RS232 to RS485

Guys, this is more of a theory question that Im trying to learn here. Any info or experiences would be great.

Have what is called a GEHO pump. This pump is controlled by Siemens S5 PLC. Comms is one way, out of the S5 to a Yokogawa DCS. The data is transmitted out of the S5 in RS232, it then gets converted to RS485 by an Adam 4520 converter module, then the RS485 cable goes into the DCS.

Is RS232 and RS485 actual comms protcols like Ehternet, DH+, Profibus?

If so, are they a non generic brand, like an industry standard type of thing?

How do I go about learning how a RS485 protocol works and why? Can any one post some good notes or a link to a good website?

Im very black box when it comes to networks, use them but not sure of the underlying stuff.

Cheers

TFC

RS232 and RS485 are electrical standards. They define the signal lines and voltage levels. The basic difference between the two is that in RS232, all voltage levels are referenced to a single common. In RS485, each signal is transmitted over two wires, and the voltage level is measured between the two wires for each signal.

Neither of those two standards is a protocol. A protocol establishes the rules for encoding data. Modbus is a protocol. You can use the Modbus protocol on either an RS232 or RS485 cable.

Lets say you want to contact a customer in Quebec. Do you call on a landline or your cell phone? That's similar to the choice between RS232 and RS485. When the customer answers, are you going to talk in French or English? That's analogous to the choice of protocols.

Thabnks for the reply Steve.

This has rather confused me a bit though regarding how the PLC communicates to the DCS in this instance. Are they communicating on a very low level using the 232 and 485, or must there be a protocol of some description working above the 485 "layer"?

In my site documentation, there is no reference to a protocol.

You are right. There is another layer. Exaclty what it is remains to be seen. The RS-232/485 is just a description of the electrical characteristics, as Steve said.

I normally don't like it when people guess in replies, but I am going to anyway.

Since you said that the communication is one way, it may be that the PLC is sending simple ASCII strings to the DCS. The commands would be in the DCS manual. There is no protocol, per se, just a bunch of commands.

A protocol implies two way communication for error checking and verification.

Rick, after reading your explanation as it is only one way data I would tend to agree.

All the critical interlocks are hard wired through outputs to the DCS. The data comms to the DCS is for non critical data such as temperatures, levels in dampner vessesl, stuff like that. If the data lead is pulled the pump will still run.

I will try and find out more

Heres a cad drawing of the pumps in general, driven by a 500KW motor. Couldnt find a real photo.

This is the only other info I can find in my manuals.

The communciations link protocol settings are:
Baud rate 9600
Parity 1
Data bits 8

The comms card for the S5 is a:
Simatic S5 Special Driver for CP 521 Basic/SI

The Yokogawa DCS data card is a ACM12

I know this stuff probaly means nothing to you guys but Im just trying to learn more. Siemens S5s and these particular pumps that it is controlled by are one of my weaker points/skills and Im trying to overcome that.

Thanks

TFC

I did a Google search using Yokogawa ACM 12 and came up with a link to the manual GS33S03L20-010E_005.pdf. It lists a number of protocol options for ACM11/12 module, including Siemens.

From my experience with RS485/RS422 converters they not always the same on RS485 side.

1. RS485 most likely 2 wire differential pair, with TRXD+/- signals. So communication is a half duplex.
But some so called RS485 converters are really 4 wire converters with separate lines RXD+/- and TXD +/- which is really RS422 standard.
In most of cases you can just connect them together, but not always.

2. Termination: RS485 required 120 Ohm terminators on both ends and some times will not work without. Also you may need pull up and pull down resistors.

3. Some isolated converters using PC port power on RS232 side and will not work with other devices.

Check schematics for your converter to see if it matches device connections.

Yes, the convertor is stamped 232 to 422 but we are using it as 485 and yep there are 120 ohm resistors at each end.

B&B Electronics has some decent information about RS-232 and RS-485 standards and how to troubleshoot these types of installations.

I have used several of their line powered converters (232-485) with success.

http://www.bb-elec.com/tech_articles/rs422_485_app_note/table_of_contents.asp?PathId=593

Marc

A very handy site, thanks Marc and everyone for the help on this topic!

The manual of your communication processor:
http://www4d.ad.siemens.de/WW/skm/f...m=99&siteid=cseus&query=cp 521&searchinprim=3

Or search for ID 1109578

Rick Densing

it may be that the PLC is sending simple ASCII strings to the DCS

Click to expand...

This processor support ASCII communication.

RK512 protocol is a Siemens protocol.