RS485 and arduino

Using Arduino Networking, Protocols, and Devices
1

Hi. i want to have communication bettwen PC and arduino (via serial monitor). to 10m distance.

So i decided to use a USB to RS485 converter. i will connect a MAX485 to arduino and i will connect a 10m cable from A to A and B to B (between each MAX485)

I'm gonna buy a simple USB to RS485 device (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=390336868348&ssPageName=ADME:L:OC:US:1123#vi-desc) for that.

my question is simple.
How can i control trasmit/recieve mode (DE/RE pins) of MAX485 from the PC?
from arduino side i know the way. simple wiring DE and PIN with an arduino's PIN. but i dont know how to do that in the other side? is it posible to do that in some way on FTDI chip?

2

That converter shows that you can use it with a multi-drop layout, in that case they must have provision for the right control. I'd get onto them for an explanation or don't buy the device.

As this isn't a multi-drop situation you can run full duplex, in which case you don't need any DE/RE control in the first place, just a converter that does full duplex.

All that said you probably don't need any of the above for 10M, you can get active USB cables that will do that length (I think). I'm posting this right now through a 5M active cable, they may make longer ones (or use 2).

Rob