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How to troubleshoot: begin by isolating a section towards the end of the communication link to reduce the total
number of devices connected to the network. This will bring down the unit load on the network and allow the connected
devices to communicate. You can then add back a certain amount of the devices to the network and determine how
many can remain connected. Alternatively, the controller manufacture may recommend installing only their devices on
one communication link and 3
easy adjustment. Otherwise, consider rewiring for max devices per multiple links.
BACnet Max_Manager (formerly Max_Master)
BACnet MS/TP now called Multidrop Serial Bus/Token Passing describes how each network device can communicate
without talking over one another. The manager (formerly master) who currently has the token is the device that can
send and receive messages. The BACnet standard requires any manager on the network send a 'Poll for Manager'
message out to discover if any new device has been added to the network. Each manager must poll 1 out of every 50
times it receives the token (Parameter 58.13). The polling then checks to see if there is a device at the next address, for
example address 10 checks for 11.
This can become an issue when multiple devices have to check multiple addresses for new devices every time they have
to poll, requiring overhead bandwidth on the link and can slow communication. To limit the impact of this polling
ensure all address are sequential by not skipping address numbers. Only the last device in numerical order then has to
check addresses up to 127. Alternatively, you may have an issue discovering new devices if the Max_Master (Parameter
58.41) value is set lower to limit this polling. While a lower Max_Master will reduce the time to finish polling, any devices
with addresses greater than the max will not be discovered by the poll and never receive the token to being
communicating.
How to troubleshoot: if a device is experiencing slow response times and is the highest address on the link, considering
lowering the Max_Master value or adjusting the network to have no skipped addresses. If devices cannot be discovered
at their current address, review and confirmed no skipped addresses or raise the 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. Physical troubleshooting and investigation using the above features of the ACH580
will guide you through identifying errors in communications network installations. If you are unable to determine
certain wiring deficiencies or parameter configurations, consider using other tools to monitor the communications
traffic such as Wireshark. Technical Notes available for additional resources are:
•
LVD-EOTKN123U-EN – Wireshark for BACnet MS/TP traffic capture
•
LVD-EOTKN076U-EN – Testing EFB port on ACH580
•
LVD-EOTKN121U-EN – RS-485 Design and install best practices
•
LVD-EOTKN044U-EN – ACH580 drive with BACnet
•
LVD-EOTKN067U-EN – ACH580 E-Clipse bypass with BACnet
LVD-EOTKN129U-EN REV A
Effective: 2023-03-31
Table 1: Theoretical devices count per RS-485 link
Unit Load
Device count
Full (1)
32
Half (1/2)
64
Quarter (1/4)
128
Eighth (1/8)
256
party devices on another link. If possible, using existing comm cabling this can be an
rd
ABB products
ACH550, ACS320
ACH580, ACH480, ACH180
E-Clipse Bypass
5
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