitive, and grounding one end to sink the electric charge to ground, is adequate shielding.Circuits were also low bandwidth and gain, unlikely to oscillate from random transmission line stubs. Except when they weren't (peppy RF tubes are more than happy to!), but grid stoppers were standard practice as well.Tim

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2020, 01:08:07 am
content: Quote from: innkeeper on January 25, 2020, 12:16:37 amThat is a real problem with audio as discussed above when RCA connections are used but there are ways to fix it. The usual solutions are to either use differential signaling which rejects the difference in the grounds/commons, or to sense the remote ground/common potential with a difference amplifier so it can be subtracted.The above also applies to differences between grounds/commons within a chassis as well. With precision circuits, it may apply on a single printed circuit board or ground plane like when you have 16+ bit converters.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: February 01, 2020, 02:48:28 am
content: Quote from: David Hess on February 01, 2020, 01:08:07 amYes, it is always a good idea to keep the shield and signal return (circuit ground) as separate points. I think the circuit reference should never be called as Ground as it creates confusion. It all started off with the main wiring connection where one of the conductors was physically connected to the earth (ground) for safety reasons. For battery powered devices, there is no connection to the physical ground but there is always a shield to keep away EMI. For systems where there is some connection to the earth, for e.g. PC, Audio Amps, or most of the line powered devices, it is always recommended to keep the circuit reference separate from the shield (Ground). The shield can carry noise current which if connected to the circuit reference, may show up as SNR degradation. By keeping them separate, you split the noise paths and the signal return paths.Take another practical example of a system which is enclosed in a metal case. The power supply to the system is completely floating (isolated using transformer). The circuit reference is not connected to the metal case. However there is a shield on the PCB which is connected to the circuit reference. And the mains Ground (also called earth in many countries, and usually a Green Wire in the mains supply) is connected to the metal case (chassis). Should the shield on the PCB be connected to the metal case of the system to make a single point ground? The simple answer is NO. The internal shield on the PCB is used to keep the EMI away by containing the fields within the shield. These carry mostly high frequency currents that are in low amplitude. The metal case offers protection for the user and in case of any breakdown, the metal case is held at the earth potential by the earth (ground) connection. By shorting the metal chassis to the RF shields on the PCB, we could induce some currents on the chassis which was never meant to flow to the earth. These RF currents could cause radiation as the earth wires are long and are very good radiators.Keep the circuit reference away from the shield (ground) whenever possible. In some cases where you use a connector like BNC or SMA where the Ground is the same as the shield, you do not have a choice but the short them. But these are mostly RF equipment where the circuit reference and the ground are coupled due to capacitive coupling. Solving the ground issues in RF designs is tricky as this is the area of black magic design, where the observations in the lab are seldom written on paper.. They remain in the minds of the engineers...

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: February 01, 2020, 02:49:55 am
content: I would recommend the new book from Henry Ott (Electromagnetic compatibility engineering)It is a bible for EMI engineers...https://cds.cern.ch/record/1188673/files/9780470189306_TOC.pdf

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2020, 03:22:40 am
content: Analog Devices has several old application notes covering this subject:https://www.analog.com/media/en/technical-documentation/application-notes/AN-202.pdfhttps://www.analog.com/media/en/technical-documentation/application-notes/6001142869552014948960492698455131755584673020828AN_345.pdfhttps://www.analog.com/media/en/technical-documentation/application-notes/41727248AN_347.pdf

