Cheap USB interface: uDMX vs. FTDI RS485 cable

[craigmw]

Wanting to do some lighting control for my church, I purchased a Lixada USB to DMX interface to connect to a bunch of American DJ Mega Par Profile Plus fixtures. Using the uDMX setting on the Input/output tab, this controller worked without any problems on both my desktop running Windows 10 and on my MacBook Pro running Sierra. However, I noticed that simple color changes and fades were very choppy, with an apparent refresh rate of about once a second or so. Having done some reading here, I was able to modify the uDMX parameter for the number of channels to 128 and this helped some, but clearly limits the capabilities of the interface. After doing a bit more reading, I found references to FTDI RS485 USB cables that might function better than uDMX cables. I also found the following Instructable on how to build a USB to DMX interface using an FTDI USB-RS485-WE-1800-BT:

https://www.instructables.com/id/Pro-US ... r-For-QLC/

This provides a very nice description about how to construct a USB to RS485 interface with a female XLR connector that functions perfectly in sending DMX signals via USB, as DMX is a subset of the RS485 communications protocol. I also built a DMX terminator from the plans described using a male XLR connector and a 120 ohm resistor. Simple soldering skills and a soldering iron are required for this, but FTDI sells other USB to RS485 cables that can be used for this purpose that have screw terminals, so it would be a simple matter to build one using a cut DMX cable connected to the interface via screws. Note that the unit I used is based on a genuine FTDI FT232R chip and an RS485 interface chip. I'd avoid other USB to RS485 cables that use FT232R clones.

First, I tried this on my Windows 10 desktop. As soon as I plugged the cable into a USB port, Windows 10 automatically downloaded drivers and configured it. After plugging in a generic RGBW fixture to the cable, I loaded up QLC and clicked on the DMX USB output checkbox. Clicking on this shows the following info in the window below:

DMX USB
This plugin provides DMX output support for DMXKing ultraDMX range, Enttec DMX USB Pro, Enttec Open DMX USB, FTDI USB COM485 Plus1, Vince USB-DMX512 and compatible devices.

No input support available.
1: USB-RS485 Cable (S/N: FT2KYMY3)
Device is operating correctly.
Driver in use: FTD2xx
Protocol: Open DMX USB
Manufacturer: FTDI
DMX Channels: 512
DMX Frame Frequency: 30Hz
System Timer Accuracy: Good

I opened up the Simple Desk and played around with the RGBW settings and found that the interface worked perfectly without any of the lag observed for the Lixada/uDMX interface. No problems with flickering were observed as with the uDMX device.

I next plugged the cable into my MacBook Pro and opened up QLC. While QLC did report a DMX USB device and the FT2KYMY3 interface, it did not appear to be able to control the fixture. I ended up realizing that while my MacBook Pro had FTDI drivers, they were not the correct ones. I downloaded the latest drivers from the FTDI site and followed directions about how to load these and also to how to disable the default Apple-supplied FTDI drivers using D2xxHelper. After doing this, I rebooted the Mac and loaded QLC as before. This time, I could see the LEDs on the USB to RS485 cable light up when QLC loaded. I checked off USB DMX in the Input/output tab and now Simple Desk works perfectly with my fixture, again with no lag or flickering.

If you are looking for a cheap and reliable USB to DMX interface to use with QLC Plus, I suggest either purchasing a good professional one (e.g. Enttec or DMX King), or building this cable. It requires a bit of work, but not too bad and it is much more reliable than the cheap uDMX cables for sale at Amazon, etc.

[GGGss]

Thanks for your extensive report.

IMHO:
Sidenote 1: using the interface does not provide galvanic decoupling - interferences introduced into the DMX strain will possibly destroy your RS485 chip.
Sidenote 2: using it in a fully addressed universe you might expect timing issues (with cheap Chinese fixtures) so a fogger on channel 510 or so is risky business.
Sidenote 3: it's the processor inside your computer who is responsible for the DMX-signal-train timing. (Over) load your processor and timing will be off.

[craigmw]

GGGss:

Thanks for your response. You bring up some very good points about the limitations of this interface. This clearly does not have the benefits of a more expensive system like an Enttec DMX USB Pro, but it is also significantly cheaper. It provides similar functionality as the Enttec Open DMX. For those of us running very small lighting rigs, I think the performance is acceptable. When our church lighting becomes more complex, I believe it would be best to upgrade to an ethernet based approach (e.g. Artnet), but for now, this works very well for our needs.