clipka wrote:
> Am 23.05.2010 12:41, schrieb Orchid XP v8:
> 
>> I took a trip to Maplin and somehow spent £25 - which is impressi
ve,
>> given that a 74HC00 is a mere £0.79 (for example).
> 
> No, actually it's not - once you get used to this effect :-P

Indeed, one of my own big insights was "Wow, I can use the computer for 
another project, if I write *software*!"  ;-)

> Pre-cut wires? 

And I would think if you're getting pre-cut breadboard wires, you *want* 

them to be stiff.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

>> Pre-cut wires? 
> 
> And I would think if you're getting pre-cut breadboard wires, you *want* 
> them to be stiff.

I have now cut some flexible wire, and it works quite well. (In 
particular, it can go around corners...)

Post a reply to this message

Hmm, this isn't going to work at all, is it?

Post a reply to this message

Attachments:
Download 'logic1.png' (7 KB)

Preview of image 'logic1.png'

PS. MS Word is utterly *horrible* at drawing circuit diagrams... (But I 
guess you probably knew that already.)

Post a reply to this message

On 5/23/2010 9:24 AM, Orchid XP v8 wrote:

>> Why not?
>
> Because when it doesn't work, I will probably end up killing myself, for
> starters... A power supply is far too complicated for me to get right.
>

A power supply is a rather simple device: A transformer that steps down 
the 240VAC to (for example) 12VAC, a bridge rectifier to convert the AC 
source into DC then a couple regulators to get (perhaps) 5 and 12 volts.

Personally, though I'm a bit skittish about using wall power, I use an 
AC adapter w/12 volts (though the one powering my headphone amp is 24v 
regulated, so the op-amp gets +/- 12v much more than is ever needed for 
boosting heaphones, but gives the op amp plenty of headroom to source 
current for the low impedance phones I have)

... One of these days I'll upgrade it to an op-amp w/ a mosfet output 
stage so I can get as much current as I can, even though this is plenty 
loud ;)

-- 
~Mike

Post a reply to this message

On 5/23/2010 9:50 AM, Stephen wrote:

> LOL, They get them from work ;-)

For a minute there I thought you were going to say you picked them off 
of discarded electronics.

-- 
~Mike

Post a reply to this message

>>> Why not?
>>
>> Because when it doesn't work, I will probably end up killing myself, for
>> starters... A power supply is far too complicated for me to get right.
>>
> 
> A power supply is a rather simple device: A transformer that steps down 
> the 240VAC to (for example) 12VAC, a bridge rectifier to convert the AC 
> source into DC then a couple regulators to get (perhaps) 5 and 12 volts.

...or I could use one assembled by professionals who actually know what 
they're doing. ;-)

I don't mind destroying a few 29p LEDs by accident, but I'd really 
rather not die a virgin...

Post a reply to this message

On 5/23/2010 6:10 AM, Orchid XP v8 wrote:
>>> I took a trip to Maplin and somehow spent £25 - which is impressive,
>>> given that a 74HC00 is a mere £0.79 (for example).
>>
>> No, actually it's not - once you get used to this effect :-P
>
> Yeah, it's like that time I was on Amazon, and I saw a textbook for,
> like, £4, so I bought it. And then I bought another one. And in short
> order, I somehow spent £80. o_O Which seems silly, given that £4 is a
> small amount, while £80 is a frickin' HUGE amount. Then again, it's only
> actually 20 books...
>
> Or, more recently, when I was buying CDs for a mere £2 each, and still
> somehow spent £100. The point being, even a small price, when multiplied
> by a suitably big number, becomes a *big* total. ;-)
>
>> Pre-cut wires? Gee, you must have too much money to spend ;-) (or
>> suffering from an utter lack of time... duh, thinking about it, it
>> somehow makes sense after all... >_<)
>
> Well, I don't know where my soldering iron, solder or wire cutters are.
> And I know from experience that trying to strip wires is approximately
> impossible, and trying to tin the ends is very tricky. And I thought £10
> for a nice set of wires with a box to keep them in wasn't bad.

This is why you buy solid wire and not stranded! Though I have learned 
that stranded is the best choice for hooking up external peripherals to 
your circuit, when used in a project.

For extra fun, do something like this:

http://www.flickr.com/photos/87959461@N00/4469974973/
http://www.flickr.com/photos/87959461@N00/4470754768/

I learned a pretty important lesson with this project:

Put the jumpers on the component side, and use stranded wire for the 
external parts (jacks, pot, power and LED) the solid wire made install 
into the enclosure a royal pain.

