3 Description of Functions and Measuring Principle User Manual PROFview XL
Especially with low baud rates, PROFIBUS plants with many or unconfigured stations, or large amounts of
data to be transferred, a measuring cycle can last longer. If it is not possible to determine all data during the
measuring time, the measurement will end with a timeout. In this case, the timeout value can be increased
via the PC program (menu "Tools" → "Settings").
3.2.6 Triggering to a station address
During a measuring cycle, the PBT must detect frames of the PROFIBUS station to be measured and evalu-
ate its physical bus signal. The assignment of the signal sent to a station is provided via the source address
(SA) contained in each PROFIBUS frame
1
. Once this address has been detected in the measurement "One
station" and if this address is identical to the station to be measured, a trigger pulse is generated at the trigger
output of the PBT (BNC socket). This can be used in conjunction with an oscilloscope to display and check
the bus signal for any station.
3.3 Online monitoring functions
3.3.1 Bus status
The bus status provides a quick overview of the status of your PROFIBUS plant. A cyclic measurement over
one second indicates whether or not data traffic (level change) is present and whether or not the PBT is con-
nected correctly to the bus. The so-called idle level of the bus is measured and displayed on an idle plant
(without data traffic). Conclusions as to any errors in the bus cabling can be drawn from the value of this dif-
ferential voltage. Such errors can be, for example, swapped lines, as well as missing, surplus or unpowered
terminators.
3.3.2 Bus cycle time
PROFIBUS functions on the basis of the master-slave communication hierarchy with token passing. A
PROFIBUS master controls the access to the bus and polls its configured PROFIBUS stations (slaves). Once
the master has executed its communication tasks, it passes the token (frame) to the next master (or back to
itself - in plants with only one master). The bus cycle time (token rotation time of a master) specifies how long
the token "travels" for a complete bus cycle. More precisely, the time measurement starts with passing on of
the token by a master and ends after a bus cycle when the token is again passed on by the same master.
The objective when configuring any PROFIBUS plant is to keep the bus cycle time and thus the refresh inter-
val of the process data as short as possible. Not only configuration problems, but also diagnostic and alarm
messages, as well as poor transmission properties resulting in sporadic frame repetitions can extend the bus
cycle time. Problems of this kind can be avoided by measuring the cycle time at regular intervals and saving
the recorded minimum and maximum times.
1 ...except in a short acknowledgment frame; there, the destination address of the immediately previous frame is stored and as-
signed.
8 PROFview XL
To Next Page
Loading...