author: bob91343
author_rank: Super Contributor
timestamp: February 01, 2020, 06:23:47 pm
content: One approach I use is to draw on the diagram the complete loop of signal current. For example, from collector to coupling capacitor to output jack to load, through load, back to low side output jack, back to emitter, through transistor and finally back where we started, collector.Any 'ground' should not be in the circuit, although it's okay to connect some point to chassis (say, output low side) for shielding purposes but not to pass any signal. And once you pick a ground point, use it for all grounds.That won't make it perfect due to cross currents in the actual ground connection but it will make it possible to avoid major loops. Sometimes when connecting several wires to a chassis ground point it will be advantageous to disconnect them and try reconnecting in a different order. Only a fraction of an Ohm can make a difference.Don't rely on rivets and screws to stay connected; they loosen and corrode. Not only do they lose some continuity but they also become diodes and rectify signals with resulting intermodulation. A large bolt and toothed lockwasher and solder lug is a good thing if tight enough. A copper strip is good; bolt it to the chassis after soldering all grounds as close to one another as possible.Shielded wire can be a trap. Use the shield as a shield and not for signal return, meaning 'ground' it at one end only. If you need a return wire, get two-conductor shielded wire and use one internal wire for signal return. Back in the day, we learned a lot when trying to connect phonograph cartridges to amplifiers. Getting the hum out took a lot of thought.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2020, 07:33:33 pm
content: Quote from: bob91343 on February 01, 2020, 06:23:47 pmAgain, broken shield means introducing 100.0% of interference picked up by said shield, into the signals inside.If you're doing a differential pair, maybe that's okay. In which case you don't even need the shield in the first place. 15V CMR is enough to handle a lot of ambient noise sources. Won't stop clicks and pops from ESD and such. Maybe those are brief enough they don't matter. Maybe you prefer the sound of random clicks, because it's like a phonograph...But you better hope your CMRR is good, including at radio frequencies. And if it's not, I hope you like the local broadcast stations...Tim

author: mansaxel
author_rank: Super Contributor
timestamp: February 01, 2020, 10:24:08 pm
content: Quote from: bob91343 on February 01, 2020, 06:23:47 pmAs T3sl4co1l writes, that is not always the good idea you describe it as. I'd limit that method to perhaps inside a device, where you have an external good RF screen and need to go from one place to another by cable. Outside, entering the system design domain, the "lifting" of shield connections was tried, erratically and with very mixed results, by those in the trade, aided by stupid and sometimes dangerous methods and measures offered by habit and box designers, like ground lift switches on devices and ungrounded outlets..Once you understand that the interference is an induced voltage that wants to become current, the solution is obvious. Let the current flow, but in a managed way, in places where it won't harm your signal.As a benefit, you'll get less harm to people too: I once worked at a theatre, where the stage managers position has got an intercom wallbox just beside the iron fire curtain. I happened to stand there and touched the intercom box while leaned against the iron curtain, and was zapped with 117VAC, not much current, but it hurt. I had my suspicions, as to why, and after the show I opened the wall outlet where the mixing desk power supply was connected and replaced the ground wire. The Y2 capacitors in the PSU will act as voltage dividers and put 1/2 * mains voltage on the ground pin if it's left floating. The entire audio system shield / 0V / "ground" (including human-touching parts like microphone pop screens etc; the mixer provides a ground connection that is continous all the way out to the vocalists mouth.) had 117VAC 50Hz on it, because the mixing desk of course was connected to the chassis of its PSU. With a suitably beefy ground connection, the problem disappears, and the Y capacitors can work again.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2020, 11:04:49 pm
content: Quote from: mansaxel on February 01, 2020, 10:24:08 pmYikes! Did you notice that everything felt weird when you lightly run your finger over it? It's not just weird--there's an electroresponsive reaction (I forget if it's the sense of touch, or the skin itself somehow moving?) to AC electric fields!Tim

author: mansaxel
author_rank: Super Contributor
timestamp: February 02, 2020, 07:46:30 pm
content: Quote from: T3sl4co1l on February 01, 2020, 11:04:49 pmOh, yes. My laptop has exactly the same problem. Portable macs, with their metal shell and wall warts that can take 2-prong plugs or 3-prong cables (at least in Europe) will need to be fitted with a 3-prong cable/plug to be safe and tolerable. If not, they will present a very special tingling sensation when you type on them. Your palms typically rest on the metal case and will conduct if you're better grounded than the floating computer. An ESD carpet is low resistance enough to set this off, and leaning towards a metal cabinet will literallyI did some testing with this setup:The laptop is plugged into the mains with a 2-prong plug, and measuring from the ground lug in the outlet to the shell connection of the headphone jack (which is the same as the case of the computer) yields this nice measurement:Altering the setup by fitting a grounded mains cable, but otherwise similar, produces a different result, roughly 3 orders of magnitude lower:

