View Full Version : Parallel ports

17-04-2002, 09:34 AM
I know what the acronyms mean, but someone explain the differences between the various options for setting up parallel ports - eg EPP, ECP, etc. What are the plus's and minus's of each. I'm having a few problems with printing from my CADD program that I need to resolve, and I suspect it may be set up issues.

17-04-2002, 12:23 PM
If I am going to explain this I may as well tell other people what the abbreviations mean for others who may view this thread.

Enhanced Parallel Port (EPP) is for peripherals that require constant two-way communications with the PC, the EPP protocol offers high-speed, two-way data tansfers with relatively little software overhead. Hardware handles the handshaking and synchronisation between the peripheral device and the PC. By removing the CPU from the handshaking process, an EPP port enables the CPU to transfer data to and from the prot with a single command, saving a significant number of clock cycles.

Extended Capability Port (ECP) was proposed in response to the need for high-performance parallel communcation for printers and scanners. ECP is considered the fastest of all the parallel standards (EPP was created before the Institute of Electrical and Electronics Engineers(IEEE) 1248 committee). ECP data transfers are loosely 'coupled,' meaning that once the data transfer has begun, the software that initiated the transfer cannot monitor the progress of the transfer. The software must wait for a signal that shows that the transfer has been completed. Even more than with EPP, this reduces the number of clock cycles used by the transfer to a bare minimum. Although it also reduces the amount of control that the software has over the process, not much control is needed. ECP is designed for operations that involve moving large chunks of data.

The ECP standard provides the same degree of flexibility to hardware manufacturers as EPP. Manufacturers do not always embrace the entire standard. On the peripheral side, this is not much of a problem. Some modes are more appropriate for some types of devices than for others. ECP excels at handling large blocks of data via DMA channels, making it ideal for printers and scanners, but not so attractive for devices such as external CD-ROMs. External devices that must frequently switch back and forth between read and write operations are better served by EPP's ability to change the direction of the data flow without additional handshaking and overhead.