USB-RS485 converter - change TX enable time?

Log in or register to post comments
Go To Last Post
2 posts / 0 new
Author
Message
kk6gm
kk6gm's picture
Level: Raving Lunatic
Joined: Fri. Sep 11, 2009
Posts: 4022 View posts
Location: Sacramento, CA
#1
Posted by kk6gm: Tue. Jan 3, 2012 - 01:20 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

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.

Tags:

Learning and Information, General Electronics
Top
Kartman
Kartman's picture
Level: 10k+ Postman
Joined: Thu. Dec 30, 2004
Posts: 24979 View posts
Location: Melbourne,Australia
Posted by Kartman: Tue. Jan 3, 2012 - 03:11 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

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.

Top