Just for the sake of curiosity, I knew DMX is a protocol to control complex lighting. What can you do by receiving and not sending it?
DMX is a one-way implementation of RS485, with the controller sending a byte value for each of up to 512 channels to be received by up to 32 physical devices on the DMX chain. So in normal operation, you have one device sending only, and up to 32 devices receiving only, Hence the need for complementary one-way devices to have a functioning DMX network.
Of course, two way communication can be quite valuable, so modern systems sometimes provide for it through RDP, but relegate device->controller communications to a separate pathway.
The spec requires a 5-pin XLR connector with two twisted pairs wired through. The first pair is used for DMX data, naturally, and originally the second pair was reserved so that a second "universe" (set of 512 channels) could be carried on the same physical network.* In modern practice however, where the second pair is used at all it's often used for RDP, which allows devices to report status information back to the controller. That way you get two-way communication, but the first pair is kept to vanilla one-way DMX to avoid confusing less intelligent devices (of which there are many *cough*Leviton DDS packs*cough* on the market).
* DMX/other control over Ethernet protocols used in modern infrastructure have largely obviated the need for 1024 channels on a single DMX physical interface. Larger installations generally use an Ethernet 'backbone' and convert to DMX near each gear position. Some applications may see Ethernet run directly to specific fixtures (such as High End's DL.2
) as well. This left the idndstry pretty much free to find other uses for the second pair, such as RDP. There might have been some products out there that used the second pair to send power to accessories as well, but the industry has standardized on different cable (one twisted pair for data, and a larger gauge pair for power) terminated with a 4-pin XLR for such things--color scrollers, pattern rotators, motorized irises, to name a few--nowadays.