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Are 5v-tolerant inputs clamped?

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I can't put '244s in series because I don't have any CPU port pins to read the parallel outputs of the '244s with. If I use one as a clamp, that will be clearly explained _on_ the schematic.

I have 16 outputs, and one spare input. Each output goes to a FET, then a load. There have been field problems with the load devices, so I want to detect that the loads are connected, not shorted, and correctly operated by the FETs. I can do that if I can monitor the output of the FETs to make sure they're pulling low when commanded, and that a node is +24v when its FET is off.

I think we need ASCIIman--ASCIIman to the rescue!

This is the circuit I want to improve:

+24v -+- | buffer .----load_1--+ .------. | | | | ||---' | | |->--|| | | | ||->-. | | | | | | | =3D=3D=3D | | | | | | .-----load_2--+ | | | | | | ||---' | | |->--|| | | | ||->-. | | | | | | | =3D=3D=3D | ~ ~ | ~ ~ .----load_16--' | | | | | ||---' | |->--|| | | ||->-. | | | '------' =3D=3D=3D

I want to detect faults in the drivers and the loads. This is the proposal I'm considering:

+24v -+- | .--o | |_ +3.3v --- _)|| -+- ^ _)|| load_1 | 'hc244 - ._)|| .----. ('Jed') | | | | +3.3v latch '--o------47k---o-----o--| |> |-- x -+- .------. |'A' | | | | | | | ||---' 10k | '----' .--------------. | |----|| | | | | | | ||->-. Q1 =3D=3D=3D '-------->----| 5v-tolerant | ~ ~ | | i/o expander | ~ ~ =3D=3D=3D ~ ~ ~ ~ | | '--------------' | =3D=3D=3D

Adding the port expander on the i2c bus gives me 16 extra port pins, which I can read through the i2c bus. No i/o pins needed. The port expander runs on +3.3v but has inputs that tolerate +5v.

Does that make more sense?

ets/ds31437.pdf

p://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3DSDM6...

That's a good point--I only need one diode (to +3.3v), not two. I hadn't thought of that.

Thanks.

-- Cheers, James Arthur

Reply to
dagmargoodboat
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use the

You don't need the 10K resistors.

think).

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more:

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...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

use the

Not is series with CPU ports, rather in series with the port expander.

This is what I'm suggesting... +24v -+- | .--o | |_ +3.3v --- _)|| -+- ^ _)|| load_1 | 'hc244 - ._)|| .----. ('Jed') | | | | +3.3v latch '--o------47k---o-----/--| |> |--+ -+- .------. |'A' | / | | | | | | ||---' 10k / '----' | .--------------. | |----|| | / | | | | | ||->-. Q1 === ///////////+--| 5v-tolerant | ~ ~ | | i/o expander | ~ ~ === ~ ~ ~ ~ | | '--------------' | ===

They don't need to.

Yes, I understood before, but as long as you're using a '244, *use* it.

think).

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more:

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Reply to
krw

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=A0 =A0 +3.3v

=A0 -+-

=A0 | =A0 'hc244

=A0 .----. ('Jed')

=A0 =A0 | =A0 =A0| =A0 =A0 =A0 =A0 =A0 +3.3v

=A0 =A0-+-

=A0| =A0 =A0 =A0 =A0 =A0 =A0 |

--' =A0 =A0 .--------------.

=A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0|

----->----| 5v-tolerant =A0|

=A0 =A0 =A0 =A0 =A0 =A0 =A0| i/o expander |

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ~ =A0 =A0 =A0 =A0 =A0 =A0 =A0~

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0~ =A0 =A0 =A0 =A0 =A0 =A0 =A0~

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0'--------------'

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=3D=3D=3D

The 10k is so an 'open load' condition reads as 0v.

Isn't there a Thompson's Commandment about not using protection diodes as circuit elements _and_ expecting the device to work, or does that not apply for 'HC gates?

-- Cheers, James Arthur

Reply to
dagmargoodboat

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Reply to
dagmargoodboat

you'll

use the

higher

have

clamps.

implying

OK

You're not using the gate function. Though I like the 13-input NAND better... you'd be using input diodes with no big output devices nearby to SCR. (Though your currents are way below maximum allowable.) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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Reply to
dagmargoodboat

BTW, I never (intentionally) use the ESD devices, particularly as free-wheeling diodes. I'd go with the discrete arrays unless it got embarrassingly complicated. I *know* what they're doing and I can count on no changes in the foreseeable future.