author: JDW
author_rank: Frequent Contributor
timestamp: July 05, 2022, 01:29:21 am
content: Here's a more specific case that perhaps some of you could offer your detailed thoughts about...I would like to present the case of a motherboard taken from a 1987 Apple Macintosh SE computer. The chassis of that computer is metal, and is earth ground when a 3-prong power cord is used. There's a metal bar atop the motherboard which makes contact with the metal chassis, to provide chassis ground to the connector portion of the motherboard. The motherboard gets it's primary ground via the main wiring harness that attaches between the motherboard and the analog board (which drives the CRT and which leads to the power supply).I have confirmed from observing the motherboard Gerber files ground plane is divided into two parts, and the parts are tied together by a single trace on the top side of the board. That single trace is shown in red in the photo above and the Gerber file below.I'm not 100% sure why it is designed like that, unless it anticipates a dead short to ground in the chassis or external connectors, such that the trace shown in red above would act as a fuse and blow (split apart) when a short occurs.Any thoughts?

author: David Hess
author_rank: Super Contributor
timestamp: July 05, 2022, 03:05:16 am
content: Quote from: JDW on July 05, 2022, 01:29:21 amApple was not the only one to use a single trace like that, and I have seen such traces blown. Sometimes those traces are intended as fuses, but another reason to do that is to prevent ground currents between the external chassis connections to stop the chassis from radiating EMI. A single connection to the external chassis will prevent this, but then what do you do when you have multiple shielded connectors connecting to the chassis?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2022, 04:54:14 am
content: They made it worse, but it didn't much matter because the enclosure is metallized IIRC.The effect is that the main area of the board, pushes itself around by the reaction force from all those damned traces crossing the gap.EMI is higher inside the enclosure. But only within logic levels give or take, so extreme coupling would be needed to corrupt logic levels elsewhere in the circuit. It has relative immunity from itself, which is fine -- it's functional.This would be a very bad idea if there were connections on the front, or extender cords to external drives (through the front slot/bays), etc. Such connections would need significant mitigations applied (heavy filtering, tons of data line CMCs, isolation). But such cases are not, I think, part of any normal build of that machine, so the fact that the enclosure is shielded, should be enough to deal with EMC.As for the connector side of the board: the slot does avoid PS currents flowing across it. It obviously doesn't prevent signal currents running around it (along the slot, necking down through the one lone linking trace), and it degrades the signal quality of the traces crossing it (which I guess the big DIP there is a SCSI controller? ref:https://upload.wikimedia.org/wikipedia/commons/6/65/Apple_Macintosh_SE_Main_PCB.jpg).Probably it doesn't matter much, because with so many chips, and trace length -- and ribbon cables and so on -- it may never pass EMC with a plastic (unshielded) enclosure, even with perfect layout. So there's not much if any cost to pay for the poor layout, as there's no cost-saving alternative.Perhapsone might argue for a 2-layer board (assuming that's 4 there), but with the PLCC socket, and wide buses, that's probably infeasible, even if the layout could be done while respecting signal quality (which it definitely can't, not in 2 layers with component density like that!).On a side note, I wonder how CRTs were for EMC, in general. TVs were certainly common enough with plastic (or wood, hah) enclosures, and most monitors as well I think. Granted, a lot of monitors have metal shields or heatsinks in place, dealing with a lot of that -- intentionally or otherwise -- at the board/assembly level already.Which begs the question, why not shield the motherboard? Perhaps they didn't have low enough EMI between cables, so still needed the enclosure shielded. Perhaps a shield would be too bothersome given the number of connections through it? Counterpoint: I have a late model (contemporary, heh) XT clone with a shielded motherboard, with plenty of holes in said shield for cables, expansion slots, etc., and a molded plastic enclosure.Tim

