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Q60717: Using IBM Adapters and DMA Support on Full Duplex Lines

Article: Q60717
Product(s): Microsoft LAN Manager
Version(s): 1.0
Operating System(s): 
Keyword(s): 
Last Modified: 08-NOV-2001

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The information in this article applies to:

- DCA/Microsoft Communications Server, version 1.0 
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SUMMARY
=======

When you use an IBM SDLC adapter for an X.25 link, a Direct Memory Access (DMA)
error appears in the connection trace in response to the SET LINK
CHARACTERISTICS command.

This warning message is being displayed because the SDLC and X.25 link services
always ask the adapter driver to use DMA. If the driver has no DMA support, or
can't provide full duplex DMA, it returns a warning to the link service in the
form of a return code to the SET LINK CHARACTERISTICS command. The link services
log this return code at level 6 and continue.

IBM SDLC adapters have only one DMA channel. X.25 is a full duplex protocol,
which means that data is normally being both transmitted and received at the
same time. This would require two DMA channels on the adapter to use it in DMA
mode. Therefore, since DMA can't be used, the driver falls back to using
character interrupts when the protocol requires simultaneous send/receive. In
practice, per-character interrupts can be sustained at 9600 baud, so this
warning does not necessarily indicate a problem.

MORE INFORMATION
================

Direct Memory Access (DMA) is a hardware feature that permits adapters to write
directly to memory rather than requesting that the CPU transfer the data. A
non-DMA adapter must interrupt the processor with each byte of information,
whereas a DMA adapter places the information in a predefined memory location and
interrupts the processor when a complete block has been transferred. An adapter
operating in DMA mode is less demanding on the CPU.

In the case of SDLC connections, a PC AT type machine can support DMA to the SDLC
adapter, or to one of the MPCA adapters. The PS/2 can support DMA to both
adapters.

Additional query words: 1.00

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Keywords          :  
Technology        : _IKkbbogus kbSNAServSearch kbDCAMSCommServ
Version           : :1.0

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