understanding multiplexing
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Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=117427
Printed Date: May 01, 2024 at 5:25 AM
Topic: understanding multiplexing
Posted By: calitech247
Subject: understanding multiplexing
Date Posted: November 02, 2009 at 5:11 PM
Hello all, Just became a member on this site but I'm not new here. I've used this site many times before to help me with wiring specs, how to wire relays and other stuff. So time has come for me to become an advanced MECP, and I got sorta puzzled here on the part on multiplexing. I'm trying to understand the whole concept and I think I got it figured out but still kinda fuzzy. So here's what I got out of it. Any feedback would be greatly appreciated. Please feel free to correct me if I'm wrong at all. A typical car these days has some 5-6 miles of wire. So multiplexing is the solution to weight and cost issues, since in multiplex circuits, one wire supports multiple functions. On top of this, multiplexing gets data from point A to point B more efficiently by widening the bandwith. And whether its 10 bits or 10 megabytes the speed in which this data transfers on this multiplex "superhighway" is always the same (roughly 66% the speed of light). So is the purpose of multiplexing to increase the bits per second, widen the lanes on the freeway, or both? And will this even be on the test? (brain nerve now swelling):errr:
Replies:
Posted By: tommy...
Date Posted: November 03, 2009 at 7:37 AM
When i took my first class certifications...Wasnt really much on that...But that was back in 2000...soooo...not sure if they have updated the test some...!?
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M.E.C.P & First-Class
Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!
Posted By: tommy...
Date Posted: November 03, 2009 at 7:38 AM
I know there were some practice tests available that REALLY helped out on the test...!
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M.E.C.P & First-Class
Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!
Posted By: calitech247
Date Posted: November 03, 2009 at 9:58 AM
Really? What practice tests?
Posted By: KPierson
Date Posted: November 03, 2009 at 1:22 PM
My personal take on it, and it may just be my opinion, but multiplexing and data signals are two different concepts. Multiplexing is is a way of encoding multiple signals on one wire, usually by varying voltage (0-5vdc or 0-10vdc analog signals), by current (0-20ma or 4-20ma are commong) or by pulse widths (servo control). In cars, generally you will only see voltage multiplexing and it's usually not 0-5vdc analog, it's usually a few precise voltages somewhere between 0 and 5vdc. An example of this would be steering wheel buttons for radios, cruise control, air conditioning, etc. If you push one button it will drop the voltage to 0.5vdc, another would drop it to 1.0vdc, and so on. In the typical car scenario there are a limited number of possibilities. Data signals are different because they follow protocols (RS232, CAN, LIN, etc). They have baud rates and have start/stop bits and data bits as well as other parameters predefined. A data signal can transmit much more data much quicker then a typical multiplexed system. ------------- Kevin Pierson
Posted By: chungo
Date Posted: November 03, 2009 at 8:01 PM
I just took the advanced test today and none of the question were that complicated. I think to only question about multiplex system was to describe the opposite of multiplex.
Posted By: calitech247
Date Posted: November 03, 2009 at 9:35 PM
So maybe I'm overstudying cuz another dude told me that same thing. That none of the questions were that complicated. And that some parts of the book aren't even on the test. So did you pass?
Posted By: chungo
Date Posted: November 03, 2009 at 10:13 PM
all the questions from the practice test at the end of the chapter are there, and the rest are similar.
Posted By: howie ll
Date Posted: November 04, 2009 at 11:41 AM
KP I think you're wonderful! Nice simplex answer (there I pun again). You should get together with Messrs Wallace and Luongo and write a book entitiled "Easily understood concise anwers to complex questions". And I'm not joking.
Posted By: calitech247
Date Posted: November 04, 2009 at 12:44 PM
chungo wrote:
all the questions from the practice test at the end of the chapter are there, and the rest are similar.
Chungo, so what sections of the MECP advanced textbook would you recommend to study the hardest? And should I memerize the Power, Ohm, Current, Voltage Pie Chart? Is there lots of math questions? Thanks for your reply...
Posted By: calitech247
Date Posted: November 04, 2009 at 12:50 PM
KPierson wrote:
My personal take on it, and it may just be my opinion, but multiplexing and data signals are two different concepts. Multiplexing is is a way of encoding multiple signals on one wire, usually by varying voltage (0-5vdc or 0-10vdc analog signals), by current (0-20ma or 4-20ma are commong) or by pulse widths (servo control). In cars, generally you will only see voltage multiplexing and it's usually not 0-5vdc analog, it's usually a few precise voltages somewhere between 0 and 5vdc. An example of this would be steering wheel buttons for radios, cruise control, air conditioning, etc. If you push one button it will drop the voltage to 0.5vdc, another would drop it to 1.0vdc, and so on. In the typical car scenario there are a limited number of possibilities. Data signals are different because they follow protocols (RS232, CAN, LIN, etc). They have baud rates and have start/stop bits and data bits as well as other parameters predefined. A data signal can transmit much more data much quicker then a typical multiplexed system.
