Multi-display clock for model railway exibitions. - Page 1
Forum section: electronics / projects

author: Alana
author_rank: Frequent Contributor
timestamp: October 25, 2013, 11:50:12 am
content: I was asked if i can do a model railway clock [clock that can be switched from real time to time x2, x3 ,4x etc] and i did - clock is working nicely on a breadboard. But now i have bit of a problem because project outgrew my level of experience.Control box and display are separate modules that are connected via 4 wire data cable - it was meant to allow to connect more than one display to a control box and on that exhibition that this is meant to be used there will be 12 display modules, each for every station with about 150-200 meters of track between first and last station. I'm unable to test it on site - its available for us only for duration of an exhibition which is in 2 weeks from now and i don't know what to expect.Data line is simple shift register with strobe, pulse duration 2ms so it should work even with TTL level signals but if i can make it more reliable i want to try.I do not want to use another micro in display modules [more micros = more bugs] so shift registers and this 4 wire data line stays.Each module has its own power and are connected by 4 wire telephone cable with wire diameter same as in typical UTP computer cable.What i thought of is:- re-wire each display so it has both data input and data output so i can use it as signal amplifier, this will limit node length to about 20m- try to make external stand alone signal amplifiers with 4050 buffers every 50m- add 2 max 232 chips on inputs and output to switch to +/- 10V levelsI do not want to go overboard with this and like i said - doing proper tests on site is not possible.Attached: schematics for master and display modules.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 25, 2013, 12:50:16 pm
content: Expect grief!Distributing clocks and data, especially at TTL levels, is not a trivial matter.I would: Make each "station" an autonomous unit such that it keeps going even when the communications fails. You obviously have the means to program the master controller, transfer this intelligence to each station. Perform the communications at a higher level, using either RS232 or even differential signalling, and use the intelligence at at station to make it fault tolerant.In industry, before it was economically or practically viable to put intelligent in each display station, I used to play this remote display game - and frequently lost. Copper losses, signal reflections, interference etc., all these become orders of magnitude more significant when distances of more than a few metres are involved.

author: Alana
author_rank: Frequent Contributor
timestamp: October 25, 2013, 01:01:09 pm
content: Then MAX232 it is. Have you ever tried to connect one output of max232 to more than one input?

author: Andy Watson
author_rank: Super Contributor
timestamp: October 25, 2013, 01:08:49 pm
content: Quote from: Alana on October 25, 2013, 01:01:09 pmYes, it does work. But I've only tried that with 3 simultaneous input, and only over a few metres of cable.

author: Alana
author_rank: Frequent Contributor
timestamp: October 25, 2013, 01:21:12 pm
content: Ill change line interface to something like this. Wont add much to costs and if you say RS232 levels is minimum for reliability then...Will lowering frequency of clock from 500 to 250 Hz add to reliability?

author: ivan747
author_rank: Super Contributor
timestamp: October 25, 2013, 04:31:33 pm
content: Not a lot I think, but in theory it does. At 50 meters per circuit you could try RS-485, since it's getting in that territory but RS-232 will do fine and you already designed something around MAX232s.

author: Alana
author_rank: Frequent Contributor
timestamp: October 26, 2013, 04:54:20 pm
content: Did some testing today. Room with working model railway, 40m of cable laid along the track as it will be on big exhibition, transmitter with max232, receiver with low pass RC filter followed by CD40106 and it works in condition its meant to be operated.In final version I'll make room for this RC filter before max232 receiver but leave it unpopulated - just in case. And I'll stick to the very same kind of cable i used in today's testing.

author: Kryoclasm
author_rank: Regular Contributor
timestamp: October 27, 2013, 03:56:46 am
content: Quote from: Alana on October 25, 2013, 11:50:12 amJust thinking out loud... you could use the track as your "wire" to send a modulated signal. The station that matches that signal would perform the attached command. Each station would need a micro.Pros:*minimum wire needed.*transmission losses reduced.*each station can be addressed individually with its own code or all can be addressed at once with a global code.Cons:*more than 1 micro.*ac signal over a dc powered line, possible noise issues.Just a quick idea.

author: Alana
author_rank: Frequent Contributor
timestamp: October 27, 2013, 06:23:31 am
content: Tracks are already DCC so it will not work.

author: Kryoclasm
author_rank: Regular Contributor
timestamp: October 27, 2013, 08:38:27 am
content: I had to look that up, DCC. I had no idea.Maybe a wireless set up?

author: SeanB
author_rank: Super Contributor
timestamp: October 27, 2013, 09:30:35 am
content: If you are using RS422 use a Cat5 screened cable, screened plugs and sockets, and use the screen as the common for the RS422 and one pair for the data. Then daisy chain them end to end and terminate only at the end. Use a terminator at each node with jumpers or make up a very short stub cable with the terminator resistors on it and use another pair to feed 5V power down another pair on each board. Reliable and relatively noise free, and a cheap cable as well.

author: Sander
author_rank: Contributor
timestamp: October 27, 2013, 08:59:59 pm
content: Doesn't it have LocoNet if it uses DCC?http://www.digitrax.com/support/loconet/home/