Here is an app note from Texas Instruments describing in depth how DMX512 protocol functions, and how to build isolated DMX512 receivers. Their new isolated transceivers use digital capacitive-isolation technology, which has proven to be more reliable, and resilient, then the opto-isolation that was common in DMX512 devices.
This type of isolation has been tested at data transfer rates of over a 100Mbps. While the voltage isolation threshold is around 560V of continuous charge, and 4kv peak charge.
A DMX512 network utilizes a multidrop topology similar to RS-422, where a single controller (master node) sends repetitive control data to multiple receivers (slave nodes)…
A DMX512 controller transmits packets of asynchronous serial data at 250 kbps (see Figure 3). A data packet starts with a break (logic low) and is followed by a mark (logic high), a sequence known as mark-after-break (MAB). Following MAB is a time slot consisting of a start bit, eight data bits, and two stop bits. The entire packet consists of a maximum of 513 time slots, 512 of which are actual data slots. The first slot, known as the start code, specifies the type of data in the packet.
I am confused…. the ISO35T parts are rated for 1 Mbps. ( not 100Mbps ) Do you know how the capacitive coupling scheme works ?
The speed for the ISO35T is for rs-485; dmx512 is an application layer for the
rs485 electrical and datalink layer (plus the isolation spec, think MIDI and ground
loops, it’s for theatre-geeks). If you chase a bunch of links around, they
describe it as a UHF transmitter/receiver pair across a capacitive isolation gap.
Digikey lists it at $9/each quantity 1 and not yet in stock. The built in driver for
a transformer to produce isolated power at the link endpoint is pretty
neat but you pay for it. Check out Analog Devices iCoupler for some cheaper
solutions.
Yeah, pretty expensive. The ADUM-Series from Analog is cheaper, but most times you need an extra isolating DC/DC converter. But even cheaper, and most times good enough, is just two fast 6N137 optocouplers, they are about 50 cent each.
that’s nice but… what kind of trasformer need i to connect to D1 and D2? i was looking for a dmx512 isolator without two dfferent trasformers…