KPierson, Wow, awesome feedback. You obviously have an indepth understanding on the topic. So, you say here that data signals can transmit greater amounts of data than a typical multiplex system. But, how can data signals tranmits "quicker," or at a faster rate when electrons on copper wire all travel at the same rate (appr. 66% the speed of light). Are you saying that the data is "processed" at a faster rate?
Posted By: t&t tech
Date Posted: November 28, 2009 at 6:20 PM
Posted By: t&t tech
Date Posted: November 28, 2009 at 6:21 PM
Data and multiplexing are different as KP said, these are some sites on the data side of things!
Posted By: oldspark
Date Posted: November 29, 2009 at 2:40 AM
KPierson wrote:
My personal take on it, and it may just be my opinion, but multiplexing and data signals are two different concepts. Multiplexing is is a way of encoding multiple signals on one wire...
I'm too late on the admiration wagon, so I'll stay technical.
Bingo! Spot on! Well put, etc etc.
I was going to say "Multiplexing is is a way of encoding multiple inputs on one wire..." (or outputs).
EG - 4 cameras, 1 input. Switch between the cameras.
Or save cables - 4 cameras, one LONG tranmission line - switch 4 cameras successively thru the line.
Then maybe switch each to their own monitor.
IE - time-switch or multiplex 4 cameras thru 1 line and then de-multiplex to their 4 monitors.
And that's a bad example.
A common example is multiplexed displays - eg 7 segment LEDs (digits) that each require 8 connections - 7 segments and a common.
Instead of a 4-digit clock having 4x8 = 32 wires to the display, why not 11 wires?
IE - 7 wires that go to or though all 4 digits.
Then the 4 separate common wires.
Display "1" on digit #1 - turn on the 2 of 7 wires for the "1" segments, and the "common" for digit #1.
Display "2" on digit #2 - turn on the 5/7 wires for "2" and the common for digit #2 (noting that common for digits #1, $3 & #4 are off!!).
Etc.
Similarly for inputs - a common modern example being several analog inputs like coolant temp, oil pressure, oil temp, voltage etc into a electronic or engine management systems (EMS).
Because Analog to Digital Converters (ADCs) are expensive or a pain to manufacture, and because most analog signals are very slow compared to uPCs (micro-Processors) etc, they often use the same ADC for all analog inputs - ie, multiplex the inputs aka channels.
I see multiplexing as a "mechanical" means of sharing a pipe. It can be called time-sharing etc.
It is not necessarily digital per se, just the sequential time sharing of different input or outputs over the same line(s).
"Data" is different. (Assuming "data" to be as in digital communications - computer speak etc.)
Data is the encoding through the pipe or line from the inputs etc.
Whether your sensors (temp, pressure etc) are multiplexed is irrelevant. You somehow collect their values, ADC them to a digital value comprising "bits" (ones & zeros), and send that "data" down the line to the <whatever> that decodes these 1's & 0's. EG - binary 111 = 7; 101 = 5 etc.
Bitrate is how fast you send the data.
High bandwidth is required for fast data because "speed is bandwidth" - but that gets a bit complicated. (Nyquist Theorem springs to mind.)
I've seen a nice diagram somewhere that shows a multiplexed system output (maybe a screen or 7 segment displays?) as parallel things and a rotating switch underneath that selects each one at a time. The input is other parallel things (temp, oil-P etc, or time-digits) with a similar selector switch. There is a link between the two switches showing that they are synchronised.
Not that I looked a the previous mentioned links!
Is it here that I should add that there is no such thing as digital transmission? Nah - maybe not.
Posted By: howie ll
Date Posted: November 29, 2009 at 3:25 AM
As my old cookney born dad would have said of Oldspark, "he doesn't half know what he's talking about". More simply in an auto environment a multiplex cable is usually a trigger cable for something, e.g. Chrysler locks. Different levels of resistance on that cable well provide multiple signals, thus saving on weight and expense of cables in a car. Usually as a reference from the nominal 12v+ to ground(earth). CAN (Convergent Area Networks) are pure data, usually used as control and feedback (for diagnostic purposes). There are quite a few variants, the GM low speed system is generally a data rather than a strictly CAN system, there are regional variants even amongst the same company e.g. GM in Europe and the US, also manufacturers are constantly changing the protocols, VAG spring to mind here, this is why we have by-pass problems.
Posted By: oldspark
Date Posted: November 29, 2009 at 8:37 AM
IOW analog encoding rather than time multiplexing.
Still the same "sharing a single line between multiple signals" concept.
(And no different to "digital transmission" lol!)
My old Sony auto-reverse twin cassette (from 1980!) used that system for its remote.
You unhooked its front panel and plugged in a 3.5mm stereo-plugged cable.
Upon investigation (  ) I found it multiplexed its 8 or so fwd, rwd, play, rec, etc buttons using different resistor values.
It was cute for the time (30 years ago), but those systems were replaced by their easier manufactured digital encoders.
I would have thought a preference for non-analog systems in cars given the harsh electrical environment etc, as well as in general these days thanks to our lovely GSM system (hey - I refused GSM and used CDMA, but the bricks shut down our CDMA!  ).
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