A breakout board for a 14-segment display. (The title is incorrect):

http://www.flickr.com/photos/87959461@N00/4487754588/

> I'm actually surprised that 10 m of wire is £2 and not £0.2, to be
> honest. (Or I could get 100 m for £6. But I actually wanted several
> colours...)
>

Copper isn't cheap....

-- 
~Mike

Post a reply to this message

>>> Pre-cut wires? Gee, you must have too much money to spend ;-) (or
>>> suffering from an utter lack of time... duh, thinking about it, it
>>> somehow makes sense after all... >_<)
>>
>> Well, I don't know where my soldering iron, solder or wire cutters are.
>> And I know from experience that trying to strip wires is approximately
>> impossible, and trying to tin the ends is very tricky. And I thought £10
>> for a nice set of wires with a box to keep them in wasn't bad.
> 
> This is why you buy solid wire and not stranded!

Trouble is, you just can't *bend* the stuff! It's like trying so connect 
components together with a coat hanger...

(Although, I bought 4 chunks of wire, and one of them actually *is* 
single-core wire. While it's rather rigid, it's not quite as bad as the 
other stuff.)

>> I'm actually surprised that 10 m of wire is £2 and not £0.2, to be
>> honest. (Or I could get 100 m for £6. But I actually wanted several
>> colours...)
>>
> 
> Copper isn't cheap....

And 10 m of wire weighs, what, about 10 grams?

Oddly, both the single-strand and multi-strand cables appear to contain 
some kind of white metal, not copper.

Post a reply to this message

On 5/24/2010 8:30 AM, Invisible wrote:

>
> ...or I could use one assembled by professionals who actually know what
> they're doing. ;-)
>

pff.. I did that, was highly dissatisfied with the result, and the thing 
ran on batteries, which was inconvenient. So, I built one to my own 
requirements. :D

-- 
~Mike

Post a reply to this message

On 5/24/2010 8:38 AM, Invisible wrote:

> And 10 m of wire weighs, what, about 10 grams?
>
> Oddly, both the single-strand and multi-strand cables appear to contain
> some kind of white metal, not copper.

tin-plated copper.

-- 
~Mike

Post a reply to this message

On 24/05/2010 2:23 PM, Mike Raiford wrote:
> On 5/23/2010 9:50 AM, Stephen wrote:
>
>> LOL, They get them from work ;-)
>
> For a minute there I thought you were going to say you picked them off
> of discarded electronics.
>

Lordy, no. Nothing but the best for the maintenance department. :-)


-- 

Best Regards,
	Stephen

Post a reply to this message

Invisible wrote:
> I have now cut some flexible wire, and it works quite well. (In 
> particular, it can go around corners...)

You're not supposed to run breadboard wires around corners. You're supposed 
to plug it into a hole and run a connected wire at an angle if you need angles.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

>> I have now cut some flexible wire, and it works quite well. (In 
>> particular, it can go around corners...)
> 
> You're not supposed to run breadboard wires around corners. You're 
> supposed to plug it into a hole and run a connected wire at an angle if 
> you need angles.

I don't follow.

Post a reply to this message

Invisible wrote:
> I don't follow.

http://www.robotroom.com/Pumpkin/LEDPulsingBreadboard.jpg

See how the black cable goes from C1 to C2?

Or how the purple cable goes "around the corner"?

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

Darren New wrote:

> http://www.robotroom.com/Pumpkin/LEDPulsingBreadboard.jpg
> 
> See how the black cable goes from C1 to C2?
> 
> Or how the purple cable goes "around the corner"?

I see...

Makes it kinda hard to rest the ends of the wires on both battery 
terminals simultaneously when they're utterly rigid though. Similarly, 
if you've got one wire that's 4 holes wide and another that's 6 holes 
wide, you have to systematically design your circuits to not need any 
connections 5 holes wide. Routing becomes formally equivilent to solving 
the Zero Sum problem. (!) Not to mention the fact that those tiny 1-hole 
wide wire loops are excruciatingly hard to manipulate.

By contrast, if you have some flexible wire, you can make a loop of any 
size you desire, and it's very easy to work with.

Post a reply to this message

Invisible wrote:
> Hmm, this isn't going to work at all, is it?

Would it work if each switch had a seperate resistor? Or does that not 
fix it?

Post a reply to this message

>> Hmm, this isn't going to work at all, is it?

