non-standard baud rates with Prolific 2303 USB-serial

["Michael G. Katzmann" <michaelk@IEEE.org>]

I am trying to use a USB serial interface with a Prolific chip to connect to an old ASR 33 Teletype at 110 Bd.

The USB device will work at 110 Bd in Windows 10 but not under the Linux driver. The USB PID/VID is 0673 / 2303 which corresponds to

static const struct usb_device_id id_table[] = {
    { USB_DEVICE(PL2303_VENDOR_ID, PL2303_PRODUCT_ID),
        .driver_info = PL2303_QUIRK_ENDPOINT_HACK },

in the pl2303 driver. The Manufacturer of this device is 'Sabrabt' (CB-DB9P) and I believe it's a genuine chip device (and not a clone).

The Linux driver does not seem to produce sensible baud rated for other than the 'supported' rates.

stty to 110 bd results in ~95,000 bd (that's not a typo but 95 thousand). Other rates like 200 also produce odd speeds although not in a logical manner.

The data sheets don't describe the formula used for deriving the four bytes used to set the speed.

I tried adding 110 to the supported rates but this did not produce the correct baud rate so I presume the Windows driver is using a formula different than the one in the Linux driver for 'non standard' baud rates.

Does anyone have any suggestions to get this working at 110bd or what other experiments to try?

- Michael