DMX switch with monitoring lights

[Pauldf]

Hi all
I was involved with running a live music show using a PC last week and had the misfortune of the whole lot locking up during a stage blackout, thankfully early enough on in the day that it didn't matter too much but still a bit embarrassing.
So over to the project, in order to avoid the issue I have a laptop with another DMX dongle as standby, so I have two active DMX dongles which constantly output during the show.
What I'd like to do is have a normal DPDT switch so I can simply switch between the two dongles which is pretty straight forwards.
I'd also like to have a couple of LEDs connected to each dongle so I can see that the data is being transmitted, I saw an LED project here http://www.bigclive.com/dmxtest.htm and wondered if I can do something similar. I tried a pair of LEDs connected directly to the DMX dongle via a 270R resistor but the light output was poo. I don't really want to use a smaller resistor as I don't want to load up the dongles and them not work properly. I have attached a drawing of an opto coupler which I have in stock.
Anyone see any issues with connecting the above to an RS 485 network (DMX)? Would I be better using some sort of OP amp to drive it?
Cheers

Paul

[Pauldf]

Just an update, having read up a little on DMX it appears that the above won't work as the impedances are too low on the 74128 emmitters so have opted to look along using a differential bus receiver type setup (12k + impedance).
I have posted the same over on the blue room forum: http://www.blue-room.org.uk/index.php?showtopic=60954

As I am trying to keep it relatively simple is there any reason why I cannot use a simple double pole double throw switch to select the relevant RS485 output switching it to the universe, or would I be better switching the signals to TTL and using gates to control them, outputting back to RS485?
I have connected the three lots of Pin 1 together (screen)
I would also like to be able to see if DMX is available on either of the two inputs, my thoughts are to have a pair of LEDs, one lit on the positive cycle of the pulse and the other on the negative. If the DMX output stops then I'm wanting one of the LEDs to extinguish. I'm thinking that the LEDs will look steady as iirc the signal is 250kb/s.
I have attached a circuit below:

The chip is rated at 60mA so is plenty for a pair of 20mA LEDs one at a time. The chip sources the 5v supply via an internal 85 ohm resistor and sinks directly to 0v hence the different size resistors I'm thinking on the output. The datasheet for the sn75186b chip is here: http://www.ti.com/lit/ds/symlink/sn65176b.pdf

This is what I have built to press, I haven't tried it live yet.




Any comments appreciated

Paul F

[piratepaul]

Hi, I am completely lost ... do you want to say that again slowly.
I will look it up and get back to you.

Tar&stuff

[piratepaul]

http://forums.prosoundweb.com/index.php?topic=124329.0

[Pauldf]

OKay dokay - I'll try again.
When I use a PC for live events such as bands I use a PC to DMX converter (dongle) which controls the lights on the stage.
The PC isn't 100% reliable, being a Windows PC can do it's own thing, it may just stop sending DMX for no reason therefore I also take my laptop as standby.
This is where my project comes into play, I want to be able to monitor both the outputs from the PCs to make sure they are transmitting DMX.
I also want to be able to throw a simple switch to switch the lights between computers.
I am not looking for a DMX relay switch thingymajig which is used to switch lights on and off without a dimming cycle.

[piratepaul]

Hi, I am realy busy early this week, I will be back later in the week or weekend... I need to look up the numbers, time...

At first sight the USB's switch the DMX's on and off, so you just need to switch +5V on the USB's... not sure if that is what you want to do.

The link was just something I found ... thought you might be interested. back at week end.

Tar&stuff.

[Pauldf]

I'm looking to do the switching on the DMX (RS485) side of things, I have two usb-dmx adaptors so can have one permanently attached to the desktop and the other attached to the laptop.
I want to be able to select Desktop DMX or Laptop DMX. I also want to have a way of seeing that there is a DMX signal being transmitted by both the devices pre selector switch.

[piratepaul]

Hi, did I say wot weekend... well did I.
I have still not looked this stuff up, I looked up DMX, sounds like a great bit of kit.
So you can use the laptop to switch switches... there must be soft ware involved.
So how many output wires have you got, and what is the max current in the wires?
Great bit of kit... tell us more.

Thanks.

[Pauldf]

I am only talking a few volts on an RS485 drop, hence the driver chip.
The switch is manual, there is only one output of RS485 which is directed to one of the two inputs via a DPDT switch.
I have had it working in a fashion but the data rate is a bit much for viewing LEDs so am looking at some sort of divide by counter to slow it down.

[piratepaul]

This is the third attempt to reply, I click submit then it tells me to log on and deletes what I wrote, may be it is time testing... testing.