author: JDW
author_rank: Frequent Contributor
timestamp: July 05, 2022, 05:28:39 am
content: Quote from: T3sl4co1l on July 05, 2022, 04:54:14 amCan you explain what you mean? For example, what would occur had they not divided up the motherboard ground plane into two parts (with that single trace joining them) and instead just kept the entire ground plane as a single piece?Quote from: T3sl4co1l on July 05, 2022, 04:54:14 amThe motherboard is shielded with a flexible metal shield which covers the entire bottom, left, right and back (connector) sides. (Naturally, the bottom is covered with insulating material so the solder side cannot short against it.)Secondary case shielding was accomplished with a metallic spray coating on the inside of the plastic case.Lastly, how do I specify a pixel width for the inline images in my posts? These full width images are annoying.

author: David Hess
author_rank: Super Contributor
timestamp: July 05, 2022, 09:10:15 am
content: Quote from: JDW on July 05, 2022, 05:28:39 amIt means the return currents for the traces which cross the gap have to go all the way around to the trace, which increases the loop area and emissions. A ground plane allows the return currents to closely follow the signal trace, minimizing loop area and emissions to minimum.Was it worth increasing board emissions to lower emissions from the chassis? I have to assume so. Maybe they took other steps which are not so apparent, like limiting the bandwidth of the signals or using differential signaling.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2022, 09:43:18 am
content: Ah OK, shield over it besides. Hmm, maybe it was worse than I expected!Quote from: JDW on July 05, 2022, 05:28:39 amLike so:Without the slot, Vcm would be, single, maybe 10s of mV. With slot, it can be 100s; maybe even several volts if a lot of the bus changes all at once. I'd be willing to bet they could've saved that internal shield, or, perhaps with adding a few more ground points (midway and/or around the board edges) to the enclosure to reduce the Vcm and ground-return impedance for the cables. Maybe some ferrite beads on the cables to help that out a bit, and likewise better grounding (to enclosure) for the various peripherals (drives, CRT board, PSU).Or maybe not; I saw apparently there's an upgrade module for as much as 50MHz? That must be within the card though... little on the main board can run that fast, surely? (Not much logic on there; I see a couple 74LS, and a couple 74F. Hard to say.) That'd all be pretty rough going, at 5V supply, 3.3V at best -- it's a lot of voltage to launch down a ~100 ohm trace, tens of millions of times per second, and indeed a lot of power as well, which is why as clock rates have gone up, signal levels have gone down.I'm not sure exactly how sharp the MC68k, or other hardware, drove their pins; not sure offhand if that's full CMOS (say 74HC ish) or TTL-ish (NMOS, HMOS) levels, in terms of both speed and voltage range. [HMOS, was an Intel thing; I don't know what Motorola did at the time.]Anyway, I'm sure it's enough trace length that -- not so much signals coming from the CPU end towards the SCSI chip, because that's just a small capacitive load -- but the return path, SCSI driving the bus, is definitely a concern. I guess I should've drawn the voltage source on the other side, but whatever it is, that's the loop that's the problem.QuoteLike HTML: put a [img width=foo] in there.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2022, 10:10:16 am
content: Quote from: David Hess on July 05, 2022, 09:10:15 amThey definitely did, like, there's a whack of AM26LS30/32 for RS-422 (that'd be AppleTalk I think?), plus SCSI is... well usually differential, I guess I don't know if it was here, I could look at a few more datasheets to tell...And they knew there would be problems. The narrow DIPs there, resistor packs. With "FILTER" written by them... though plain resistors aren't much for filtering. (If they were ferrite arrays, that might be something, but I'm pretty sure Bourns never made one of those, at least not in the common yellow or blue plastic like that? 115-0002 is just resistors, in any case.) There's a handful of caps and such, but not nearly enough for all the signals going around, I think; the shielded D-subs (and, the others are DIN connectors, right?) are definitely mandatory for meeting EMC with devices attached.Granted, that's maybe not such a smoking gun, as D-subs and DINs have always been very popular.A cost-reduction argument for unshielded differential cables might be made; AppleTalk could be similar to Ethernet in that regard, but with DC coupling and ordinary UART signaling (or, whatever else they wanted to use!). And it could still stomp all over the PC's pitiful RS-232 serial; assuming they can get data to it fast enough, of course (buffering and/or DMA would probably be mandatory)....Which, oh neat, the SCC does a bit of link layer protocol actually, but apparently doesn't do almost any buffering, so that kinda sucks for sustained data rates.Not sure what's the display on this thing, I mean it's internal obviously, but does that just go up the main power connector and harnessing links that to the CRT board? I suppose it's just a bitstream anyway, that thing was monochrome. A schmitt receiver on the far end will clean up whatever ringing/droop the harness incurs. Only a... what, 10ish MHz pixel clock? Oh, or maybe that 15.6672MHz can.Tim

