Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Explain circuit usb to RS485 auto transmit

H

haohaodk46

Member level 4
Member level 4
Joined
Apr 8, 2012
Messages
76
Helped
1
Reputation
2
Reaction score
2
Trophy points
1,288
Activity points
1,804
Hi everyone ,
I have curcuit USB485 converter like this
64psjaxhkdap784i5dotokpwm.png


When TXD level = 1 => DE/RE = 0 => Max485 is receiver
When TXD level = 0 => DE/RE = 1 => Max485 is transceiver
=> So Why can it transmit automatically

This circuit is working
Thanks !
 
Sort by date Sort by votes
Hi,

haohaodk46 said:
=> So Why can it transmit automatically
Click to expand...
because of this:
haohaodk46 said:
When TXD level = 1 => DE/RE = 0 => Max485 is receiver
When TXD level = 0 => DE/RE = 1 => Max485 is transceiver
Click to expand...
These lines are the answer to your question. .. because it "automatically" controls DE.
(RE is not that important. Indeed I´d add a pullup resistor at the RE line. (even if there may be high ohmic one inside))

What else do you need to know?

Klaus
 
Upvote 0 Downvote
The circuit can be used for short range RS-485 connections. disadvantage that it drives TX idle (mark) state only with the help of bias resistors. With given 4.7k resistors, the driven level is below specified MAX485 input threshold and thus doesn't guarantee reliable operation. The circuit works only by chance.
 
Upvote 0 Downvote
Many USB-to-serial bridges support RS-485 auto enable, e.g. FTDI, Prolific. They enable transmitter for full character frame including stop bit.
 
Upvote 0 Downvote
KlausST said:
Hi,


because of this:

These lines are the answer to your question. .. because it "automatically" controls DE.
(RE is not that important. Indeed I´d add a pullup resistor at the RE line. (even if there may be high ohmic one inside))

What else do you need to know?

Klaus
Click to expand...
No I mean that :
When TXD level = 1 => DE/RE = 0 => Max485 is receiver . IT should be When TXD level = 1 => DE/RE = 1 ( because DE = 1 It is enable transmit )
When TXD level = 0 => DE/RE = 1 => Max485 is transceiver. When TXD level = 0 => DE/RE = 0 ( RE = 0 It is receiver mode )
 
Upvote 0 Downvote
Hi,

TxD idle is HIGH. And when idle, the driver needs to be switched OFF.

So this (not the best solution as FvM explained) is
* only driver ACTIVE when transmitting a LOW
* for the rest of the time (sending_HIGH or receiving or idle) the driver is OFF and the external resistors pull (too weak) a HIGH level on the bus.

***
If I had to do the job:
* I´d use a different USB_to_UART bridge with RS485 capability
* AND I´d choose external resistor values for valid levels.

Klaus
 
Upvote 0 Downvote

LaTeX Commands Quick-Menu:

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top