Hardware design questions

[ericthegeek]

Originally Posted by ELMVideo

1. I'm designing a one in two or more output circuit. Just to verify - the input whould be terminated to 120 ohms and each output should have the biasing network?

What you're describing is usually called an RDM Hub or RDM Splitter.

It's generally a bad idea to have a single DMX input with fixed termination unless you're absolutely certain your device will always be the last device in the DMX daisy chain. Generally it's best to have a male input connector with a passive female "through" connector (sometimes called "loopthrough").

You can then offer switchable termination, or end users can plug an external terminator into the loopthrough when it's the last device in the chain.

Originally Posted by ELMVideo

2. Using the 75176 IC's, the output of each IC "R" (receive data) outputs will need to 'simulate' a collision merging all of the data streams, (for the discovery mode). My intention is to connect a 100ohm resistor to each output merging into a single bus-would this be an good approach? Or would a multi input AND gate work?

A resistive combining network by itself won't work. For an N output splitter, you'd have N-1 outputs driving the line high and only 1 driving low during a normal RDM response.

You need some kind of logic managing the "Transmit Enable" and "Receive Enable" of each 485 line driver. I've seen the logic implemented with a microcontroller, with a PLD, and with a state machine.

Once you have that, you can use either a passive combining network, or an AND gate for the data signals.


When designing a RDM Hub, make sure you pay attention to the break shortening restrictions in the E1.20 document. Also, during the discovery response period, you either have to shorten the discovery response by one full character (lop off an entire 0xFE from the beginning of the response), or by a minuscule amount (the 75ns limit in section 4.2.3).