author: JDW
author_rank: Frequent Contributor
timestamp: July 06, 2022, 02:19:05 am
content: Quote from: David Hess on July 05, 2022, 09:10:15 amI am aware about loop area and emissions (EMI considerations), but it seems you are saying that the deliberate design decision by Apple engineers to break the motherboard ground plane into 2 pieces -- 2 pieces connected to each other only by that single trace -- "lowers emissions from the chassis" by way of the very slight resistance and parasitic inductance inherent to that single trace, allowing that single trace to somewhat act like a very low end ferrite bead, filtering HF noise coming into the motherboard from the chassis (or connectors).Quote from: T3sl4co1l on July 05, 2022, 09:43:18 amRight, which is why I don't really understand why the broken up ground plane is needed at all. Just keep the entire ground plane ground, with no breaks, and add ferrite beads, etc. at the noise points, which would be where the metal bracket solders to the motherboard, and of course the connectors, which are all on the same edge of that motherboard.Quote from: T3sl4co1l on July 05, 2022, 09:43:18 amNot 50MHz for the Macintosh SE. The SE (8MHz 68000 CPU), like the superior SE/30 (16MHz 68030 CPU), has a PDS slot for accelerators, but the accelerators for the SE are a bit more low end than the SE/30. There are 25MHz 68030 PDS accelerators out there, but the 50MHz 030 or 40MHz 040 accelerators are reserved for the SE/30. The design of the SE/30 motherboard is similar insofar as there is a break as shown below, but the broken parts are connected across different planes via a few thru-hole IC pins from what I can see inthe Gerbers.Quote from: T3sl4co1l on July 05, 2022, 09:43:18 amTo reduce inline image width, per your suggestion, I tried adding "[img width="500px";]" & [img width=500px]" to no avail. Any further specifics would be greatly appreciated.Quote from: T3sl4co1l on July 05, 2022, 10:10:16 amIt is plain old SCSI-2 on the SE and SE/30 computers.Quote from: T3sl4co1l on July 05, 2022, 10:10:16 amThe 4 "FILTER" chips are BOURNS RC networks, part number4120R-601-250/201. They are becoming harder to find, so someone has made arecreation here.Quote from: T3sl4co1l on July 05, 2022, 10:10:16 am9" B&W CRT displaying 512x342 pixels.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2022, 05:43:06 am
content: Quote from: JDW on July 06, 2022, 02:19:05 amThe one they they probably do get, is less current across (laterally) the connector area -- though I'm still not very confident in this because of the sheer number of traces crossing the gap. To the extent that the outer metalwork and metallization are imperfect, this could help to reduce the voltage drop between them, as seen from the outside (where it matters, to EMC) -- it just seems a very cut-off-your-nose-to-spite-your-face kind of move.QuoteMaybe they read one of those trade mags back in the day, discussing slotted grounds, and got way too excited about the trick.I see it all the time; far more often than is deserved. 99.5% of the time you think about slotting a plane -- no, it's actually worse that way, don't do it.It's those tiny few cases remaining, where it does help out. But knowing which ones, requires a lot of experience.No idea who built this board, how much experience they had -- maybe it was intentional and expertly done, maybe it was ignorant, we can't tell at this point. (Unless you get lucky and they're on Twitter and watching, or something.)Quote...Where at? I don't see an edit...Like this (quote my post to see the code):Also, fortunately the forum shrinks images to fit width, so those of us using modestly sized browser windows don't have to strain our eyes to look at things. Also, you could consider sparing a few megabits on your viewers by reducing the images in the first place.QuoteAh, so parallel it is (I guess? Wikipedia still isn't terribly specific, at a glance..), and that explains the lack of terminators.QuoteAh, cool! So they do get some attenuation there, that helps a lot.QuoteSo without porches, and at 60Hz, 10.5MHz pixel clock; shouldn't be much higher than that, actually, I would be shocked if they have much overscan... maybe they put a 2/3 PLL in that system chip or something? Which I guess is handling graphics too, and, hmm would it have onboard VRAM for that then? At least being monochrome it doesn't need a whole lot I guess.PCs were much simpler in that respect; graphics was either a pile of chips around a CRTC (6845 usually), or some ASIC doing the same things (probably most EGA+?). The attached VRAM of which is very obvious.(My XT clone indeed has 128k Paradise EGA; what luxury! Too bad the ISA bus is so dreadfully slow, along with the 8086, to do very much with it...)Tim

