DPU Bus Troubleshooting
Metso Automation MAX Controls
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If backup control is anticipated, the backup station address must be the next
sequential station address above the address of the primary. If the backup
stations are not being installed at startup, the appropriate station addresses
should be left unused to avoid readdressing existing stations when the
backup stations are finally installed. For example, assume that the several
DPUs controlling temperature are to have future backup, and that you have
chosen to override the automatic alternating low-loop assignment of
addresses described in the preceding paragraph. The addresses initially
assigned to these DPUs could be 40, 42, 44, 46, and 48. When the backup
DPUs are installed, they would be assigned addresses 41, 43, 45, 47, and 49
where 41 is backup for 40, 43 is backup for 42, etc.
If the number of stations exceeds 31 for system monitors or 32 for DPUs,
some station addresses will have to be duplicated. This is permissible if they
are not on the same DPU Bus. In this case, it is possible to assign the same
station address to a system monitor in each DPU Bus to which it is
connected. For example, assume a
DBM is connected to two DPU Buses. It
could be given the same address, e.g., 02, in each of the two DPU Buses.
Any reference to that station address would indicate the same DBM.
Token Passing and DPU Bus Mastership
A maxDNA
DPU Bus
network uses token passing topology. The token is
defined as a virtual symbol of the right to initiate communications on the
DPU Bus. Token passing is the rotation of mastership among stations on a
DPU Bus. Mastership is defined as possession of the token. The token
passing procedure guarantees mastership to every station in the system and
assures efficient DPU Bus utilization under high traffic conditions in large
systems.
Communicating on the DPU Bus requires at least two participating stations.
At any given moment, one station is master. A master station has possession
of the token and the authority to initiate messages. When a master station
passes the token to the next station, that next station becomes the master.
Initially, token passing to all possible addresses is attempted. Every sixteenth
pass an attempt is made to pass the token to all possible addresses. If no
response is received after three tries, subsequent station addresses are tried
until communications are established. This ensures that any new station or
any station that has been repaired is once again included in the rotation for
mastership.
Stations which are not present or have malfunctioned are removed from
normal token passing using an IGAP procedure. (Refer to "IGAP and UGAP
Procedures" later in this chapter.) After IGAPing, the token is passed only to
stations actually present.
