Long Range transmission
I’m interested in communicating between stamps over a long distance. Say 100 meters or so. Would it be ok to use the built in serial transmission to accomplish this? Or would I need to come up with something fancier to transmit over this distance? I would most likely be using cat5e cable. Any thoughts?
Comments
An alternative is to use RS423 chips (sorry, don't have any numbers) which should be good to 4,000 feet (1300 meters).
I suspect the reference should have been to RS-422 not RS-423.
Regards,
Bruce Bates
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Chris Savage
Parallax Tech Support
csavage@parallax.com
http://forums.parallax.com/showthread.php?p=595191
RS485 is half duplex differential on one pair of wires and RS422 is full duplex differential on two pairs of wires. Both operate from a single 5 volt supply without a voltage multiplier or +/- power supply, which makes RS485 or RS422 chips simpler as well as more capable for distance and speed. Fault protection is a desireable feature no matter what protocol you select.
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Tracy Allen
www.emesystems.com
·· Please feel free to correct me if I'm wrong...It has been awhile.· I thought RS-422 was a single-driver, multi-receiver network, whereas RS-485 is multi-driver, multi-receiver?· I guess I always thought the RS-485 was the better differential scheme, but if I am incorrect I need to do a little more homework, because I need to update and old project using it.· That project used half-dulplex communication, but with the Propeller may in fact become a full duplex scheme.· Oh, it's been too long...
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Chris Savage
Parallax Tech Support
csavage@parallax.com
In RS485, the driver is capable of going into a tri-state mode, so the same pair of wires can both transmit and receive, which makes it most suitable for a multdrop system with both multiple drivers and receivers. A simple RS485 system operates with one driver and one receiver operating in turn over a single pair of wires, i.e. half duplex. The RS422 setup would requrie two pairs of wires, with one pair dedicated to each direction, but both directions could be talking at the same time, i.e. full duplex.
There are other details of the specifications, such as the impedance levels and common mode mode voltages allowed and the number of receivers. RS485 receivers are supposed to have higher input resistances and therefore the system can have a larger fanout.
A lot of modern driver chips are advertised as good for either RS485 and RS422, because they include the tristate enable for the driver, which is the essential difference. For RS485 you use it, but for RS422 you leave it enabled all the time and treat driver and receiver sections of the chip separately. As receivers they have quite high input resistances and leave it up to you to add an impedance matching line termination if and where necessary.
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Tracy Allen
www.emesystems.com
Post Edited (Tracy Allen) : 8/22/2006 1:49:14 AM GMT
·· Thanks for elaborating...I used an RS-485 system in one application (Alarm System) which had remote panels.· These required both isolation and distance.· I figured as long as we were on the subject I would make sure my memories were right.· I guess I will stick to RS-485 for simplicity and since I've used it at least once before.
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Chris Savage
Parallax Tech Support
csavage@parallax.com
···We built the network part of·a security·system (with help from·some colleagues) back in 1997 (even though the main system had been ready since 1994)...I almost think we used chips from Maxim, but that may be just because there is so much talk about them.· I know at the time I was using at least 1 Maxim part consistently (MAX232) and several Dallas chips in various designs.· Of course, that system (if still in operation) is in NY so I have no way to go back to it and see what we did.· But check the Maxim website as they have RS-485 ICs.· I will be doing this myself to upgrade another system to have network (remote station) capability.· I will, of course, be testing the system in my own place with cables strewn all over.
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Chris Savage
Parallax Tech Support
csavage@parallax.com
I suspect you should consider the Isolation features if you really want to 100 meters. Dr. Tracy Allen was discussing this in another thread and apparently there can be huge differences in ground at separate locations. Also, long wires may invite a lighning strike to enter your locations. There is one $10 chip that includes an isolaton transformer and it seems far superior for that purpose.
If isolation is not a consideration, it is mostly about how many lines? If you want to mimic RS-232 with handshaking in RS-422, you will require two TX and two RX. This is not a bad choice as Local Area Network cable provides 4 twisted pairs with modular wiring connectors. Being able to plug and unplug really adds to convience.· You would only be using two twisted pairs, so the others could be grounded or provide the remote site with DC.· You might even get away with using 4 wire telephone line at slow speeds [noparse][[/noparse]the twisted pair may bean ideal, but not absolutely needed]
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
Post Edited (Kramer) : 8/22/2006 4:00:06 PM GMT
In this recent thread,
http://forums.parallax.com/showthread.php?p=595191
there was discussion of the MAX1490, which is an isolated (1600 volt!) RS485 tranceiver hybrid.
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Tracy Allen
www.emesystems.com
They only use one twisted pair, but you have to tell the chip that it should be in transmit or recieve mode.
Generally, you have to provide a Master / Slave relationship to know when a transmit or recieve is okay.
Full-duplex is much nicer, but twice the wire and all the transmitters and receivers remain constantly available.
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········