IBM PS/2 Computer Hangs When Start SNA Server SDLC Connection

• Microsoft Windows NT Server versions 3.5 and 3.51
• Microsoft SNA Server for Windows NT, versions 2.0, 2.1, 2.11, and 2.11 Service Pack 1

SYMPTOMS

In certain instances, when you activate an SNA Server SDLC or X.25 connection on an IBM PS/2 computer configured with an IBM MPA/A (or the Microgate equivalent), the computer freezes and fails to respond to keyboard or mouse input.

CAUSE

There are two problems which cause this problem to occur:

• The IBM PS/2 abios disk controller tries to allocate all available map registers for large data transfers. The standard SDLC device driver allocates 2 when the SDLC connection is activated and never releases them.

  The disk controller hangs while waiting for the map registers. Slowly the whole system hangs as paging and disk access are stopped.

• The IBM PS/2 abios disk controller performs DMA data transfers at interrupt processor level. Microsoft Windows NT defines this as illegal (this may work on a single processor architecture, but in a multiprocessor environment it is dangerous, so Windows NT disallows it). The SDLC driver allocates a DMA resource, the disk controller generates an interrupt, and tries to acquire this DMA resource, and cannot, and never will be able to (the disk controller has just locked out the driver that owns it permanently). This causes the system to lock solid.

WORKAROUND

Both problems are solved by applying the alternate Ibmsync.sys driver (intended for PS/2 computers using the IBM MPA/A adapter or Microgate Microchannel equivalent), described below. If the default Ibmsync.sys driver is used, the first problem is solved through the support of a special DontUseMappedRegisters registry setting for the IBMSYNC driver.

As documented in the SNA Server 2.11 Release Notes (Readme.wri), a special version of the Ibmsync.sys driver supplied on the SNA Server 2.11 compact disc can also be used to solve hang problems that may occur on IBM PS/2 computers. To replace the existing driver, start the Windows NT Command Prompt and enter the following commands:

  cd <snaroot>\system
  rename ibmsync.sys ibmsync.211
  copy <sna_cdrom>:\i386\system\hwsetup\ibmsdlc\ibmsync._sy  ibmsync.sys
  cd hwsetup\ibmsdlc
  rename ibmsync.sys ibmsync.211
  copy <sna_cdrom>:\i386\system\hwsetup\ibmsdlc\ibmsync._sy ibmsync.sys

Using the default Ibmsync.sys driver, the following registry setting causes the driver to not request the use of mapped registers (though this version of the driver does not correct the other hang problem observed with PS/2 computers).

WARNING: Using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.

Open Regedt32.exe and locate the following key in the system registry:

   HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/IBMSYNC/Parameters

   DontUseMapRegisters: REG_DWORD: 1

MORE INFORMATION

If problems persist with the SDLC connection, a special traced version of Ibmsync.sys is included on the SNA Server 2.11 compact disc that logs additional failure conditions to the Windows NT system event log.

To apply the traced driver, start the Windows NT Command Prompt and enter the following commands:

  cd <snaroot>\system
  rename ibmsync.sys ibmsync.211
  copy <sna_cdrom>:\i386\system\hwsetup\ibmsdlc\debug\ibmsync.sys

This special version of the driver logs internal driver errors when sending/receiving information to/from the IBM SDLC/MPAA or Microgate adapter. These new errors are logged in the Windows NT system event log as event 33 or 34.