DMX512 and BACnet MS/TP Termination: Stop Flicker and Dropped Tokens
Two buildings, the same disease. In a theater, moving heads twitch and LED walls flash single frames of the wrong scene. In an office tower, VAV controllers drop off the building network and thermostats go stale. DMX512 runs the first. BACnet MS/TP runs the second. Both ride RS-485 wiring, and both fail the same way when the last device on the line has an empty terminator port.
This guide covers termination for both protocols, the cable mismatch math, and the RJ45 pinouts that building and lighting equipment expect.
The flickering rig: a story every lighting tech knows
An architectural installation runs 180 m of Cat5e through a building to a chain of fixtures. On the bench, every fixture works. Installed, the rig flickers. Fixtures execute scenes late or fire a channel at random. The crew swaps drivers and re-patches the console for two days. The fault: the pass-through port on the final fixture is empty. DMX frames at 250 kbps hit the open socket and reflect back down 180 m of cable, colliding with live frames.
One 120 Ω plug in the last port ends the problem. That is the entire repair. Watch the frame travel the chain and die cleanly in the terminator:
DMX512-A (ANSI E1.11) fixes the data rate at 250 kbps. The standard expects a 120 Ω terminator across Data+ and Data− at the end of the line. ANSI E1.27-2 maps DMX onto RJ45: Data 1+ on pin 1, Data 1− on pin 2. Every daisy chain has exactly one last device. That device needs the plug.
The tower that dropped its tokens
BACnet MS/TP devices share the bus by passing a token. Each controller talks only when it holds the token. Reflections corrupt the token frame itself, so the whole segment stalls, re-polls, and stalls again. The symptom reads as software: token-pass timeouts, devices that appear and vanish, a segment that slows as the day warms up.
A facilities crew in a 12-story building chased exactly that for three days with software tools. The oscilloscope told the real story in minutes: rounded edges and ringing from missing end-of-line resistors, made worse by long untwisted tails at terminal strips. Two 120 Ω EOL resistors and tight re-terminated pairs stabilized the segment.
ASHRAE 135 specifies the MS/TP physical layer on shielded twisted pair. It requires termination at both extreme ends, plus network bias to hold the idle line defined. Many controllers include EOL DIP switches. The failure mode is human: switches left off, or left on at three middle nodes.
Wrong cable, real numbers
Lighting installs inherit whatever cable is in the wall. Microphone cable and 75 Ω coax look fine and fail quietly. The mismatch between cable impedance and terminator reflects part of every edge:
A 120 Ω terminator on 75 Ω audio cable still reflects about 5% of every edge, on every transition, all day. Proper 120 Ω data cable with a matched terminator reflects nothing. The lesson: terminate, and run the right cable when you can.
Pinouts for building and stage networks
| Protocol / equipment | RJ45 data pins | Termination |
|---|---|---|
| DMX512-A over RJ45 (ANSI E1.27-2) | Pins 1 + 2 | 120 Ω at the last fixture |
| BACnet MS/TP controllers (common map) | Pins 1 + 2 (vendor-specific, check the manual) | 120 Ω at both segment ends |
| Modbus RTU building meters | Pins 4 + 5 | 120 Ω at both segment ends |
Our RJ45 terminators come wired for either map, with the pins stated on the listing:
- RJ45, pins 1+2 (DMX / BACnet), 120 Ω, 1%, soft silicone — pair, $20 for stage racks and office risers.
- RJ45, pins 1+2, 120 Ω, 1%, shielded, hard silicone — pair, $28 for plant rooms and roof-top units near VFDs.
Orders ship as a pair. A DMX chain uses one and keeps a spare in the console case. A BACnet segment uses both. For XLR-5 terminators, 6P6C, or custom values, request a build.
Quick answers
Why does my DMX flicker without a terminator?
Open-ended cable reflects every frame back into the line. Reflections collide with live frames, and fixtures decode corrupted channels. The longer the run, the worse the collision timing. One 120 Ω plug at the last fixture removes the reflection.
What resistor value does a DMX terminator use?
120 Ω across Data+ and Data−, rated 1/4 W or better. Our plugs use 1% resistors, measured before shipment.
What is the EOL resistor value for BACnet MS/TP?
120 Ω at each extreme end of the segment. Middle devices keep their EOL switches off.
Can I use an Ethernet patch cable for DMX or MS/TP?
Cat5e carries these signals well and ANSI E1.27-2 permits it for DMX. The connector does not make it Ethernet. Keep the network on its own cabling, follow the pinout map, and terminate the ends.