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View Full Version : USB vs. Serial vs. Parallel port data rates



jgkeil
03-20-2002, 02:04 PM
Is there someone out there who could explain to a PC novice why the data rates for USB ports are so much faster then either Serial and/or Parallel ports?
Thanks, John

Ghost_Hacker
03-21-2002, 10:41 AM
Each has a different method of Input/Output using different protocols and hardware signaling.

Serial ports transport one bit at a time using one data "wire". (hence the name "serial" port)The speed of the transfer is controlled by a UART chip.


Parallel ports send one byte (8 bits) at a time using 8 data wires.(hence the name "parallel" port) Speed increases between the different parallel port modes comes about due to different signaling and the use of buffers.


USB has 3 types of data transfer methods.


Bulk
Isochronous
interupt



Each method of data transfer use a different packet size and tranmission timing. But USB can send from 64 to 1023 bytes at a time.

hope this helps http://www.PCGuide.com/ubb/smile.gif

jgkeil
03-21-2002, 11:22 PM
Dear Ghost_Hacker:

This is just what I was looking for. Clears up the packet size issue for me. I missed that!

Thanks,
John Keil



Originally posted by Ghost_Hacker:
Each has a different method of Input/Output using different protocols and hardware signaling.

Serial ports transport one bit at a time using one data "wire". (hence the name "serial" port)The speed of the transfer is controlled by a UART chip.


Parallel ports send one byte (8 bits) at a time using 8 data wires.(hence the name "parallel" port) Speed increases between the different parallel port modes comes about due to different signaling and the use of buffers.


USB has 3 types of data transfer methods.


Bulk
Isochronous
interupt



Each method of data transfer use a different packet size and tranmission timing. But USB can send from 64 to 1023 bytes at a time.

hope this helps http://www.PCGuide.com/ubb/smile.gif

Flick
03-22-2002, 09:46 PM
Ghost_Hacker...so does that mean that if you have an open USB port (I do), it would be better (and faster) to connect my printer via USB rather than the parallel port?

iisbob
03-23-2002, 12:46 AM
Not necessarily, as the USB port is sporadic in it's support of Bi-directional communication with some peripheral's-so you're printer can't tell your PC when it's out of paper/ink/etc.

What you have to remember is that USB is part of the original serial bus port ( it's actually a variation on the PCI standard ), and though the current USB standard supports up to 12 Mbps ( about 1.5 MB's per sec ) transfer rate, rarely do most peripheral's get above 300-500 K persec consistant data transfer rate. Parallel ports { in ECP mode ) generally run about this same rate, and provide consistant Bi-directional support { with the proper cable }.

So even though USB would be more convienant, until the peripheral supports the new 2.0 standard ( which is supposed to offer up to 500 Mps constant thruput-about 62 MB's per sec data transfer ) your best bet is the standard Parallel port, just make sure that you have ECP in your BIOS enabled.



------------------
iisbob

The man who sets out to carry a cat by its tail learns something that will always be useful and which never will grow dim or doubtful. -- Mark Twain.

Flick
03-23-2002, 04:35 PM
Thanks iisbob! I have an HP 970 Cse DeskJet and it can be hooked up either way (i.e. parallel or USB). Since I bought it last year, I have had it hooked up to the parallel port and have been very impressed with its speed and print quality. The feature I really like is its ability to print on both sides of the paper. I always use high quality 24 lb. stock so I don't like to waste paper. Currently the only items I have hooked up to the USB are the keyboard, mouse and my scanner.