• ECP stands for Extended Capability Port
• Hewlett Packard and Microsoft created it as an advanced mode for connecting printers, scanners and other peripherals
• Like EPP, ECP provides bi-directional high speed, half-duplex operation
• ECP supports a run length encoding feature, (RLE). This real time compression of data allows compression ratios of up to 64 to 1. This is especially useful for printers and scanners dealing with large hi-resolution images. RLE can only be used if both the peripheral and the BIOS or adaptor host supports this facility
• It supports Channel Addressing. This is different from EPP in that it enables the host device to see several peripherals connected as ‘logical’ devices:
  
• For example, if you have a modem, printer and scanner connected to one parallel port, it would be possible to receive information from the modem and still have the printer busy processing a print image (the printer is considered a totally separate ‘logical’ device from the modem)
• With EPP, if the printer is busy there will be no communication to any other peripheral until this print job has been completed. With ECP, the software or port driver uses ‘another channel’ and communication would continue transparently
  
  
• ECP is a much more ‘loosely coupled’ protocol. The software driver isn’t aware of the current state of the DMA data transfer to the peripheral. It doesn’t care if it’s the first or last byte; it is only concerned with whether or not the data transfer was successful or if it failed.
• Another difference between ECP and EPP is the reading and writing of data between the host adaptor or BIOS and the peripheral:
  
• With EPP both read and write operations can be performed without any overhead or handshaking
• ECP requires the host adaptor or BIOS to request a change and then wait for the peripheral to acknowledge the request before continuing. If the previous transfer of data was under DMA control, this would mean the host must either wait for the DMA to complete or interrupt it. The host software must then determine the amount of information already transferred so it may continue in a proper fashion without causing disruption to the information being transferred. For peripherals requiring frequent reading and writing of status registers or buffers this can add a substantial overhead to the whole process.

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