Beebem, Serial Ports, Modems and Baud Rates

[marrold]

Hi all,

I've been experimenting with Beebem and an external USB adapter connected to a Modem. It's working reasonably well but it has raised a question.

Modern serial adapters (an FTDI in my case) don't support different baud rates for Rx / Tx, and don't support 75 baud. 300 is as low as they'll go.

When I use *CVIEW in Commstar II it automatically sets the Baud rate to 1200/75, so I'm curious how its working when the USB adapter doesn't support it.

This might be a more of a generic serial question but if anyone has any insight it'd be appreciated.

Cheers
Matthew

[SteveF]

I'm definitely no expert here, but I have an idea that PCs in general didn't handle split baud rates like 1200/75, even back in the day. (Because in the US 300/300 and 1200/1200 were common, but 1200/75 wasn't?) So modems designed for those PCs would work at (say) 1200/1200 talking to the local PC, but 1200/75 on the phone line. It might be something like this happening with your modem/adapter here. (Edit: the more I think about it, the less convinced I am that this is what's happening here. But I'll leave the post up anyway...)

I think modems intended for use with the Beeb probably didn't do this, as there was no need for the extra complication when the Beeb itself could do 1200/75.

[marrold]

Modern modems will do some buffering so they can cope with the serial baud and connection over the telephone being at different baud rates. In this instance I was using a "modern" USR Courier to connect.

I think you're right about the modems of that period (Pace Nightingale, Miracle WS2000 etc) though, they'd just operate at the same rate as the connection over the line.

Cheers

[sweh]

When I use *CVIEW in Commstar II it automatically sets the Baud rate to 1200/75, so I'm curious how its working when the USB adapter doesn't support it.

I looked at an old version of the Unix beebem code (maybe based on v3.2 of the windows version?), and it has a section

Code: Select all

        Tx_Rate=Baud_Rates[(Data & 7)];
        Rx_Rate=Baud_Rates[(Data & 56)>>3];
        // Note, the PC serial port (or at least win32) does not allow different transmit/receive rates
        // So we will use the higher of the two
        if (SerialChannel==RS423) {
                HigherRate=Tx_Rate;
                if (Rx_Rate>Tx_Rate) HigherRate=Rx_Rate;

[marrold]

Thanks for that, that would make sense.