PCI/AGP bridge latency causes serial port CE_OVERRUN error..

Archived from groups: comp.sys.laptops,comp.sys.ibm.pc.hardware.chips (More info?)

Hi Gurus,

I have an IBM ThinkPad T21 Laptop (Pentium III-750Mhz) running Windows
2000. Using serial port COM1 at 115kbaud, it experiences CE_OVERRUN
errors from ClearCommError in the Win32 API. This indicates that the
serial port is losing bytes due to a FIFO overrun in the
16550-compatible UART. It happens even with "Use FIFO buffers" and the
lowest Receive Buffer threshold setting in the Advanced Settings
control panel for COM1.

Minimising/maximising Internet Explorer, resising the window or
dragging it around is enough to cause the occasional CE_OVERRUN error.
By running SiSoft SANDRA (http://www.sisoftware.net/) I found that the
PCI Latency Timer for the PCI/AGP bridge appears to be set too high
(128), which would cause servicing of the UART to be blocked for too
long when performing video accesses. I can't find a BIOS setting to
reduce this latency timer value, and none of the PCI Latency Timer
setting utilities I've found on the 'net so far seem to run on Windows
2000. Does anyone know any way to adjust the Latency Timer setting, or
another fix to this problem?

Thanks,
Graham
Computer Systems Engineer

Archived from groups: comp.sys.laptops (More info?)

bobb wrote:
> Do it HAVE to sustain this data rate? Can you not just enable
hardware
> flow control?

Tragically, the device I'm communicating with doesn't support hardware
(or software) flow control; so I'm stuck with finding out why an
only-moderately-old laptop can't sustain a lousy 115kbps without
dropping the occasional byte or two.

Thanks,
Graham

Archived from groups: comp.sys.laptops (More info?)

> Tragically, the device I'm communicating with doesn't support
hardware
> (or software) flow control; so I'm stuck with finding out why an
> only-moderately-old laptop can't sustain a lousy 115kbps without
> dropping the occasional byte or two.

Windows is not intended for realtime applications. This is not a bridge
issue, it's an OS issue.

By the way, if your HDDs are not currently using DMA, this can
exacerbate the problem.

Archived from groups: comp.sys.laptops (More info?)

Well, my theory about excessive PCI bus latency settings sounded good
at the time. But I set all the latency timers in the system to 0 with
the PCI Latency Tool; and I can still cause CE_OVERRUN errors in the
UART by scrolling windows on the screen. I've already upgraded the BIOS
and the video drivers. I'm clean out of ideas; anyone got any other
suggestions?

Thanks,
Graham