General questions for remote communication

[GUEST]

I'm interested in designing a system that would update a display every second and be controlled from a keypad up to 25 ft away. There could be from 1 up to 16 displays connected in parallel that are displaying the same information. This system needs to be portable so the cables will most likely have RJ45 termination.

I've had experience with programming pics for reading a keypad on the product and serial communications with the 232 port, but this is going to require more. Would it be possible to scan the keypad with the pic even though the keypad is 25' away or would I need a communication link of some type?

Since the keypad will be powered from the display it is connected into, should I make this display as the master and additional displays as the slaves? I would like for all the displays to be the same so that you do not have a special connection or different control boards for the master and the slaves. You could just connect the remote panel to any single display and then connect any # of the slaves from there.

The major concern is with communication between the displays. They could be a considerable distance between them so RS 232 is probably not a choice. RS485 will most likely be required. I have not written code for RS485 nor that familar with the required circuits and termination since there could be a different # of devices on the bus.

Any input from both the software, hardware and overall approach would be appreciated.

Thanks,

[jecottrell]

Get rid of the cabling issues, use:

http://www.maxstream.net/products/xbee/xbee-oem-rf-module-zigbee.php

They're cheap and simple to use. I have very little experience and have had no problems.

Sounds like all your other questions are solved in a simple protocol for the comms.

John

[asmallri]

Yes you need a communication link.

At low baud rates RS232 is fine. At 25' you can realistically expect 19200 buad without problem. The main disadvantage is you now require point to point RS232 connections from every display to a central communications unit.

RS485 would indeed be a good candidate because it will enable all comms across a common bus. The main issue you will have to content with is that of common ground. If each of your displays is power from its own "110/240 volt mains" power outlet the is a chance you may have a probel with ground differential.

Without knowing more about you project, one thing you could consider is to make all displays slaves, the keypad is the master. Alteratively, lets say you had a common 3 or 4 wire communications bus, you could use a 5 pin header, The first 3 or 4 pins are run between the displays, the 5th pin is only available on the keypad connector. Whichever display it is plugged into becomes the master.

Another option is Ethernet. Use a small Ethernet switch and star out to each display.

If you want to outsource some of this development, feel free to drop me a line or check out my site.
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Regards, Andrew

http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!!

[Guest]

RS485 will be a good way for a start if you never do with display before. It will over kill the use of RS485 but for a start I will say this is okay, since you actually don't need to have 2 way communication for a display. I do did some display project before which communication with 10-20 display with different address, but they don't need to reply with the master. This mean the master will keep sending the same signal, it will not tie down for you polling reply from each display.

[Guest]

Thank you for all of the good feedback. It will be very helpful in deciding what I need to do as the next step.