Hello everyone!
Long time lurker, first time poster on the forums, so lovely to meet you all.
I am currently working on a relatively simple "adapter" circuit to incorporate Lumen Radio's TimoTwo DMX module. It's an integrated device that facilitates a wireless DMX512 link and a TTL level, serial DMX output. It handles all of the 2.4GHz magic within itself and doesn't require much intervention beyond being a bit careful about the PCB layout.
The only thing left to the user is the circuitry for driving, biasing and terminating the actual differential signal.
BACKGROUND
The device can be configured to act as a RF_receiver (2.4GHz wireless -> wired DMX) or a RF_transmitter (wired DMX -> 2.4GHz wireless). From the bus signal perspective, the device must be based on ANSI E1.11 - 2008 (R2018) and ANSI E1.20 - 2010 (for use of Remote Device Management or RDM).
Unlike just DMX (simplex), RDM uses a half duplex RS485 communication, with specific timings. Because the command port (in our case the RF_receiver) needs to switch between driving and receiving data on the bus, a bias circuit must be implemented to leave the bus in a known default state:
This is done simply enough with a resistor network.
When the device is operating as an RF Transmitter, it is on the receiving end of the bus, and likely at the end of it. Therefore, it requires a means of termination, most commonly, a 120Ω resistor.
The mode selection on the TimoTwo device, in a way I'm planning to implement it, depends on a single I/O line being high or floating (RF Receiver) or pulled to GND (RF Transmitter). To avoid a misconfiguration of the biasing circuitry and the current mode, I would like to have both Mode and Bias/termination defined by the state of a single switch.
Basically switch between this
The only implementation I could find online was based on Maxim's app note AN212, using SSR's for connecting and disconnecting a terminator resistor.
Long time lurker, first time poster on the forums, so lovely to meet you all.
I am currently working on a relatively simple "adapter" circuit to incorporate Lumen Radio's TimoTwo DMX module. It's an integrated device that facilitates a wireless DMX512 link and a TTL level, serial DMX output. It handles all of the 2.4GHz magic within itself and doesn't require much intervention beyond being a bit careful about the PCB layout.
The only thing left to the user is the circuitry for driving, biasing and terminating the actual differential signal.
BACKGROUND
The device can be configured to act as a RF_receiver (2.4GHz wireless -> wired DMX) or a RF_transmitter (wired DMX -> 2.4GHz wireless). From the bus signal perspective, the device must be based on ANSI E1.11 - 2008 (R2018) and ANSI E1.20 - 2010 (for use of Remote Device Management or RDM).
Unlike just DMX (simplex), RDM uses a half duplex RS485 communication, with specific timings. Because the command port (in our case the RF_receiver) needs to switch between driving and receiving data on the bus, a bias circuit must be implemented to leave the bus in a known default state:
ANSI E1.20 - 2010, 2.4.1The command port shall provide a means to bias the termination of the data link to a value of at least 245 mV [...] If the line biasing network is enabled, the differential input impedance shall be 120ohms +/- 10% [...].
This is done simply enough with a resistor network.
When the device is operating as an RF Transmitter, it is on the receiving end of the bus, and likely at the end of it. Therefore, it requires a means of termination, most commonly, a 120Ω resistor.
The mode selection on the TimoTwo device, in a way I'm planning to implement it, depends on a single I/O line being high or floating (RF Receiver) or pulled to GND (RF Transmitter). To avoid a misconfiguration of the biasing circuitry and the current mode, I would like to have both Mode and Bias/termination defined by the state of a single switch.
Basically switch between this