Depends what you mean by "work".  Seems like the two input pins on the IC 
are actually tied together (to the top side of the resistor), so it's just 
going to work like a NOT gate.

Did you also do the calculations to check that it's ok to use a single 360 
Ohm resistor with those two LEDs for every combination of output states?

> Would it work if each switch had a seperate resistor? Or does that not fix 
> it?

Dude, really, you are going to fall over yourself over and over again if you 
refuse to learn at least some basic analogue circuit electronics.  You must 
have realised by now that even to implement the simplest logic circuits you 
absolutely must have some basic level of understanding.

Also try to draw your circuit schematics the same way as others do (you will 
pick this up automatically whilst you learn about them, eg ICs like this are 
usually drawn vertically, not horizontally).  Others will be more likely to 
help you if they don't have to spend time deciphering which wires are 
connected to what.

Post a reply to this message

> Ironically, the biggest problem was actually *wires*. I bought a box of 
> nicely cut wires, and that was £10 on its own. (In fairness, it comes in a 
> nice compartmental box and there's quite a lot of wire in it, each length 
> in a different colour, with the ends neatly bent to a right-angle.) 
> Unfortunately, the wire is so utterly stiff and unbendable that I had a 
> lot of trouble using it.

It is meant to be like that, so you can plug it straight into the 
breadboard.  If you really need to bend it then get a pair of needle nose 
pliers and make a neat bend.  Those pliers are also very useful for plugging 
in and removing the very shortest wires.

> Makes it kinda hard to rest the ends of the wires on both battery 
> terminals simultaneously when they're utterly rigid though.

That's what battery holders are for.

Post a reply to this message

scott wrote:

> Dude, really, you are going to fall over yourself over and over again if 
> you refuse to learn at least some basic analogue circuit electronics.  
> You must have realised by now that even to implement the simplest logic 
> circuits you absolutely must have some basic level of understanding.

So what you're saying is that there is basically no way I will ever get 
this to work, and I should just give up now?

Post a reply to this message

>> Dude, really, you are going to fall over yourself over and over again if 
>> you refuse to learn at least some basic analogue circuit electronics. 
>> You must have realised by now that even to implement the simplest logic 
>> circuits you absolutely must have some basic level of understanding.
>
> So what you're saying is that there is basically no way I will ever get 
> this to work, and I should just give up now?

No, you can probably get this circuit to work by experimenting yourself, 
asking here and googling.  Some people might even be nice enough to give 
some detailed explanations, and you might learn some stuff.  You might even 
be able to get a slightly more complex circuit to work ok.

But in the end you're doing things backwards, learn the basics *first*, it 
will make your life so much easier, and you won't get stuck at every point 
when things don't work as expected.  You will be able to answer your own 
questions if you have the knowledge of how basic electronic circuits work.

For example the next thing will be relays to control things that need higher 
power or voltage, you're going to connect one of your outputs to a relay 
coil and then wonder why it's not working, and then complain that the relay 
is destroying all your ICs :-)

Post a reply to this message

scott wrote:

> But in the end you're doing things backwards, learn the basics *first*, 
> it will make your life so much easier, and you won't get stuck at every 
> point when things don't work as expected.  You will be able to answer 
> your own questions if you have the knowledge of how basic electronic 
> circuits work.

Funny, I thought I *already* had a basic knowledge of how circuits work...

I guess what's throwing me is this new "logic low = negative" thing. I'm 
used to thinking of logic low as "nothing connected here". If you were 
making a bunch of logic out of switches with either connect or don't 
connect a wire to the positive rail, everything would be quite a bit 
simpler. (E.g., connecting the output of two gates together couldn't 
possibly create a short-circuit condition, since each output is 
connected to the same battery terminal or to nothing at all.)

I guess I'm still trying to get my head around all the consequences of 
that. For one thing, it looks like every single [external] input and 
output is going to end up needing a resistor on it. Yay. :-/

> For example the next thing will be relays to control things that need 
> higher power or voltage, you're going to connect one of your outputs to 
> a relay coil and then wonder why it's not working, and then complain 
> that the relay is destroying all your ICs :-)

Uh, no, even I'm not crazy enough to want to do that. :-P

1. I'm turning LEDs on and off. What the hell do I need relays for?

2. Relays are comparatively expensive.

3. Relays presumably use about 100x the current that an IC can supply.

4. PROFIT!

5. Relays move vastly slower than even the slowest electronic logic 
gate. Why bother?

Post a reply to this message

> Funny, I thought I *already* had a basic knowledge of how circuits work...

