USB-RS485 converter - change TX enable time?

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I've got a generic ebay USB-RS485 converter that uses a PL2303 chip. No doc whatsoever. It seems to wirk fine (I can enumerate it, config it, open it and send data from a C# test program), but the TX enable is based on some internal timing (digital? RC network?) and the TX enable turns off when multiple '1's are sent. I can see this as the T+ line droops down after 2-3 bit times (19200 baud).

I'm not even sure this is a problem in a real RS485 network with weak pull-up and pull-down resistors on the lines. But it does make me cringe. I'm wondering if anybody has experience with PL2303 devices and whether there is some way to tie the TX-disable time to the baud rate.

Any and all comments welcome.

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I use the Advantech ADAM 4561 drivers for RS485 work (these have a pl2303 in them). I've never looked closely at the bus turnaround probably since I've not had a problem! Mind you, most of my work is using MODBUS RTU that frames the data with a 3.5 char time idle.