[miljnor]

I just purchased (just to check it out) a BOB that is buffered by chip sn74hc245N and sn74ahct245n.

basical using an Ic to produce a signal identical to the input signal but driven by the chips power supply rather than the Computers Parallel port.

what effect (if any) does this have on isolation. Or in another way does this provide any kind of isolation?

[Al_The_Man]

This type of buffer does not provide electrical isolation, IOW the output has the same electrical reference as the input.
Another usefull one is the DS2003, TTL to Darlington driver. Unlike the 74*** series which is TTL (5v) input/output, TTL loads only.
The 2003 has high voltage/current capability on the outputs with TTL inputs.
Al.

[miljnor]

Well the board doesn't state that it isolates, I was just wondering. This board was designed so that you could use Pins 2-9 on the parallel as Input or output.

which could be handy.


Thanks Al

[NC Cams]

Isolated vs non-isolated analogy;

A signal is transferred between two ships.

A non isolated signal is carried t'ween the two with a wire and they share a common ground as a result.

If the signal is transferred via non-wire, non common grounded radio communication, satelite uplink or signal flags, you have isolation .

Buffering takes a low power signal and puts "OOMPH" behind it - sort of like an amplifier does a sound wave.

An optoisolator (IE: 4N25, 4N37) transfers the signal optically while concurrently NOT using a common ground between the sender and receiver.

For more info, look up "parallel port interfacing" on the internet and/or this website. The subject of optoisolating the parallel port as a driver is pretty easy to find.

Hope this helps...

[pminmo]

My 2 cents,
You can have isolation without an opto coupler. Even an optocoupler has a breakdown voltage. A buffered interface provides a level of isolation on the active signals, but usually not ground which typically is the root of the problem. You can have failure modes that optical isolation doesn't protect against. In a correctly wired system optical isolation isn't necessary. But it does give you room for error that a nonisolated system doesn't. Understand that system is all encompasing including how your PC is plugged in and the AC for the PC vs. the AC for the machine.

A buffered board may fault if a motor driver fails and protect the PC, but it may not also.

The one advantage your HC buffered board will have, is an increased noise immunity, and an output level that will drive opto isolated drivers at a full 5V swing.

[miljnor]

So for the ultimate in protection, I can make an opto Isolation board to recieve all the Inputs/outputs and plug this buffered board into them.

is there fault is this logic (other than time and money to build)?

[ViperTX]

I guess the only good isolation is sign language, smoke signals....maybe even wireless....most opto-isolators have a breakdown voltage of at least 1500 volts...so opto-isolators are the way to go...

[NC Cams]

When you get to the level of the breakdown voltage (IE: 1500 ), the TTL or HSTTL or or HS-CMOS, CMOS or just about any logic series chips are toast anyway if they see that much voltage.

Regarding the plug in of buffer to opto isolator, the answer is pretty much YES.

However, depending on the pull-p resistor value used on the open collector side of the opto-isolator, you may not even have to use a buffer IC. If you're worried about noise, however, use a gate that is of a Schmidt trigger config. It tends to square up lazy or "dithering" signals.

[ViperTX]

NC Cams....sounds like we are saying the same thing....

[gar]

060213-2233 EST USA

Consider a source, possibly a computer, battery powered in a totally enclosed conductive box. The common of the internal circuitry is connected to the enclosure. The destination also has its electronic common connected to its chassis.

(edit 060215-2047 for the purpose of this experiment this paragraph was correct, but for other purposes wrong and could lead someone to incorrectly wire their nuetral)

(Badly written paragraph) Connect the chassis of the destination to the 120 V AC neutral and its electrical safety ground wire. This would be standard procedure at a CNC machine, but more likely the source is 3 phase 240. The 3 phase 240 is of no importance for this experiment, just the neutral and ground are important.(end of incorrect paragraph --- pminmo in a later post indicated my error)

(correction for this paragraph)
Connect the chassis of the destination to the 120 V electrical safety ground wire, the safety ground wire is connected to the neutral at the main circuit breaker box. But neutral must not be connected to the safety ground at the destination. This would be standard procedure at a CNC machine, but more likely the source is 3 phase 240. The 3 phase 240 is of no importance for this experiment, just the neutral and ground are important.
(end edit)

Next take a hot 120 wire and connect it to the chassis of the source. The source is on an insulator. There is no or negligible current flow from the hot wire and nothing is damaged.