The circuit diagram you posted here makes it look like you don't.

> I guess what's throwing me is this new "logic low = negative" thing. I'm 
> used to thinking of logic low as "nothing connected here".

OOC why are you used to thinking that?

> If you were making a bunch of logic out of switches with either connect or 
> don't connect a wire to the positive rail, everything would be quite a bit 
> simpler. (E.g., connecting the output of two gates together couldn't 
> possibly create a short-circuit condition, since each output is connected 
> to the same battery terminal or to nothing at all.)

This doesn't work, because as soon as you connect a "not connected" output 
to something, it becomes "connected" and will have a specific voltage.  The 
severely limits how many things you can connect to a single output.

> Uh, no, even I'm not crazy enough to want to do that. :-P

It was just an example, you might one day want to control a motor, a 
brighter lamp or a heater or something.

> 3. Relays presumably use about 100x the current that an IC can supply.

Hehe no, I've got a tiny surface mount IC here that is driving 6W of LEDs 
directly.

> 4. PROFIT!

Huh, what are you planning to sell? No more free advice :-)

Post a reply to this message

>> Funny, I thought I *already* had a basic knowledge of how circuits 
>> work...
> 
> The circuit diagram you posted here makes it look like you don't.

Well, I noticed it wasn't going to work *before* I assembled it, right? ;-)

>> I guess what's throwing me is this new "logic low = negative" thing. 
>> I'm used to thinking of logic low as "nothing connected here".
> 
> OOC why are you used to thinking that?

I guess it's the way most books refer to circuits as "on" or "off". 
Usually if something is "on" it means it's connected to a power source, 
and if it's "off" then it isn't connected. That's how, e.g., a light 
switch works. It doesn't connect the output to the other pole when you 
turn it off, it just doesn't connect it to *anything*.

Of course, strictly speaking, logic high and logic low can be 
_anything_. So long as it's documented what they are and all the 
circuitry is expecting the same thing, it'll work. But still, most 
people casually refer to "on" and "off".

>> Uh, no, even I'm not crazy enough to want to do that. :-P
> 
> It was just an example, you might one day want to control a motor, a 
> brighter lamp or a heater or something.

Fair enough. But my immediate plans only run to wiring up lots and lots 
of logic gates, and observing their output.

>> 3. Relays presumably use about 100x the current that an IC can supply.
> 
> Hehe no, I've got a tiny surface mount IC here that is driving 6W of 
> LEDs directly.

Sure. But I'll bet it isn't a general-purpose logic gate. It'll be some 
special driver IC or something.

>> 4. PROFIT!
> 
> Huh, what are you planning to sell? No more free advice :-)

OOC, what's your hourly rate?

Post a reply to this message

On 5/25/2010 6:33 AM, scott wrote:

>> 3. Relays presumably use about 100x the current that an IC can supply.
>
> Hehe no, I've got a tiny surface mount IC here that is driving 6W of
> LEDs directly.
>

It's more about the relay's coil being an inductor. Once the IC 
switches, it will send a high voltage pulse back through the IC, frying 
it nicely. There are ways to get around that, but I'll leave that as an 
exercise to the reader to figure that out.

Hint: The circuit simulator has an example of this. :)

-- 
~Mike

Post a reply to this message

> It's more about the relay's coil being an inductor.

This was exactly the point I was trying to make, that wiring up a relay 
would "seem" to work (assuming your IC could source enough current) but then 
in reality it's going to blow the IC :-D

Post a reply to this message

Mike Raiford wrote:

> It's more about the relay's coil being an inductor. Once the IC 
> switches, it will send a high voltage pulse back through the IC, frying 
> it nicely. There are ways to get around that, but I'll leave that as an 
> exercise to the reader to figure that out.

More importantly, if the *whole* circuit is relays, there's nothing to 
fry. ;-)

(Other than your electricity bill, anyway...)

Post a reply to this message

> Sure. But I'll bet it isn't a general-purpose logic gate. It'll be some 
> special driver IC or something.

Of course, same as your hex buffer you mentioned.  But you still can't 
connect a relay directly to it without risking damage to the IC (as Mike 
explained).

> OOC, what's your hourly rate?

How much you got?

Post a reply to this message

>> Sure. But I'll bet it isn't a general-purpose logic gate. It'll be 
>> some special driver IC or something.
> 
> Of course, same as your hex buffer you mentioned.  But you still can't 
> connect a relay directly to it without risking damage to the IC (as Mike 
> explained).