author: mansaxel
author_rank: Super Contributor
timestamp: July 06, 2022, 05:44:55 am
content: Quote from: JDW on July 06, 2022, 02:19:05 amThe proper syntax is:Code:[img width=300 height=300]picture.jpegSizing arguments are without units (pixels are assumed), and are optional.

author: David Hess
author_rank: Super Contributor
timestamp: July 06, 2022, 01:06:45 pm
content: Quote from: JDW on July 06, 2022, 02:19:05 amIt is not the motherboard ground plane necessarily, but the multiple connections to the chassis from the motherboard ground plane. Without a single point ground, return currents through the motherboard ground plane would cause return currents through the external chassis between the shielded connectors, which would then cause the chassis to radiate EMI. When cables are connected, the return currents would travel through their shields as well.Alternatives include using only one external shielded connection so there is only one point connected to the chassis, or isolating the external shields from the chassis which has its own problems. A secure seamless connection between the connectors and chassis to keep the currents on the inside of the chassis would be ideal but is difficult to achieve; this usually means finger stock and this solution can be found on many PCs to varying levels of thoroughness.

author: wizard69
author_rank: Super Contributor
timestamp: July 06, 2022, 02:31:28 pm
content: Ground loops suck and yes there is a possibility here of a problem. Beyond ground loops if the "grounds" are not at the same potential you have real posibility of frying cabling and possibly other hardware.As for your post you do not mention what sort of transducers you are using. It makes a difference. For example thermoucouples can come in variants that have the junction grounded and those that are ungrounded. As somebody has already mentioned there are so many factors here that no one can provide an absolutely correct answer. Personally I'd ground the supply at one point on the chassis and work from there.Quote from: innkeeper on January 25, 2020, 12:16:37 amYou are not wrong but if you are talking a large installation where ground potential can vary you might end up with excessive current on your ground. In extreme cases you could burn up your cables. Supposedly these sorts of industrial systems have equal potential grounding installations but that never seems to be perfect in practice. So in a modern facility you would have less of a chance of excessive ground currents. If you are limited to a lab or small installation, ground currents should be less of a problem.Noise on the other hand is funny. You can follow recommendations precisely and have noise problems. Ideally you would have the ability to lift the ground at one end of the sensor cable if needed.Quote