Note that I was suggesting using the inputs (of the '244), also.

Reply to
krw

Extra?

Reply to
krw

U,

That's a decent idea--it'd take two parts, and eliminates the i/o expander. It's clean except for using the ESD diodes of the active device, which annoys our IC designer gurus (Jim Thompson and miso).

It also takes one output pin to clock it, and one input port pin to read the '165. Alas, I don't have them. Getting them would be inconvenient. Possible, but inconvenient.

I'm still thinking about the whole thing. My current scheme can't distinguish between a shorted driver and an open load. A shorted FET is one of the most likely failures[*], so it'd be nice to detect it.

[*] or it was, anyhow. I've added short-circuit protection, so those failures should be greatly reduced.

-- Cheers, James Arthur

Reply to
dagmargoodboat

te:

.

That takes two '244s. With a dedicated dummy, I can tri-state the drivers and use the outputs as clamps too--that's 16 clamps, only one part needed.

-- Cheers, James Arthur

Reply to
dagmargoodboat

s

t on no

Truthfully that's my preference (discrete diodes) by far. I'd need 26 diodes, though, so 6 pieces of even that nifty Diodes 5-diode array you linked.

As I mentioned in another post my current scheme can't distinguish between a shorted FET and an open load. I'm thinking of changing course altogether--I can sense current easily, and digitize it. Doing that I can cover more failures, diagnose with better specificity, and with less hardware. Fewer changes to a crowded board is a bonus.

Ironically, it was beating things out in this thread that got me thinking about how I could do that other thing. Thanks guys.

The IC-as-clamp idea, though, that's gotta come in useful sometime, somewhere.

-- Cheers, James Arthur

Reply to
dagmargoodboat

Oh, *NOW* I see. You're using both the inputs and outputs. I thought you were just using the outputs.

Reply to
krw

The IC-as-clamp idea, though, that's gotta come in useful sometime, somewhere.

-- Cheers, James Arthur

I agree, that is a nice creative one. However, even if you document the hell out of it, sometime in the future someone will delete that IC from the BOM as a cost cutting measure because it obviously doesn't do anything! Art

Reply to
Artemus

e

he

LOL! Yes, anything's possible.

-- Cheers, James Arthur

Reply to
dagmargoodboat

There is the SP720 that John L. and I discussed a week or so ago--14 bullet-resistant protection diodes per package.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net
Reply to
Phil Hobbs

6
g

Hey, those are neat--SCRs, not diodes, and sold by Littelfuse of all people:

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That's a keeper. Roughly $1.60 in qty. Thanks.

James Arthur

Reply to
dagmargoodboat

no

Yes, that is slick (though pricey). I'll keep it in mind for keypad ESD protection.

Reply to
krw

snipped-for-privacy@yahoo.com:

Well, actually I meant to use an I/O expander, but an 8-bit or less one, from which you can get the 3 I/Os needed to control the HC165, (that I thought had to be 4 for 32 inputs).

The input diodes of the HC165 are, in my experience, very robust, also the datasheet says they can swallow up to 25 mA. A trivial resistor allows you to protect the inputs very effectively.

This arrangement does not contradict the gurus, since those diodes are meant, in my opinion, for exactly the purpose you're looking for, since 25 mA are way too much for an ESD buildup.

Yes, the SH/LD pin can be tied low.

That's why I suggested to have the I/O expander anyway. But an 8 bit one would be cheaper and smaller (and, at least from Farnnel, much more readily available) that a 16 bit one, and, with just three 16 pin chips you would be all set.

In this case my solution is even more convenient, because you just need two more HC165 to read that information.

Reply to
F. Bertolazzi

F. Bertolazzi:

Well, let's say 20 mA, but exactly as much as the 244 suggested by krw, solution that Jim Thompson, if I understood well, did not dismiss.

By the way, I've seen an 8086 motherboard that used the 244 in exactly that way, just for protecting the inputs of a big MOS chip.

With the HC165 I made some 300 boards (all of the firedoors of italian ferry-boats and many of french, dutch and greek ones are monitored by that board) and never had a single failure, despite the fact that ship's service engineers that installed them often are retarded monkeys.

Reply to
F. Bertolazzi

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