And I still wouldn't try it in the first place. ;-)

>> OOC, what's your hourly rate?
> 
> How much you got?

I think we can take it as read that it's peanuts compared to what you 
usually make. ;-)

Post a reply to this message

>>> OOC, what's your hourly rate?
>>
>> How much you got?
>
> I think we can take it as read that it's peanuts compared to what you 
> usually make. ;-)

So?  Depends how fun it is.  If you wanted me to test drive Ferraris then 
you needn't pay me very much :-)

Post a reply to this message

On 25/05/2010 1:07 PM, Mike Raiford wrote:
> It's more about the relay's coil being an inductor. Once the IC
> switches, it will send a high voltage pulse back through the IC, frying
> it nicely. There are ways to get around that, but I'll leave that as an
> exercise to the reader to figure that out.

As Scott said, if you knew more about electronics then you would know 
that a back emf diode is an integral part of these circuits and relays 
used in process control come with the diodes fitted.

-- 

Best Regards,
	Stephen

Post a reply to this message

Invisible wrote:
> those tiny 1-hole 
> wide wire loops are excruciatingly hard to manipulate.

Needle-nose pliers.

And I don't think you're supposed to use that wire for the battery.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

On 5/25/2010 2:55 AM, Invisible wrote:
> Darren New wrote:
>
>> http://www.robotroom.com/Pumpkin/LEDPulsingBreadboard.jpg
>>
>> See how the black cable goes from C1 to C2?
>>
>> Or how the purple cable goes "around the corner"?
>
> I see...
>
> Makes it kinda hard to rest the ends of the wires on both battery
> terminals simultaneously when they're utterly rigid though. Similarly,

Seriously, go down to the local electronics supply store and buy a 
packet of 9-volt battery clips. It makes life much easier!


-- 
~Mike

Post a reply to this message

On 5/25/2010 7:08 AM, Invisible wrote:

> I guess it's the way most books refer to circuits as "on" or "off".
> Usually if something is "on" it means it's connected to a power source,
> and if it's "off" then it isn't connected. That's how, e.g., a light
> switch works. It doesn't connect the output to the other pole when you
> turn it off, it just doesn't connect it to *anything*.

Strange... Here's how my mind worked when I encountered the floating 
gate = high issue, and not truly realizing logic zero really needs to be 
tied to ground:

First I checked the data sheet, and found the information that a 
floating input would be interpreted as high. Then I stared at my dip 
switch and lamented the fact that it wasn't a SPDT switch. The I 
searched the electronics websites for SPDT dip switches, and came up 
frustratingly empty. Then started thinking to myself, "How can I make 
this work if I only have a SPDT switch" after staring at the circuit I 
thought to myself, well, if I connect it to ground with a suitable 
resistor, then that should allow the chip to see ground without dumping 
tons of current through the switch when its in the on position.

I tried it, and with some trepidation flipped the tiny switch to the on 
position and ... it worked just as it was supposed to!

Then I go an research after the fact, playing with the circuit 
simulator, and prodding various things in it to find out that yes, 
indeed, it does do what I thought it would do. and it makes sense. With 
no current from the positive rail, the gate's input can source to 
ground. While a voltage is on the positive rail, the gate sinks some of 
the current. Exactly what I needed.

A bit more research later and I discovered if I had read a tutorial on 
working with logic gates, I would have known about this solution, and 
it's called a pull-down or pull-up resistor, depending on which rail it 
hooks to.

:)

-- 
~Mike

Post a reply to this message

Mike Raiford wrote:
> f I had read a tutorial on working with logic gates, 

Heck, if you've ever used a SCSI device you know *that* much. :-)

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

Mike Raiford wrote:

> Seriously, go down to the local electronics supply store and buy a 
> packet of 9-volt battery clips. It makes life much easier!

I've *got* a battery holder. But I still need to connect the terminals 
of the battery holder to the beadboard.

Post a reply to this message

Darren New wrote:

> Heck, if you've ever used a SCSI device you know *that* much. :-)

I've used several SCSI devices. Your point?

Post a reply to this message

> I've *got* a battery holder. But I still need to connect the terminals of 
> the battery holder to the beadboard.

Solder one of your single-core breadboard wires to each wire of the battery 
holder.