Next from the source make a direct connection, meaning no electrical isolation, from a serial or parallel port to the destination. Then run the same experiment as before. Interconnect wires burn out and also all sorts of other devices are destroyed. IC buffers would offer no protection and thus be of no value.

Third introduce optical or other non-conductive isolation on all wires between the source and destination. Run the experiment and nothing is damaged and data is successfully transferred while the fault voltage is present. Virtually no fault current flows.

See my discussion on my web page on NOISE and GROUNDING at www.beta-a2.com . In particular the section on "Why is a DIRECT RS232 to RS232 connection risky?"

.

[ViperTX]

gar you are quite correct if you don't have a lightning strike....and then alot of other factors come into play...which I'm sure you are cognizant of.

[gar]

060214-1040 EST USA

ViperTX and others:

On lightning --- a reference that I listed on my NOISE and GROUNDING page --- www.harvardrepeater.org/news/lightning.html --- has some very interesting information.

If we can keep lightning current outside our home or building, then we do not have much to worry about, even though the building might rise 100,000 volts above ground potential from a lightning strike. A steel structure building with a "good" grounding point outside the building, and effective filtering on conductive circuits coming into to the building can be extremely effective as discussed in the above web site.

Years ago we had a Calcomp plotter connected to a computer with about 50 ft of cable, no isolation. Everything was on the same circuit breaker box. A lightning strike close by destroyed all the RS232 chips at both ends of the link. We were lucky and the damage did not propogate further than the interface chips. A shorter cable does not necessarily protect you.

A different type of situation. At a Chrysler axle plant on a pinion shim selection machine we had an LVDT on a verifier in a convenient location for the operator. The physical mounting was not directly attached via a very low resistance joint to the machine. In other words the column holding the verifier was bolted to the main machine and not welded to it.

A welder put his ground wire on the main machine instead of on the part he was welding and did some welding on the post that held the verifier. ZAP. The LVDT was destroyed, the LVDT cable was burned up because it carried a substantial portion of the welding current, and LVDT signal conditioning circuitry was destroyed. There was no way we could isolate the LVDT except at greater expense than the damage done, nor in this case should we have thought it necessary. You can not economically protect for every possible fault, but it is wise to do it for probable ones.

.

[Al_The_Man]

In a correctly wired system optical isolation isn't necessary. But it does give you room for error that a nonisolated system doesn't.

I will second that. :wave:
Over several years I have designed and installed several Servo based systems which included PC based that work in harshest environment imaginable.
Although these are not parallel port based systems, they are nevertheless PC based using a motion card and TTL interface involved.
On PC based systems, I have never used isolation except for opto isolators to translate TTL to 120~240 AC but this power is not truly isolated due to all the common supplies in the system, common-ed up to a ground bus, together with all shields etc, including the PC supply common.
I have found it extremely rare in the components that I have used, that it is only possible to use them with some form of isolation.
I have found that it pays to design the complete system with every component linked together this way, instead of treating everything as a separate entity trying to isolate everything into separate parts.
As Phil said, .' In a correctly wired system optical isolation isn't necessary'
I know there are others that have a different philosophy, that the best way is to keep everything separated.
All I can say is the method I have adopted has worked for me, so far without problem headaches.
In any machine, alternate ground paths can always exist inadvertently, but you design your system so that your planned ground paths offer the least resistance possible.
Al.

[pminmo]

060213-2233 EST USA

Connect the chassis of the destination to the 120 V AC neutral and its electrical safety ground wire. This would be standard procedure at a CNC machine,
.

I'd like to politely disagree. :nono: The only place AC neutral is connected to ground is at the power distribution box. The only exception is if you isolate the power source via a subsystem such as an isolation transformer and you have to rereference the output to 0V.

This is from the EC&M website:

"The NEC requires you to ground the neutral at only one location in a system:usually at the utility service entrance. Two grounds on a grounded system (e.g., 1-phase, 3-wire; 3-phase, 4-wire; or grounded delta) would result in parallel neutral and ground paths. Current will divide between the two paths and be a safety/coordination problem. By definition, system grounds are not part of ungrounded systems."

Phil

[gar]

060215-2038 EST USA

pminmo:

You are absolutely right. It is good that you caught my mis-statement.

I did not say what I should have said:

Connect the chassis of the destination to the 120 V electrical safety ground wire, the safety ground wire is connected to the neutral at the main circuit breaker box. This would be standard procedure at a CNC machine.

Thanks.

[HillBilly]

buffered vs Opto Isolated

Buffered is usually used when the drive (or other device.) allready incorporates opto isolation. It would be redundant to use the output of a opto coupler to feed the input of another opto coupler.

