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Technical Note 129
RS-485 Troubleshooting guidelines
Identify common issues with communications
Network communication protocols such as BACnet MS/TP or Modbus use the RS/EIA-485 electrical standard for serial
communication. These are just two network options available as embedded fieldbus (EFB) on the ACH580 series of
variable frequency drives (VFD). Often field installers, technicians, integrator and even tech support encounter
intermittent or persistent communication errors. This technical note will detail troubleshooting guidelines using
features available within the ACH580 interface.
The following topics are common issues with serial communication that we'll break down into causes, methods to
identify and how to correct:
1.
UART and CRC errors
2.
Isolated devices, 3-conductor wiring
3.
Addressing
4.
Baud rates
5.
Wiring Polarity
6.
Biasing
7.
Number of devices per network, unit load
8.
BACnet Max_Master settings and Token counter
UART, CRC error counters in ACH580 tools for troubleshooting
The ACH580 Primary settings menu organizes the configuration and diagnostics for communications parameters, see
Figure 1. Access this menu by navigating to Menu > Primary settings > Communications > Embedded fieldbus >
Communication setup or Diagnostics > EFB data from client.
A universal asynchronous receiver-transmitter or UART is the transceiver type used in serial communication circuits
utilized for network communication. The UART transmits data packets with a start and stop bit. If these do not match,
an error has occurred and increments the UART errors counter (Parameter 58.11). If this number is increasing rapidly,
there are too many data packets being dropped and there will not be successful communication on the link. This is
typical when device configuration problems exist on the network.
Cyclic redundancy check or CRC is a value included in a data packet that is checked upon receiving the data and if the
value does not match what was transmitted it is expected there was interference in the comm link and increments the
CRC errors counter (Parameter 58.12). If increasing quickly there is too much degradation of the data signal, and a
LVD-EOTKN129U-EN REV A
Effective: 2023-03-31
Figure 1: Primary settings, Embedded fieldbus
1
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Related Manuals for ABB RS-485
Summary of Contents for ABB RS-485
- Page 1 — Technical Note 129 RS-485 Troubleshooting guidelines Identify common issues with communications Network communication protocols such as BACnet MS/TP or Modbus use the RS/EIA-485 electrical standard for serial communication. These are just two network options available as embedded fieldbus (EFB) on the ACH580 series of variable frequency drives (VFD).
- Page 2 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).
- Page 3 Figure 3: Modbus config Figure 4: BACnet MS/TP config How to troubleshoot: using the UART errors counter, watch to see if the number is increasing rapidly. This is potentially a sign of duplicate or incorrect addressing. Check the communication setup for MAC address, Station ID, Node address, Device object ID, Instance ID, etc.
- Page 4 Bias, too much or too little Known as the failsafe state, having a bias circuit in the RS-485 network pulls the transceivers to known high and low voltages. The failsafe prevents transient voltage levels from being interpreted as network traffic. Newer designed transceivers, like those used in the ACH580, are internally biased and do not require networking biasing.
- Page 5 Max_Master value to 127 (upper limit). Summary Network protocols using serial communication following the RS-485 standard can be a robust and reliable means to connect many devices. Following the install guides for each device and setting the configurations properly will provide a reliable communications network.