Hope this is the correct forum..

We have sporadic communication problems on serial. To test this, I connected two ports of the same card with an null modem cable, set up the interface (4800 8N2, because this is the setting of the device we have trouble with) and send a small binary file (~270KB) with zModem over this.

In this setup I got 1-2 CRC errors per minute - so, the data is corrupted. (This is much more worse then in the 'good old' mailbox time with much faster modems!)

I tested this with three different cards (with different chips: SUNIX, Moschip, Oxford, all PCI-e 1x, no IRQ conflict) and two different cables.

If I connect one end to an USB serial adapter (Asix), there is no problem, also when connecting the two ports on the USB serial adapter.

I can not believe that three different cards are defect. Can anybody give me an hint where to search? (OS: Windows 10)

  • Have you tried using different slots? A different PC box? "an 0 modem cable" -- Do you mean a null modem cable? This side of the Atlantic reads "0" as "zero". How long was the cable? Did you try just short wires between pins?
    – sawdust
    Jun 27, 2018 at 7:02
  • Thanks for the suggestions. A co-Worker brazed a cable for me (connect Pin 5 and crossed 2 and 3) with a 1 Meter cable. The data flow was much more worse (I could not get the exact numbers of CRC because I need another program not displaying the number). Another Slot (with an original 1,5 meter fabric cable) on main board did not help - the same CRC rate.
    – Ralph Erdt
    Jun 28, 2018 at 6:35

1 Answer 1


It seems, the program I've used so far is shit. I've written a test on my own: It send a big buffer (4K, every round another random numbers) in both directions (every direction other random numbers!) at the same time a few 100 times. So the ports are at maximum at there transfer possibility. No data problem so far.

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