# Newb question

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October 24, 2011 9:21:02 PM

Ok, I’d like to state that I know this may be an elementary question, but if I don’t ask, I won’t learn right? So let’s say data is being transferred over say a SCSI bus from one HD to another... I understand that it may have for instance a bus width of 8 bits, and that each bit has its own data signal. So for a bitstream of 00100100 that is sent, a certain voltage (or no voltage) is sent down each data line. My question is, if a voltage indicating a digital value of 1 is to be sent, where does the voltage come from? Does the voltage come from the power supply? How is it manipulated (in a broad generalised way) to send a 1 or a 0 over the bus?

Thanks,

October 24, 2011 9:49:53 PM

Yes, the voltage comes from the PSU but it is likely stepped down from say 5V.
A logic circuit for this purpose is a gate. It either lets a signal through (1) or it doesn't (0). There may be a bank of circuits that perform this function (a register) and each gate is responsible for an integer place.

This is a simplification of course. Computer circuits can be very complex and involve billions of transistor circuits, but for digital information like you suggested this is how its performed.

Voltage in general is stepped down by transformers, for instance the transformer in your neighborhood that steps down thousands of volts to the stuff that you power your place with. On a smaller level, there are circuits that do the same thing. A limiter can limit the amount of voltage in a circuit. So basically the PSU takes the power from your outlet, transforms it to voltages it can use and turns it to DC power. These different voltages are sent to different areas on your motherboard, which in turn also transforms these voltages via solid state circuits into usable voltages that are circuit specific.
October 24, 2011 9:54:51 PM

In very simplistic terms the voltage originates from the PSU, goes to the mobo VRMs, which distributes it to the CPU, RAM, etc. When you enter a command the CPU sends directions to the HD or RAM to process the data and send it where you want it to go by sending the proper voltage to equal a "1" or a "0".

http://en.wikipedia.org/wiki/Logic_level
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October 26, 2011 5:40:04 AM

Guys, thanks for your replies! Starting to make sense now. Just another thing -
the data signal that comes from the HD controller... I read on the net that when the head within the HD hovers over a magentised portion of the platter, it induces a current in a coil on the head (or something along those lines). This then passes onto the controller which then passes it to the CPU, other drive, etc..

So, for my SCSI HD to HD signaling example, does this mean the voltage/data signal to be sent out from one HD to another emanates from the magnetised platter and not the power, which for the HD is solely used to power the motor?

October 26, 2011 6:48:44 AM

No, there isn't a way for it to work that way. To my knowledge the magnetized bit causes the voltage in the read head to fluctuate, which the drives controller then passes off as a 1 or 0. The head or platter itself doesn't generate electricity.
October 27, 2011 5:18:36 AM

4745454b said:
No, there isn't a way for it to work that way. To my knowledge the magnetized bit causes the voltage in the read head to fluctuate, which the drives controller then passes off as a 1 or 0. The head or platter itself doesn't generate electricity.

Ok, so the PSU supplies a constant stepped down voltage to the circuits within HD. A "1" or "0" is read by the head, causing a specific fluctuation in the voltage which the controller recognises (i.e. knows whether to pass on a 1 or 0).

October 27, 2011 6:40:52 AM

As I understand things, yes. The voltage reading changes because it ran over a magnetized bit on the platter.

I'd like to take a second to laugh at your thread title btw. Newb? We get those a lot with the "rate my build" questions. Asking questions like these are the ones that few on this forum can answer, and stretch my limits of what I know. Basically, this is hardly a newb question.
October 27, 2011 7:10:29 PM

4745454b said:
As I understand things, yes. The voltage reading changes because it ran over a magnetized bit on the platter.

I'd like to take a second to laugh at your thread title btw. Newb? We get those a lot with the "rate my build" questions. Asking questions like these are the ones that few on this forum can answer, and stretch my limits of what I know. Basically, this is hardly a newb question.

Thanks for the info. Was just curious to know the intricacies - hate not knowing (even if this is generalised, it's good to get an overview).

Ha, thought maybe this was some 101 stuff. Glad i'm not the complete idiot I thought I was. Thanks again (along with all the other posters; much obliged).
October 28, 2011 12:34:54 AM

Usually most are just concerned that they are getting the fastest transfer rates or something like that. Few ask exactly HOW something works. I usually have a rough understanding about these things, but sometimes it hurts the brain trying to type it out.
!