rs485/rs422 and rs232

Javalin

Hello all,

Does anybody here have experience with RS485/RS422 networks?· Reason for asking is I am looking at using these standards to implement a bus with several rs485/rs422 - to - rs232 devices.

Thanks

James

stamptrol

·· RS485 is the way to go for a rugged and expandable network. You'll need to understand a few basics and I recommend Jan Axelson's book "Serial Port Complete". It will very quickly get you up to speed on whether to go half-duplex or full-duplex and some of the cabling requirements.

· The interface chips are cheap and work well with Stamps. The 75176 chip is a good choice to experiment with and there are others which offer more overvoltage protection.

·· We've built several large Stamp based monitoring and control systems which communicate via RS485 on several thousand feet of twisted cable. See the "Wastewater Treatment Plant" in the Customer Application section of the Parallax site.

·Cheers

· Tom Sisk

Javalin

Hi Tom,

Thanks for the info.

I had a look at the article you suggest, but there is no technical detail...

Couple of questions if I may:
1) I guess the RS422/RS485 provides only the physical layer, addressing, stop and start have to be added manually?
2) Can you run different speeds on the bus? I.e. have a serial (via line driver) at 115k and other serials (again via line driver) at 19k? I guess some of this is dependant on answer 1.

Do you know of any on-line (i.e. free) sources of info on practical utilization of the standards?

Thanks

James

stamptrol

··· Hi,

····· You're right, when we speak of RS485, etc we're really describing the physical characteristics of the network. That includes the voltage levels, the cable characteristics, etc. The speeds can be anywhere from 300 baud all the way up to 115k and above, depending on how fast your processor can process things. Obviously all devices on one bus must talk/listen at the same speed.

··· It sounds like you're more interested in a protocol, which describes the way that data is transmitted/received. That would include things such as how many bytes make up a message, any type start-of-transmission byte, the type of end-of-transmission byte, how the checksum (if used ) is calculated.

·· You're going to find a great number of protocols out there. Unless you're trying to interface with a specific device which has a particular protocol, it really up to you to generate a suitable protocol. Some popular ones include· Modbus, Datahighway, and Toslink to name only three from large manufacturers. Each of these can be implemented in RS485, RS232, or fibre-optic, depending on what teh system will be used for.

· Tom

Javalin

Hi Tom

Thanks. Probably going to adapt the SNAP protocol - its free and a bit generic!

http://www.hth.com/snap/

Cheers

James