I have a few general questions about communications. i am currently pursuing my Engineering degree. However, there are too many things to focus on to. I hope by answering these questions I know where the road will lead to.

1. Are there standards for every type of communications (RS232,RS485 etc.)? If no, anyone can actually decide on whether they want which pins to represent. True or false? If yes, how important is it to learn all the different types of communication?

2. I know that serial communications exist. But what about parallel?

Thank you.

 

Parallel comms are available for some things but generally no.

Serial comms are available for a host of things. There are standards for RS232, 422 and 485 but the pinouts for industrial devices are quite often different. For example, I work regularly with devices that have Rx and Tx, RTS and CTS crossed over from the normal way of doing things. Instead of making a crosover cable, the cables finish up pin to pin.

Then there are all the protocols that run serially. Modbus RTU is probably the most common industrial serial network. Go here for details:
http://www.modbus.org. Then there is X15, X25 etc etc. A minefield.

 

Hello Steven;

The "RS" in RS-232 (and others) means Recommended Standard; it was meant to be an administrative agreement between comm. product manufacturers to enable easy links for serial comms. Since manufacturers tend to vary in their compliance to the standards (they are not enforced, as European standards are), you must adapt and study the connection diagrams from every model of comm. equipment you want to connect to your network. I can suggest at least one site that discusses the standard and links to more (there are a gazillion of them):
http://www.integrityusa.com/specs.htm

As far as parallel comms, there were attempts to develop them, such as IEEE488 (from HP). But the cost of the cable (up to 48 wires instead of the 2 or 4 in serial comms), crosstalk interference and short distances could not compensate for the gains in speed. IEEE488 is still used for high-speed data acquisition, mostly by National Instruments.

Hope this helps,
Daniel Chartier

 

There are standards for all of the serial communications I am aware of (that is quite a few), Since there are standards, you just can't go and decide which pin you want for what function - that is way there is a standard, which also relates to voltage levels. I would say you don't need to learn them all, but the primary ones, RS-232, RS-485, RS-422, these are good places to start. any others, pick up as your exposed to them. (that is what I do - unless you want to learn them all right now.)

There are standards for parallel port communications as well, you can search the web on these standards and become familiar with a few of them.

Matt

 

So, I would say that communication is really a broad area to cover, right?

 

On July 10, 2003 18:03, Daniel Chartier wrote: <clip>
> IEEE488 is still used for high-speed
> data acquisition, mostly by National Instruments.
<clip>

IEEE 488 (also known previously known as GPIB or HPIB) is used by many more companies than just National Instruments. It was originally developed by Hewlet Packard, which is why it was originally known as "HPIB" (HP Instrumentation Bus). It is mainly used in laboratory applications however, and is very rarely seen in industry except as part of a computerised test system.

In addition, I would not agree that it is suitable for "high speed data acquisition", except perhaps with very special instrument hardware which can buffer data locally. Most IEEE 488 instruments are really only useful for slower data rates, and where readings do not have to be coordinated between instruments. While the theoretical bus bandwidth is fairly high, the actual sampling rate is limited by the slow communications response time of typical instruments. Typically, the time required to actually transfer the data over the IEEE 488 bus is insignificant compared to the internal delays of the individual instruments, which have to be polled each in turn.

What IEEE 488 is particularly good at is allowing you to assemble groups of independent instruments quickly and easily into a working system. In other words, it is good for putting a lab set-up together quickly for an experiment.

--

************************
Michael Griffin
London, Ont. Canada
************************

 

The communications you mentioned are only the tip of the iceberg, RS-232/422/485 are as was mentioned here 'Recommended Standards', but they only specify voltage levels and signal characteristics, not mechanical connections. So you can see anything from terminal blocks, to 4 pin round connectors, 9 pin D shell, 25 pin D shell or anything else being used for serial connection. In many cases it is whatever would fit on the piece of equipment being built. Saying both devices use RS-232, just tells you they wont damage each other when plugged together, baud rate, parity, stop bits, and protocol form the language both are speaking. Then if you get the right signals headed the right way, things can communicate. RS-232 can happen with as little as 2 wires, Transmit and Ground, (assuming the other device has nothing to say back). Most often you need 3 wires or 5 if you want to use hardware handshaking. USB is becoming more prevalent on laptops, so you may need a USB/serial converter to talk to many industrial devices from your computer. As one fellow said, learn about what you need at the momment or else it can be overwhelming.

And you didnt mention Ethernet, another direction to think about.

 

RS232C was replaced a long time ago by TIA/EIA-232 (no longer RS = Recommended Standard, but a real one) - see http://www.eia.org and http://global.ihs.com
Likewise with TIA/EIA-422, 423 and 485.

232, 422, and 423 are for a 1-to-1 connection.
485 is for a multidrop connection of up to 32 bus members.
The story of how each is used (and misuesed) is long.

Meir

 

I quite agree with you, Roger about this phrase. I guess experience goes a long way then. But, when you need it do you get time to learn it? Thank you.

 

Take a look at Jan's excellent site: http://www.lvr.com/ He's written books on serial, parallel, USB, and even 8051 uP's

Regards
- LynnL, www.digi.com

 

If you are building both ends of the comm link, you *can* go and decide which pin you want for what function. The standards exist so you don't have to, not so you won't be able to.

The standards also exist so that one manufacturer can indicate, briefly and unambiguously, what is required to talk to their device. And so you can buy devices and controllers and cables from various vendors (that all adhere, more or less, to the same standard) and have a better-than-random chance of the pieces being able to communicate with one another.

Greg Goodman Chiron Consulting