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Re: [SOLVED] Serial Connection

On 03/04/11 02:07, Stephen Powell wrote:
> On Fri, 01 Apr 2011 06:02:38 -0400 (EDT), MAROUNI Abbass wrote:


> I also used to have an IBM PS/2 Model 70.  The UART on its motherboard
> is a 16550A.  According to the manufacturer's specifications, the
> 16550A is capable of operating at speeds of up to 256 kbps.  Due to the
> hardware design of the PC et al, 115 kbps is the maximum bit rate that
> can be programmed by software.  But by trial and error experimentation,
> I found that this UART could be driven at a maximum rate of 38,400 bps.
> If I tried to go any faster, I would get garbage or dropped characters.
> In this same machine was an expansion board (32-bit microchannel bus)
> which contained an extra serial port.  It also has a 16550A UART.  But I
> had no problems driving it at 57,600 bps.  Perhaps it would have been
> capable of higher speeds than that, but I didn't have a fast enough
> device to test it with at the time.
> The point is that all serial ports are not created equal.  The most
> common PC serial ports use 16550A UARTs, but older and slower UARTs
> are used in some older boards.  Even if the UARTs are the same,
> the actual maximum bit rate can be lower than the manufacturer's
> specs, depending on what limitations are imposed by the supporting
> circuitry.

Ahh the PS/2!
The precise scenario I was thinking of when I said "There are/were some
problematic serial chipsets" - I couldn't remember off-hand the exact
chipset, and was hoping I wouldn't have to dig through old notes to find
it. (thanks Stephen)

I owned a number of PS/2s - I still regret parting with the 9595 :-/
It was on those machines that I first used home-made three-wire cabling
to transfer data.

One day I'll have the time to rebuild my PS/2 collection and make use of
MCA cards from IBM mainframes - and maybe fix/update the site in the


A site about PS/2s
It's old, un-maintained, and you'll need to enable javascript - and wait
to get past the IBM diagnostics screen :-)
Note: the email address is defunct, as are many of the links.

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