HP 2500C Series - Manual (Page 21)

Product Information 1-13
32 nPeriphRequest During ECP mode the peripheral may drive this pin low to request
communications with the host. This request merely “hints” to the host; the host
has ultimate control over the transfer direction. This signal provides a
mechanism for peer-to-peer communication. This signal is valid in the forward
and reverse directions.
36 1284 Active Driven high by host while in ECP mode. Set low by the host to terminate ECP
mode and return the link to the Centronics (uni-directional) mode
The 1284 compliant cable supports the ECP (Extended Capabilities Port) mode in the
transfer of data. The ECP protocol includes a series of protocols that differ from standard
Centronics parallel port operation. These additional signaling methods allow the host and
peripheral to negotiate any of faster transfer modes (e.g. DMA, FIFO and RLE
decompression). The protocol is hardware driven and the performance is limited by the
ISA bus bandwidth. The primary advantage is that once data transfer is negotiated, data
can flow without the need of an acknowledge or a return status signal. This can result in a
transfer rate of up to 10 times faster than that of the compatibility mode that the
Centronics parallel cable supports.
To transfer data, the host first goes through a negotiation phase, which allows the host
and peripheral to select a mutually-supported communications mode. During the
negotiation phase, the host indicates which communication mode and options it would
like to use via the Extensibility Request Value. If the peripheral device does not support
the requested mode or options, it sets the Extensibility Flag low and the interface returns
to Compatibility Mode.