Post a reply to this message

scott wrote:
>> I've *got* a battery holder. But I still need to connect the terminals 
>> of the battery holder to the beadboard.
> 
> Solder one of your single-core breadboard wires to each wire of the 
> battery holder.

That's what I did - except, as noted, I went for multistrand wire that I 
can actually bend so that I can route it to anywhere on the breadboard. 
Indeed, since I went with multistrand, everything has become a lot 
easier. (Except stripping the wires. I suck at that!)

Post a reply to this message

> Indeed, since I went with multistrand, everything has become a lot 
> easier. (Except stripping the wires. I suck at that!)

Get a pair of wire strippers!

Post a reply to this message

"Invisible" <voi### [at] devnull> a écrit dans le message de news: 
4bfcd758$1@news.povray.org...
> easier. (Except stripping the wires. I suck at that!)

Get a pair of strippers!

Post a reply to this message

Gilles Tran wrote:

> Get a pair of strippers!

Now that's more like it. ;-)

Post a reply to this message

On 5/26/2010 3:58 AM, Invisible wrote:
> Gilles Tran wrote:
>
>> Get a pair of strippers!
>
> Now that's more like it. ;-)

Haha ... Makes it a bit hard to concentrate though.

-- 
~Mike

Post a reply to this message

On 5/25/2010 11:52 AM, Darren New wrote:
> Mike Raiford wrote:
>> f I had read a tutorial on working with logic gates,
>
> Heck, if you've ever used a SCSI device you know *that* much. :-)
>

Wait ... I thought those were terminating resistors? Entirely different 
function.

-- 
~Mike

Post a reply to this message

Mike Raiford wrote:
> Wait ... I thought those were terminating resistors? Entirely different 
> function.

They're not pull-down resistors at the end of the chain?

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

On 5/26/2010 10:43 AM, Darren New wrote:

>
> They're not pull-down resistors at the end of the chain?
>

Just did a quick read on SCSI termination. Yes, they are pull down 
resistors!

-- 
~Mike

Post a reply to this message

Mike Raiford wrote:
> On 5/26/2010 10:43 AM, Darren New wrote:
> 
>>
>> They're not pull-down resistors at the end of the chain?
>>
> 
> Just did a quick read on SCSI termination. Yes, they are pull down 
> resistors!

I never looked it up specifically, but it's pretty obvious from looking at 
the SCSI protocol specs that the addressing and bus mastering and such works 
by pulling down zeros and letting ones float. Hence the "SCSI Priority" 
being the LUN and having "0" always be the host.  It's a pretty common way 
of working a shared bus, from ISDN to those one-wire protocol capsules.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

On 5/26/2010 5:16 PM, Darren New wrote:

> I never looked it up specifically, but it's pretty obvious from looking
> at the SCSI protocol specs that the addressing and bus mastering and
> such works by pulling down zeros and letting ones float. Hence the "SCSI
> Priority" being the LUN and having "0" always be the host. It's a pretty
> common way of working a shared bus, from ISDN to those one-wire protocol
> capsules.

Yep... Except often times when I think terminating resistor, I'm 
thinking a resistor connected at the ends of the line to damp reflected 
signals.

-- 
~Mike

Post a reply to this message

Mike Raiford wrote:
> Yep... Except often times when I think terminating resistor, I'm 
> thinking a resistor connected at the ends of the line to damp reflected 
> signals.

Given that SCSI allows cables up to 50' or something, I suspect they serve 
both purposes.


-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

Well let me see now... I bought a couple of switches and resistors, and 
spent the morning soldering them together. I now have two switches - a 
rocker switch and a push switch. In one position, you get logic 0, in 
the other position, you get logic 1, and no floating inputs. Yay, me!

It's kind of frustrating how many problems I'm having with this whole 
exercise - and most of them aren't electrical. They're mundane physical 
issues. (E.g., one of the switches I bought has metal terminals with 
small holes to poke the wires through. Except they're too tiny to poke 
any wire through!)

Still, it was gratifying to flick some switches and watch an LED light, 
correctly implementing the truth table of a NAND gate.

I was about to try rigging up a simple binary half adder - and then I 
realised that this requires 7 NAND gates, and I obviously only have 4. >_<

I've bought a small plastic box, and I'm hoping to somehow mount some 
switches on it, common-up the power rails, and end up with a box which I 
can just connect to the power and have a nice set of output wires. But 
this requires drilling and soldering and all sorts.

It's times like this where I miss my old electronics kit. :-( Everything 
seemed so much easier back then...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

Post a reply to this message