Darek

[gar]

060215-2157 EST USA

All sorts of things could be defined as a buffer. But most of you are using buffer to mean a power amplifier. This might be simply greater current output, and/or it could include greater voltage. An optical coupler is generically a buffer also.

If there is a long wire run and there is the possibility of a high power source shorting to the interconnect cable, then you very well might want an optical coupler at both ends of the cable. Probably on most machines you do not need both ends isolated. So you would isolate at the most expensive component.

A TTL device has a moderately low impedance in both the high and low state. Different TTL devices will have different output impedances. Compare and LS gate with standard gate, etc. An open collector output will have a lower on state impedance than its off state. An open collector output is a very good driver for an optical coupler because the optical coupler is a rather low impedance to turn on, 1.5 V at 10 MA or 1.5/0.01 = 150 Ohms. Whereas an open collector with pull-up resistor, really meaning it is an RTL circuit, driving a TTL input will probably not provide the same noise immunity as the open collector driving an optical coupler. There are several assumptions back of this statement.

.

[miljnor]

I am a little rusty on my A/C wiring terminology.

Is the Ground the "current carrier" or is the Neutral the "current carrier"?

I believe it is the neutral that is current carrier and thus the "saftey" part of "saftey ground" is for acidents to clear a fault?

[gar]

060216-0950 EST USA

miljnor:

Hot, neutral, and ground --- These describe wires to your machine or wall outlet.

Unfortunately "ground" is used to mean too many different functions. I like to use it for a physical ground. Meaning electrodes or conductors directly in contact with a substantial amount of the earth. The NEC has specifications relative to resistance to ground.

At your service entrance to a building there must be an earth ground connection that has a resistance to earth of less than 25 ohms. If one ground rod will not meet this requirement, then there are additional criteria to be met. Note 10,000 Amps thru 25 Ohms is 250,000 Volts.

Generally your main circuit breaker box is very close to the service entrance. Within this main breaker the the earth ground is connected to one wire from the pole transformer. This power wire that is tied to earth ground at the breaker box is called the nuetral wire. Any other wires from the pole transformer are hot wires.

Because the nuetral is connected to the earth ground its electrical potential will be near ground potential under normal conditions. Meaning within a few volts of ground, and therefore also relative to things like water pipes, and steel building structure, or the cement floor.

In recent years, the last 50, there has been the requirement that an explicit safety ground wire be run from the breaker box to each destination. The intent is to provide a normally non-current carrying wire to provide a means to connect conductive parts of the load (destination) to earth ground thru a relatively low resistance path. This is to provide some degree of protection for users of the load. This safety ground wire can not be connected to the nuetral at the load because this would defeat its intended purpose.

A hot wire is any power wire that is not a neutral wire. Load power current flows thru the hot wire or wires, and maybe thru the nuetral, but only under fault conditions thru the safety ground wire.

Consider my DeWalt radial arm saw at the end of about 100 feet of #12 wire. Our line voltage is about 120 V, and very solid at the breaker box. When the saw starts the voltage at the saw drops to about 70 V. Thus, there is a 25 V drop in the neutral wire and 25 V drop in the hot wire. There is no current flow in the safety ground wire.

The frame of the saw is connected to the safety ground. If I am touching the saw and a water pipe and the saw is started I will not get a shock because the saw frame is at 0 potential relative to ground.

Another consideration. If there is an internal leakage from the hot wire to the motor to the machine frame, then this leakage current flows thru the safety ground wire. Now if that leakage is just under the trip point of the circuit breaker and the motor has no mechanical load, then the frame of the saw can approach 1/2 of the source voltage at the breaker box, or in this case about 60 V RMS or 84 V peak. This would be a sustained voltage because the breaker will not trip, and is dangerous. This is why ground fault breakers were developed and required in certain locations. These detect very small ground fault currents and trip out the hot wire.

None of these breakers will protect electronic circuits such as RS232 or TTL gates.

.

[miljnor]

A hot wire is any power wire that is not a neutral wire. Load power current flows thru the hot wire or wires, and maybe thru the nuetral, but only under fault conditions thru the safety ground wire.

This is the answer I wanted! I had to ask a freind (an electrician/sparky) After reading your post, I was so overwhelmed by excess data I missed it the first time.

most of these guys (read "ME myself and I") only want a common sense answer. But I understand you got to cover you but for the Lawyers in the house!

Thanks as always for the info!