Serial communication vs Parallel communication

Discussion in 'Computing and Networks' started by Reshma, Feb 18, 2008.

  1. Reshma

    Reshma Thread Starter Active Member

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    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?
  2. studiot

    studiot E-book Developer

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    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.
  3. beenthere

    beenthere Retired Moderator

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    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.
  4. Dave

    Dave Retired Moderator

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    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
  5. hgmjr

    hgmjr Moderator Staff Member

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    And then there is LVDS.

    hgmjr
  6. studiot

    studiot E-book Developer

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    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.
  7. studiot

    studiot E-book Developer

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    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.
  8. Dave

    Dave Retired Moderator

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    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
  9. beenthere

    beenthere Retired Moderator

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    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.
  10. n9352527

    n9352527 AAC Fanatic!

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    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.
  11. Dave

    Dave Retired Moderator

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    I've never bought a USB cable of any kind, so wouldn't know what is good value and what is not - I just looked at the cable being 2/3s the value of the printer.

    I could do with find one of the those pound-shops that actually sells electronics items. I want as short a possible USB-USB cable for my laptop cooler (~12 inches or less), but I cannot find one for love nor money. Currently I have a spool of cable on my desk which connects the lappy to the cooler. I'm tempted to make my own.

    Dave
  12. studiot

    studiot E-book Developer

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    Since this experience has generated some interest, I will confess to publishing the worst case.

    We went to four big national chains of tin sheds. Two are electrical retailers, one pc retailer and one is an office supplies specialist. I also looked in the Argos catalog and on the internet.

    The printer was generally priced at £29.99, although Comet has it for £19.99 and Morgan for £25.

    The cables varied from fancy pink and green ones, placed by the printers as a 'now you need this oprional extra' between £15.99 and £19.99. I think they were a bit longer than the average 0.5 to 1 metre cable that comes free with USB gadgets.

    Comet were offereing a cable for £1.50.

    I should also add that most printers now seem to require a USB2 cable (is there a difference?).

    I remember that there was a similar markup on Iomega parallel and SCSI cables with their zip drives.

    Short USB cables? Just Google to get a local supplier eg

    http://www.usbfirewire.com/Parts/rr-usb2-6in-ab.html
  13. Dave

    Dave Retired Moderator

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    In the interests of staying on-topic, I have copied the post about the printer/USB cables to a new thread where we can continue the discussion without deviating from the question asked in the OP.

    The thread can be found here.

    Dave
  14. Reshma

    Reshma Thread Starter Active Member

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    Thanks, studiot! The reason this question came to my mind is ever since I have been studying digital electronics especially Shift registers, the main advantage of parallel over serial communication as put in most books is the ability to transfer multiple bits at the same. So it was my general assumption that parallel is faster over serial.

    But in the recent few years, the channels between the computer and disk drives have been switching from parallel to serial. I recently purchased a new motherboard for my PC and it had only one PATA (parallel) port and six SATA ports. My old hard disk and CD writer were PATA compatible and I was finding it difficult to connect them in the Master-slave configuration using just one Port. Ultimately I had to replace my Disks with SATA devices.

    Now I know, although it would seem that a parallel cable with multiple lines for data would always yield a faster data transfer rate than a single data line, keeping the bits aligned in a parallel channel requires more complex electronics. Signals on multiple data lines can arrive at the receiving side at different times and must be aligned in order to turn the bits into meaningful bytes.

    I am still unaware over which one is more expensive but I still require the old Standard Parallel Port since my coursework involves study of simple parallel port interfacing experiments and it has been a herculean task to get hold of an old PC which still retains the old Parallel port. :D
  15. Reshma

    Reshma Thread Starter Active Member

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    Fortunately I was able to purchase my HP deskjet 2460 for $60 with the USB cable at no additional charges. :cool:

    Yes, most USB devices require a USB 2.0 cable nowadays. USB 2.0 has a raw data rate at 480Mbps, and it is rated 40 times faster than its predecessor interface, USB 1.1, which tops at 12Mbps. Originally, USB 2.0 was intended to go only as fast as 240Mbps, but USB 2.0 Promoter Group pumped up the speed to 480Mbps.
  16. studiot

    studiot E-book Developer

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    Since you are taking a serious interest in the subject you might like to consider further the following.

    With serial signalling it is the transition which is important and marks an event. A serial receiver may wait for years until a low/high transition occurs. This transition may be noise or it may be the beginning of data. At the transistion it will start counting (into a shift register) for a number of transitions to find out whether data is being received and what the data is. The exact length of the pulses can vary and still be valid data. The only way a serial receiver can distinguish between noise and data is by the use of start and stop bit patterns. Finally a serial receiver can only distinguish between data and instructions by coding convention, both arrive in the same fashion over the same line. This is why coding schemes include several start and stop bits and sometimes separation bits.

    By comparison, for most parallel schemes it is the logic levels that are important.
    The transmitter holds one or more control lines at a known logic level.
    After a suitable time to allow the control level to establish it sets the data lines to known levels.
    After another suitable time interval it sets another control line to a specific logic level to indicate the end of transmission.

    Of courses this appears as pulses on the lines, but any designer must face the question
    If there is logic 1 on line 1 and 0 on line 2..... how does the receiver know whether this is correct or just that the signal on line 2 has not yet been received?

    This is why the timing is much more critical with parallel systems and events must occur within set timeframes. There can be no equivalent of start and stop bits.
  17. theamber

    theamber Active Member

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    I thought USB was about 8 times faster than parallel. But I have seen this:
    Speed. USB 2.0 supports a maximum data transmission speed (the rate at which data travels along the bus; also referred to as throughput) of 480Mbps (megabits per second), which is equivalent to 60MBps (megabytes per second). That's more than 300 times faster than the maximum data transmission speeds of a parallel port and more than 4,000 times faster than the top data transmission speeds of a serial port. (USB 1.1 has a much slower throughput than USB 2.0, and we'll discuss this in more detail with the following question.)
    Also when manufacturers when they refers to the speed of data transfers, do they actually refer to the real speed being send finally to the device or the "throughput" data transfer before the buffers? Because is confusing since I have seen different speeds claims. While is attractive to see these differences what actually matters is what is the real output right?

    Attached Files:

    Last edited: Jun 28, 2008
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