DPU Bus Basics, an Introduction
Metso Automation MAX Controls
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Token passing follows a logical sequence. The token, rotated from station to
station in the high loop, is periodically passed to the station present with the
lowest station address in one of the low loops, A or B. Token passing
continues in address sequence among each station present in that loop and
then returns to the high loop.
The token rotation starts at the lowest station number present in a loop. The
token is passed to the next higher station number present in that loop. If the
loop is the high traffic loop, the token continues around the loop going to the
highest station number present, then back to the lowest station number
present and around the high loop again until one-quarter second has elapsed.
Then, regardless of where it is in the high loop, the token must make one
pass through each station present in one of the two low loops. When the
highest station present in that low loop has completed its transactions, or
runs out of time, the token is passed back to the high loop again to continue
the cycle. One-quarter second later the token is passed to the other low loop.
Note that token passing involves a station number (address), not the physical
location of a station. No two stations may have the same address on a given
DPU Bus.
Token passing proceeds from station to station without any intervention.
However, the token is monitored by the system monitors to ensure proper
operation and to report and correct any malfunctions.
Stallout
A stallout is defined as the loss of token passing on a DPU Bus which exists
for more than 500 microseconds. Only system monitors can detect and
correct a stallout.
A stallout recovery occurs when a system monitor detects a stallout and
restarts the token. The time necessary for a system monitor to detect the
stallout and restart the token is a function of the address of the system
monitor. The time is calculated by using the following formula:
500 ms + (200 ms) * (station address #)
This technique is used to insure that multiple system monitors do not restart
the token simultaneously.
To effect the fastest possible recovery in the event of a DPU Bus
communication stallout, at least one system monitor (DBM) should have a
low DPU Bus address, e.g., 1, 2, 3, 4.
When a stallout occurs, each system monitor becomes a potential master.
Because its waiting period is shortest, the lowest station number detecting
the stallout will try to restart the token. By having at least one system