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device may go offline. Interference is most commonly caused by improper comm cable shielding, grounding, or lack of a
reference connection using a 3-conductor cable. Other common names from controls providers for UART or CRC
counters might be Bus Health Index, Network Health, Check Sum, etc.
Isolated, 3-wire devices not connected to reference
For more background on isolated versus non-isolated transceivers, see (Technical Note 121: RS-485 Design and install
best practices). Noise induced onto a communication network is often electromagnetic or high frequency interference.
Cable shielding has a significant impact on limiting EMI if installed properly. That is a continuous connection along the
comm cable but only terminated to ground at one end, preferably the supervisor controller. To further reduce the
amount of noise into the network, devices such as the ACH580 and ACH180 include isolated transceivers that physically
separate the internal circuitry from the communication link, see Figure 2. These devices need to be referenced to one
another for the RS-485 serial communication to still function properly.
Three wire, twisted pair, comm cabling is the latest standard for some network protocols but still is beneficial without
being a requirement if there are devices with an isolated transceiver. When working with both non-isolated (2-wire) and
isolated (3-wire) networks, each should be separately wired and terminated at the supervisory controller. Mixed
networks of 2-wire and 3-wire devices are possible but should have the isolated devices referenced to an appropriate
ground typically with a small, 100 ohm resistor. ASHRAE BACnet Standard 135-2008 Addendum Y details the variation of
networks with diagrams that could be useful when needing to understand or explain making the reference connection
for isolated (3-wire) devices.
Figure 2: Mixed devices on a 2-wire, twisted pair with shield
How to troubleshoot: while a physical connection, the symptoms seen in the communication link will be intermittent
interference to communications going offline. This can be seen by watching the CRC error counter increase. This is
commonly observed when a controller is communicating with a VFD while not running. Parameter adjustments are set
but once a start command is sent the VFD, several devices or entire network will go offline. If there happens to be a 3-
conductor cable pulled for the network, simply connect the reference terminals. If it's a 2-conductor network with mixed
devices, the preferred recommendation is to add a 3
wire for reference and connecting to ground with a small, 100
rd
ohm resistor. Another option, but less immune to interference thus shall be treated as a last resort in troubleshooting,
is tying into the comm cable shielding with the resistor back at the controller.
Addressing
Each device on an RS-485 network requires a unique physical device address, commonly called the MAC (media access
control) address, so its commands or instructions can be applied to the corresponding client. In the ACH580, this
address can be found as Station ID in Primary Settings, see Figure 3, or Node address (Parameter 58.03). When initially
connecting devices to a network there may be duplicates as most devices come with a default address. For some
protocols, such as BACnet, there are additional Device object IDs, see Figure 4, that need to be unique across an entire
site connected to one or more supervisory controllers. Also, see the BACnet Max_Master section for additional
addressing troubleshooting.
LVD-EOTKN129U-EN REV A
Effective: 2023-03-31
2
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