serial communication(more clarifications)!!

1.Why RS 232 cannot be used for multidrop applications?
2.Why RS 232 can be used only up to 15m but RS 485 for longer distance?
3.Which factor determines the data rate of serial communication, electrical standards(like RS232,485 etc ) or protocols(modbus,TCP/IP..)?

4.Why there are seperate protocols for each level in communications used in a process industry?
eg:
Enterprise--------Ethernet
Control-----------HSE,ControlNet
Fieldbus------------Fieldbus,Profibus PA
Devicebus-----------DeviceNet,Profibus DP,Interbus
Sensorbus-----------CAN,ASI,Seriplex

why not a universal protocol with all the good features of existing ones??
Thank you,
Abhilash

Abhilash

1. Why can my car not load 25 tons or even more? I could save money on transportation costs.
2. Why can my car not drive at speeds over 160 kph? I could get anywhere much faster.
3. Why can my car not fly? I could surely use this ability to avoid traffic jams.
4. Why are there separate means of transportation such as cars, trucks, vans, airplanes, helicopters, boats and not one transporter which combines all possibilities?

These simple questions should be answer enough. The question you asked cannot be answered, because there is no simple answer.

jvcande,
Limitations of your car- smaller size,less power and no wings.
Hence it can't do whatever a truck/plane can.

Limitaions of RS 232- X,Y,Z...
Hence it won't support whatever RS 485 does.

If there are corresponding terms for these X,Y,Z..then that would clear my doubts in a better fashion.(Is that the difference in signal level or ....)

Thanks,
Abhilash

I don't think you understood Jean Pierre's Analogy...

THIS LINK should provide hours of enjoyment for you. It will also answer a lot of your questions to boot!... :nodi:

beerchug

-Eric

Eric,

thanks a lot for the link. It's a very interesting one, especially since no-one (and that does include me ) knows everything.

Abhilash,

I simply meant there is no simple answer. I deal with some of your questions in a half-day seminar I'm giving when people come here to study PLC communication. But I'm getting paid for that task. I don't think my employer would be happy if I would spend time on the internet giving away stuff for free when he's making money with it. I would soon be in the same state the majority of my pupils is in: unemployed.

Regards,

ONE MORE TIME! You must not have read this document, which I linked to in another thread by one of your classmates yesterday. It won't just jump out and answer your questions, but #2 and #3 are in there.

#1 is a function of number 2 and 3. You don't multidrop RS232. But, it's not because it's impossible. It can technically be done. But RS232 has very little noise immunity and will only travel short distances (without too many errors). If you load up the electrical system, you will add more failures.

#4 - Think about the message block of each protocol. Some have special messages (Profibus, H1, ASi-Bus) that make them BETTER SUITED to specific types of devices. ASi-Bus is intended for discrete signals like switches, for example. So you can wire up a string of contactors and start/stop them. The other cool thing about ASi-Bus is that it uses a flat cable and the devices have vampiric connections. There's no screwing around with wires. This makes field installation a breeze (in theory).

For higher level communication, like HMIs, you want large blocks of data to transfer at once. Fooling around with one bit at a time just wouldn't cut it. So forget using Asi-Bus between your PC and your PLC network.

To answer the final part of your question: it's about OPTIMIZATION. One size fits all (modbus) is nice, but it's not the most efficient way to transfer data. Sometimes you care more about the "quality" of the network than you do about the friendliness of it.

AK

jvdcande said:

Eric,

thanks a lot for the link. It's a very interesting one, especially since no-one (and that does include me ) knows everything.

Click to expand...

I posted that link as a distraction for Abhilash (and it seems to be working!), but it does have a lot of info. Glad you found it interesting.

Actually, that whole ePanorama site has lots of good links (on lots of stuff!). I have it bookmarked...

beerchug

-Eric

hi guys,
Thanks a lot for all the links.'epanorama' site is really a good source of information.
cheers,
Abhilash