Stuart Baker Software offers high quality consulting services and products to users of IBM PCs and compatibles (DOS and Windows)
    as well as Interdata, Perkin-Elmer and Concurrent Computer Corporation 3200 series machines (OS/32).

Overrun Errors


We ran into an odd problem on a large order (over 300 copies of PC-Passport) from the National Weather Service. They were using Compaq Deskpro model 4000 PCs running Windows95, and they began getting frequent overrun errors almost as soon as they started testing. The problem showed up in both conversational mode and block mode, and persisted even when they cut the communication speed all the way down to 300 bps. These machines were using communications ports equipped with 16550 buffered UARTs, so the serial ports were not to blame. We were unable to reproduce the problem on any of our in-house machines, not even our ancient IBM XT, and when the customer substituted another brand (Gateway) for the Deskpro the overruns went away.

Obviously, then, the problem was in hardware, but convincing Compaq tech support to fix it turned out to be a time consuming process. (Unfortunately, the NWS had already paid for all the machines, so they had no leverage.) We found that the problem was related to the power saving ("Energy-Star") feature, because after we disabled it in the BIOS, PC-Passport ran perfectly -- but only under bare DOS; under Windows it still got overruns. At last we found that overruns would also occur in the Windows95 application Hyperterminal. That convinced Compaq tech support to stop claiming that the problem was in PC-Passport, and after several tries they came up with a new "bootleg" BIOS that cured the problem. In addition to installing the new BIOS, we had to remove one of their support programs that is run when Windows95 starts.

If you have a Compaq Deskpro 4000, and are having trouble with overruns, contact us and we can put you in touch with the proper people at Compaq. The corrected BIOS applies only to the Compaq Deskpro model 4000. The BIOS update is not a released product, and has no Compaq part number. It will never be publicly released, because if your system is using revision H or newer of the system board it does not exhibit the overrun problems.

If you are having similar problems on any other machine, especially one with Energy-Star features, contact us anyway -- maybe we can help.

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This site was last modified: Wednesday, December 29, 2010