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[SOLVED] 750C serial port max bit rate?

Posted: Sun Oct 06, 2019 10:29 am
by MooNWalker
I've been playing around with some very old OS/software on my 750C, and part of the setup was getting network to work. Since the OS (very early, pre-2.0 Linux) doesn't yet support any of the PCMCIA network cards I have I've tried using PPP/SLIP with null modem cable. The setup works, but the maximum bit rate I was able to set without crazy high (50% and up) packet loss was 19.2Kbps. As I don't yet have any other way to test the cable (I'm using this one) and it seems other customers don't have the same speed problems I've been suspecting the laptop itself, but I can't seem to find the specification for the maximum bit rate the serial port on 750C supports.

So my question is this - is it reasonable to suspect that the problem is with the serial port limitations on the TP750C, or it should work at something like 115.2Kbps without any problems, and any packet loss at speeds higher than 19.2Kbps is most likely due to a low quality cable?

Re: 750C serial port max bit rate?

Posted: Mon Oct 07, 2019 10:52 am
by MooNWalker
K, I'm not sure why, but while set to 38.4Kbps I've been getting a buttload of lost packets, but bumping the speed up to 115.2Kbps suddenly everything works fine, with packet loss being mere 1-2%. I don't understand why the packet loss would peak out at certain speed and then drop when speed is raised further, but that's the behavior I'm observing. Regardless, since 115.2Kbps is definitely better than 38.4Kbps and 750C obviously supports that speed this question is effectively answered.

Re: [SOLVED] 750C serial port max bit rate?

Posted: Mon Oct 07, 2019 11:29 am
by axur-delmeria
That's a strange finding indeed. The only idea that comes to mind is some kind of packet timeout issue due to the slow (but not too slow) 19.2-38.4 kbit/sec speed. 115 kbit/sec is the usual maximum for serial ports using the 16550A UART (or functional equivalent). I remember maxing out at this speed transferring files between our Compaq Contura Aero and Thinkpad 760XD using Fastlynx. If you need faster speeds you could try PLIP.