connecting multiple RS232's to one PC?

[panic mode]

lookup "rs232 optical isolation" using google for example.

we have done many applications that used PCs with multi port serial cards (usually eight port cards making for total of ten ports per pc). we have found out that using non-isolated cards works sometimes but often means asking for trouble. (chance of something going wrong is proportional to number of connected ports).

one part of the issue is that 99% of the rs232 cards are not isolated. bigger problem is that most of those claiming to be isolated are not truly isolated, they usually only isolate two or four signals of the port (each port has 8 signals, which means that most handshake signals not isolated).

 

[danw]

one part of the issue is that 99% of the rs232 cards are not isolated. bigger problem is that most of those claiming to be isolated are not truly isolated, they usually only isolate two or four signals of the port (each port has 8 signals, which means that most handshake signals not isolated).

Ooooh. That's a disaster waiting to happen. I was unaware of vendors'claims of isolation, but the failure to actually produce such.

Do you have a recommendation on fully isolated multi-port RS-232 (or 485) cards?

Dan

 

[panic mode]

i like ME9000 from meilhaus (US distributor is www.icsdatacom.com).

note that there are different versions ("blank" for no isolation, "I" is isolation, "P" is potential-free; --> hint go with "I" instead of "P" because although "P" isolates PC from field devices, all field devices share one common).

 

[danw]

Thnaks, Dan

 

[Brian123]

B+B Electronics also offers isolated serial cards. www.bb-elec.com

Brian

 

[panic mode]

if you are thinking of 3PCIOUx cards, maybe - they are new and I don't see much details. i would be careful and suggest getting enough of information before making purchase.
same sparse information (from B&B) was reason we got bitten once. same problem, they were also skipping handshaking lines as mentioned above (this was before they got fully isolated solutions like 232OPDRI).

 

[sabihakber]

greetings

as a relative beginner in this field, can some1 plz tell me wat the after effects are of using non-isolated RS232s

secondly, the DATEXEL Analog to Digital Convertor im using has a built-in RS232 port, will the isolation problem occur in the port or in the cable with which i'll connect the ADC to my PC board?

 

[danw]

RS-232 is voltage based with a ground reference, not differential, like RS-485

Grounds at various points in the plant can vary in potential with respect to one another. If the grounds vary in potential from one another, current flows between the grounds, creating a ground loop.

The multiple RS-232 port cards offer increased possibilities of ground loops.

Ground loops can cause data errors or with high enough potentials, damage a port.

The problem is connection of grounds at different potentials, so the cable or wire between the grounds are conductors, not the cause of the problem. The evidence of the ground loop problem shows up at the port, as either data errors or blown ports.

 

[panic mode]

wat the after effects are of using non-isolated RS232s

well it may:
- just work fine
- cause one or another device to lockup
- smoke one or the other device

so far didn't have the last case but plenty of the first two.
so far locking up would not happen during boot etc. it would happen once the application tries to use the port. the only way out is to press power button on PC for 4 or 5 seconds to force shut down.