Specifications

Table 19: Bus Device Configuration List
Possible Values/Exception ValuesDescriptionLabel
Boot indicates that the device is starting and
is not completely functional.
Main indicates that the device is functional and
operating.
Unknown indicates that the device is not
responding or does not support the Code State
attribute.
Indicates the state of the operating code within
Johnson Controls® field devices. This field is
meaningful only for Johnson Controls field devices.
Code State
True indicates the FEC, FAC, IOM, and VMA16
EOL switch is On.
False indicates the FEC, FAC, IOM, and
VMA16 EOL switch is Off.
Note: All other devices on the bus display
False regardless of the status of their
EOL status.
Displays the end-of-line termination status of a
Metasys an FEC, FAC, IOM, and VMA16 (only).
EOL
Parameters That Affect MSTP Communication
The following parameters can be adjusted to tune MS/TP communication.
Important: Do not adjust attributes with which you are unfamiliar. If set improperly, the following parameters can
adversely affect MS/TP communication.
The Baud Rate Selection attribute specifies the baud rate. If the baud rate is forced to be different from other devices
on the bus, communication ceases. To change the baud rate, see Changing the Baud Rate of an Entire Bus.
The Max Master attribute specifies the highest address that can be a master on the bus. By adjusting this value,
you can prevent some devices from coming online as bus masters. All devices on the bus must have the same Max
Master attribute value to prevent the token from going to a device with an address above the Max Master attribute
value.
The Max Application Protocol Data Unit (APDU) Length Accepted attribute specifies the largest data packet that
is allowed on the bus. If set improperly, this parameter can cripple the bus.
The APDU Timeout attribute determines how long the NAE waits for an acknowledgement from a device when the
message can be sent in one transmission. If set too low, many messages fail. If set too high, error recovery is
delayed. You can change this value in special situations when dealing with third-party devices or overloaded networks.
The APDU Segment Timeout attribute determines how long the NAE waits for an acknowledgement from a device
when the message is sent in multiple segments. If set too low, many segmented messages fail. If set too high, error
recovery is delayed. You can change this value in special situations when dealing with third-party devices or
overloaded networks.
The APDU Retries attribute determines how many retries are allowed when trying to recover from an error. If changed
improperly, the network may slow down or become more sensitive to noise. You can change this value in special
situations when dealing with third-party devices or overloaded networks.
Duplicate Addresses
Two or more devices on a bus cannot have the same address. If two devices on the same bus have the same
address, performance can degrade or serious communication problems can occur, including devices not coming
online and the cessation of all communications on the bus.
52MS/TP Communications Bus Technical Bulletin: Appendix: Maximizing and Troubleshooting the MS/TP Bus