All About Circuits Forum  

Go Back   All About Circuits Forum > Software, Microcomputing, and Communications Forums > Computing and Networks

Notices

Computing and Networks Discussion forum for general computing technologies, personal computing, data networking, information technology, and computer systems engineering.

Reply   Post New Thread
 
Thread Tools Display Modes
  #1  
Old 02-18-2008, 11:52 AM
Reshma's Avatar
Reshma Reshma is offline
Member
 
Join Date: Mar 2007
Location: India
Posts: 54
Default Serial communication vs Parallel communication

Why is the Parallel port getting replaced by USB ports lately? When information is sent across one wire, one data bit at a time, its called serial. In the case of an 25-pin parallel port, you have eight data-carrying wires so that eight bits can be sent simultaneously. Because there are 8 wires to carry the data, the data finishes being transferred eight times faster than a serial connection. Maybe my understanding could be flawed but the USB port essentially makes use of serial communication...so why is serial communication replacing parallel communication even though parallel is faster?
Reply With Quote
  #2  
Old 02-18-2008, 01:01 PM
studiot's Avatar
studiot studiot is offline
E-book Developer
 
Join Date: Nov 2007
Location: Somerset UK
Posts: 3,946
Default

Parallel is not necessarily faster Reshma.

The problem with parallel transmission is that although you can send many bits at once (yes 8 in many systems) receive 'at once' is not a true statement.

Each bit travels a slightly different path down the cable and to a different pin on the chip.
The small differences in path length that each bit travels becomes more and more significant as speed increases. Thus the bits are not all received together.
As a first step you can counter this by holding the bits in a buffer until all are received (thereby loosing some of your speed gain), but eventually the bits become so out of step that garbage is received. This effect limits the max speed at which you can transmit parallel data.

With serial data each edge transition can represent only one bit so you can push them down the line just as fast as you are able. This can be more than 8 times faster than the parallel transmission limit, thus giving you a net speed increase.

Solve the synchronisation problem and we can go back to faster parallel.

This is why the latest computer bus PCI express, (PCIe) is serial.
Reply With Quote
  #3  
Old 02-19-2008, 12:23 AM
beenthere's Avatar
beenthere beenthere is offline
Senior Member
 
Join Date: Apr 2004
Location: Missouri, USA (GMT -6)
Posts: 15,815
Blog Entries: 10
Default

That is the reason serial is the way to go now - it's cheaper. The interface is no problem, but the cabling would have to be bulky and expensive to accommodate differential signaling. 10 MHz should not be particularly difficult - imagine 10 megawords/sec. versus 12 megabits/sec. It's the $50/ft (probably more for 64 bit words) for the cabling that makes it impractical.
Reply With Quote
  #4  
Old 02-19-2008, 12:34 AM
Dave Dave is offline
Senior Member
 
Join Date: Nov 2003
Posts: 6,961
Blog Entries: 17
Default

You might also want to consider something called "cross-talk" which is an issue in parallel communications, and has knock on effects to data validity across the interface.

Ultimately, the answer is that serial comms are generally cheaper and simpler (to wire/route).

Dave
Reply With Quote
  #5  
Old 02-19-2008, 12:35 AM
hgmjr's Avatar
hgmjr hgmjr is offline
Super Moderator
 
Join Date: Jan 2005
Location: Tennessee, USA (GMT-6)
Posts: 9,030
Blog Entries: 11
Default

And then there is LVDS.

hgmjr
Reply With Quote
  #6  
Old 02-19-2008, 12:45 AM
studiot's Avatar
studiot studiot is offline
E-book Developer
 
Join Date: Nov 2007
Location: Somerset UK
Posts: 3,946
Default

Quote:
That is the reason serial is the way to go now - it's cheaper.
I wish.

Last weekend I took my daughter to look for a new printer. I was suprised to find that you now have to buy your own cable with the lower end models.

eg HP Deskjet £29.99; USB cable £19.99 also required.

I can buy a standard parallel printer cable for £1.50 and a ribbon one for £0.50 in the same store.
Reply With Quote
  #7  
Old 02-19-2008, 12:54 AM
studiot's Avatar
studiot studiot is offline
E-book Developer
 
Join Date: Nov 2007
Location: Somerset UK
Posts: 3,946
Default

Quote:
Why is the Parallel port getting replaced by USB ports lately?
This is not such an idle question as both serial and parallel transmission have unique features.

Undoubtedly serial is the fastest, using fibre optic links. Current technology does not allow parallel transmission this way.

Parallel scores in that it is not possible to send both control signals and data at the same time with serial. Most parallel systems employ control lines, in addition to data lines.
Some can be quite sophisticated. The GPIB bus is one such, used to link and control instrumentation.
Reply With Quote
  #8  
Old 02-19-2008, 12:56 AM
Dave Dave is offline
Senior Member
 
Join Date: Nov 2003
Posts: 6,961
Blog Entries: 17
Default

Quote:
Originally Posted by studiot View Post
I wish.

Last weekend I took my daughter to look for a new printer. I was suprised to find that you now have to buy your own cable with the lower end models.

eg HP Deskjet £29.99; USB cable £19.99 also required.

I can buy a standard parallel printer cable for £1.50 and a ribbon one for £0.50 in the same store.
That's pretty bad news. I suppose they have to make their money somehow, or at least give you incentive to consider the high-end models. I'll be in the market for a printer in the next couple of weeks so will bare this in mind.

Mind you, that is why I hoard cables from the many PC projects I work on.

Dave
Reply With Quote
  #9  
Old 02-19-2008, 02:43 AM
beenthere's Avatar
beenthere beenthere is offline
Senior Member
 
Join Date: Apr 2004
Location: Missouri, USA (GMT -6)
Posts: 15,815
Blog Entries: 10
Default

The upper end HP printers don't come with USB cables, either. Do look for USB cabling at stores other than for computer equipment. What costs $40 in the store may be had for $5 online.
Reply With Quote
  #10  
Old 02-19-2008, 08:52 AM
n9352527 n9352527 is offline
Senior Member
 
Join Date: Oct 2005
Posts: 1,198
Default

Quote:
Originally Posted by studiot View Post
I wish.

Last weekend I took my daughter to look for a new printer. I was suprised to find that you now have to buy your own cable with the lower end models.

eg HP Deskjet £29.99; USB cable £19.99 also required.

I can buy a standard parallel printer cable for £1.50 and a ribbon one for £0.50 in the same store.
If the cable is standard A-B USB cable, then £19.99 is an exorbitant price for it. I bought mine (2m length) from one of those one pounder shop. I tend to cut and use them a lot when developing USB systems, certainly far cheaper than buying the connectors and cable from Farnell! The quality is not bad either.

Last edited by n9352527; 02-19-2008 at 08:56 AM.
Reply With Quote
Reply   Post New Thread

Tags
, ,


Related Site Pages
Section Title
Worksheet Digital communication
Video Lecture Hardware: IO, RTC, PS - Microprocessors
Video Lecture Hardware: (Computer) - Microprocessors
Textbook Electrical signal types : Digital Communication
Textbook Networks and busses : Digital Communication
Textbook Introduction : Shift Registers


Similar Threads
Thread Thread Starter Forum Replies Last Post
serial communication th3play3r The Projects Forum 8 08-27-2007 01:48 AM
HELP: serial communication between two 7495s glitterized_dreamz The Projects Forum 1 03-10-2007 07:34 PM
serial to parallel converter VHDL help pinka Programmer's Corner 1 12-08-2006 06:28 PM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 11:21 AM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin
Copyright ©2000 - 2014, vBulletin Solutions, Inc.