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This article describes a technique for use in a personal computer (PC) system which prevents overruns (data transferred too fast) for direct memory access (DMA) parallel port transfers in the receive mode.
English (United States)
This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately
52% of the total text.
Optional Local Receive Start Fix during Direct Memory
Access Receive Operation
describes a technique for use in a personal
computer (PC) system which prevents overruns (data transferred too
fast) for direct memory access (DMA) parallel port transfers in the
Conventionally, for DMA parallel port transfer in receive mode,
if transmission does not stop at the DMA terminal count, or the
transfer rate is too fast for software to set up, DMA will overrun.
The optional local receive start was implemented so that the parallel
port controller could resume receiving bytes from the remote device
in DMA mode after a terminal count. The
hardware would receive the
last byte sent, acknowledge receiving the byte and then prepare to
receive subsequent bytes. The problem is
that the hardware removed
the '-AUTOFD' signal before it received the last byte. This would
cause the remote device connected to the parallel port (i.e., a
printer or scanner) to assume the system was ready for the next byte.
With this assumption the remote device would send the next byte
before the system had a chance to process the previous byte, hence
overwriting the last byte.
technique disclosed herein a description of the signals
involved for the optional local receive start is as follows:
+START_DMA During a receive from the
device attached to the
parallel port connector, this signal is
used to initiate an
'Optional Local Receive Start'.
+EOD This signal indicates that
this is the end of data
+DMA_REQ DMA request
-DACK DMA acknowledge