Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Why USB is faster than parallel port communication?

Status
Not open for further replies.

magnetra

Full Member level 5
Joined
Apr 21, 2005
Messages
265
Helped
10
Reputation
20
Reaction score
7
Trophy points
1,298
Location
27.45N, 85.20E KTM, NP
Activity points
3,390
USB

USB despite being a serial communication medium is faster than parallel port communication. Sounds odd. Any explanation??
Regards
Saurav
 

IanP

Advanced Member level 5
Joined
Oct 5, 2004
Messages
7,942
Helped
2,310
Reputation
4,622
Reaction score
531
Trophy points
1,393
Location
West Coast
Activity points
66,416
Re: USB

Speed
Q: How can Parallel Port ISA card support quoted rates of data transfer in excess of 1MB/sec ? As far a I know every in/out command for an ISA bus address takes at least 1 microsecond to perform REGARDLESS of processor speed ?

A: A simple parallel-port read or write does take one ISA-bus cycle, and on most systems, the bus speed is around 1.3 Mhz, so one cycle is a little under 1 microsecond. A complete data transfer on the original parallel port usually takes at least 4 cycles, however: check the Busy status, write the data, bring Strobe low, then high. On ports that support EPP and ECP modes, in these modes the port hardware does the handshake, and a complete transfer can take place in 1 bus cycle.

There are at least two ways of getting faster data transfers: Some ports support a "fast mode" that uses the ISA bus's NOWS (no wait state) signal to cause the CPU to skip 3 wait states. This mode is twice as fast, or around 2.7 Mhz. ECPs also include hardware support for data decompressing (& sometimes compressing, though that's often left for software), so the effective rate of data transmission can be much faster than the number of bytes written or read per second. Also, ECPs may be able to transfer data between the host's and peripheral's ECP buffers at rates greater than 1 Mhz, though the buffers are typically just 8 bytes.
I think the above explains why it is much slower than USB. I can just add that it is just an old standard..
Regards,
IanP
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top