Knowledge Base

Article ID: 60717 - Last Review: December 28, 2014 - Revision: 1.2

This article was previously published under Q60717
This article has been archived. It is offered "as is" and will no longer be updated.


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.


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.

  • DCA/Microsoft Communications Server 1.0
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