RS422

Jeprox · Guest

If I have a source with rs422 connection and I need to connect this to 2 other devices with rs422 connections as well, what do I need?

Can I just directly wire up both to the same source? ie TX+ of source connects to both RX+ of the receivers, TX- to RX- etc. without a need for other external components?

Source TX+ --------------------------Destination1 RX+
|
|--------------------Destination2 RX+

Source TX- --------------------------Destination1 RX-
|
|--------------------Destination2 RX-

This relates to an interface project. I'm going to use a 16F877 and use its hardware UART, and translate the rs232 into an rs422 with external devices.

Ttelmah · Guest

Guest · Posted: Wed Jun 30, 2004 4:32 pm

Thanks.

Firstly, I'm not sure I understand by you saying "at each end of the entire link (not at the individual devices), with 120ohm between the lines, and a 1k2 from the 'A' line to your +ve supply, and a 1k2 from the 'B' line to the -ve supply."

Perhaps, if it's not too much of an ask you could provide a schematic on this or point me to a site that shows this.

Secondly, what about this terminator chips? How is this used?

The last thing I want is not getting it right. The dilemma I have is that I don't have the receivers with me and the only time I can get this to test is at the customer site. So, there is no room for error (basically) as the interface will have to be made beforehand.

Thanks again, Ttelmah.

Jeprox · Guest

The previous post was from me. Sorry.

Neutone · Joined: 08 Sep 2003 Posts: 839 Location: Houston

Think a single run of wire the stops at one device and goes on to the next. This is multi drop and should be terminated on the ends. The MAX485 data sheet should have some pictures you could look at.

Jeprox · Guest

Which line would have the "three resistors at each end of the entire link" - TX or RX or both?

By saying "at each end of the entire link" does it mean that both at the Master and Receiver end (I assume it's the last reciever in the daisy-chain) will have the "three resistors."?

I've seen "general" drawings on the net that only shows termination at the last Slave and none on the Master side.

I'm still a bit confused.

Jeprox · Guest

Ttelmah, on the slave sides I don't think I would have access to the +ve and -ve.

I've looked at the following site and an example. see fig. 1.7 Page 8 of the following link that shows an example of multidrop.
Is it also necessary to wire up the ground side (it shows the chassis gnd, power gnd and signal gnd)

I would assume Rt would be 100R. I intend to use a CAT5 cable.

http://nle.ixbt.com/hard&soft/485appnote.pdf

Thanks in advance.

dyeatman · Joined: 06 Sep 2003 Posts: 1912 Location: Norman, OK

Here is a very comprehensive site on RS485/RS422 that should give you a much better understanding of RS422. It covers all the details...

http://www.bb-elec.com/technical_library.asp

Especially look at the RS485/RS422 Application note near the top of the list