PLC Communication Channels

B

Thread Starter

Brian

I am an undergrad industrial engineer that needs to find the baud rates and applications for the following communication networks: RS-232 (Serial), Data Highway, Ethernet, DeviceNet, and ControlNet. I have some information from the search I performed, but feel that I'm not
getting everything. I would appreciate any help you can provide.

 
Serial can be anything from 300 baud to 115k baud. Uses are endless. One website with some serial info is www.modbus.org.

DeviceNet can be one of 3 speeds, based on distance requirements. 125kbps is the slowest, 250kbps next, and fastest is 500kbps. Look at
"www.odva.org":http://www.odva.org for more detailed info.

Ethernet can be 10Mb or 100Mb depending on the devices used (hubs, routers, etc.) and cable quality. Uses for ethernet are almost as
diverse as uses for serial, but it's much faster. Not sure what website can give you more detail on ethernet

CK
 
L
> > Searching for baud rates and applications for the following
> > communication networks: RS-232 (Serial), Data Highway, Ethernet,
> > DeviceNet, and ControlNet...
>
> Serial can be anything from 300 baud to 115k baud.

True, but do note this trivia ....

EIA-232 limits speed to 19.2 to maintain the correct wave form (rise-time, width of bit 0/1 level, and fall-time). Anything over this requires either a non-standard "slew rate" (shortened rise/fall-time) or it becomes a saw-tooth. I've played with RS-232 and an oscilloscope and it's interesting to see the variations from various vendors. I'd say a
generic example is 9600 baud makes a wonderful +/-9v near-square wave, while 115kbps is a wimpy +/-3v saw-tooth wave.

So RS-232 at speeds above 38K I'd say "Buyer beware" - you may discover subtle vendor-X to vendor-Y reliability issues. There are companies out there advertising RS-232 at 460kbps and above! It would be interesting to see what that looked like on an oscilloscope.

Also, many PLC put serious surge protection/filters on their RS-232 pins. These tend to also limit speed. For example, the one big-league PLC I tested (brand not important) claimed a max speed of 19.2 - but one could fudge it to 38K where you could see that it had an 18% bit-error rate due to the surge suppression (& why they said 38K and above wasn't supported).

Best regards

Lynn August Linse, Senior IA Application Engineer
15353 Barranca Parkway, Lantronix Inc, Irvine CA 92618
[email protected] www.lantronix.com
Tel: (949)300-6337 Fax: (949)453-7152
 
I agree completely that using higher serial baud rates is a buyer beware type of situation. I don't try to go above 19200 with my PLCs. I prefer to just leave it at the default 9600. I was simply stating the maximum parameters of
serial communications.

It's like saying my car can go 140mph. Chances are you're not going to try it...

CK
 
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