The timing is designed to provide 3 cable
round-trip times for data setup if Data is driven
simultaneously with HostClk (nStrobe).
ECP PARALLEL PORT TIMING
Parallel Port FIFO (Mode 101)
Reverse-Idle Phase
The standard parallel port is run at or near the
peak 500KBytes/sec allowed in the forward direc-
tion using DMA. The state machine does not
examine nACK and begins the next transfer based
on Busy. Refer to FIGURE 29.
The peripheral has no data to send and keeps
PeriphClk high. The host is idle and keeps
HostAck low.
Reverse Data Transfer Phase
ECP Parallel Port Timing
The interface transfers data and commands from
the peripheral to the host using an interlocked
The timing is designed to allow operation at
approximately 2.0 Mbytes/sec over a 15ft cable. If
a shorter cable is used then the bandwidth will
increase.
HostAck and PeriphClk.
The Reverse Data
Transfer Phase may be entered from the Reverse-
Idle Phase. After the previous byte has beed
accepted the host sets HostAck (nALF) low. The
peripheral then sets PeriphClk (nACK) low when it
has data to send. The data must be stable for the
specified setup time prior to the falling edge of
PeriphClk. When the host is ready to accept a byte
it sets HostAck (nALF) high to acknowledge the
handshake. The peripheral then sets PeriphClk
(nACK) high. After the host has accepted the data
it sets HostAck (nALF) low, completing the trans-
fer. This sequence is shown in FIGURE 30.
Forward-Idle
When the host has no data to send it keeps
HostClk (nStrobe) high and the peripheral will
leave PeriphClk (Busy) low.
Forward Data Transfer Phase
The interface transfers data and commands from
the host to the peripheral using an interlocked
PeriphAck and HostClk. The peripheral may
indicate its desire to send data to the host by
asserting nPeriphRequest.
Output Drivers
To facilitate higher performance data transfer, the
use of balanced CMOS active drivers for critical
signals (Data, HostAck, HostClk, PeriphAck,
PeriphClk) are used ECP Mode. Because the use
The Forward Data Transfer Phase may be entered
from the Forward-Idle Phase. While in the Forward
Phase the peripheral may asynchronously assert
the nPeriphRequest (nFault) to request that the
channel be reversed. When the peripheral is not
busy it sets PeriphAck (Busy) low. The host then
sets HostClk (nStrobe) low when it is prepared to
send data. The data must be stable for the
specified setup time prior to the falling edge of
HostClk. The peripheral then sets PeriphAck
(Busy) high to acknowledge the handshake. The
host then sets HostClk (nStrobe) high. The
peripheral then accepts the data and sets
PeriphAck (Busy) low, completing the transfer.
This sequence is shown in FIGURE 29.
of active drivers can present
problems in Compatible Mode (the control
signals, by tradition, are specified as
compatibility
open-collector), the drivers are dynamically
changed from open-collector to totem-pole. The
timing for the dynamic driver change is specified in
the IEEE
1284 Extended Capabilities Port
Protocol and ISA Interface Standard, Rev. 1.14,
July 14, 1993, available from Microsoft. The
dynamic driver change must be implemented
properly to prevent glitching the outputs.
229
SMSC [ SMSC CORPORATION ]
