Performance
Section 3-2
21
Message cycle time
A message consists of an action frame (request or send/request frame) and a
reply frame (acknowledge or response frame). The message cycle time is
composed of the frame transmission times, the transmission delay times, the
station delay time and the bus idle time.
T
MC
= t
ACTION-FRAME
+ t
REPLY-FRAME
+ 2 x t
TRANSMIT_DELAY
+ t
STATION-DELAY
+ t
IDLE
The station delay time is the time the slave needs for decoding the request
and generating an acknowledge or response frame.
The bus idle time is the time between an acknowledgement or response of
the slave and a new request from the master. Part of this time is used for
synchronisation (t
SYN
= 33 t
BIT
).
PROFIBUS has different formats for the action frame and the reply frame.
The frames can have a fixed format (with no data field or with a data field of 8
octets) or a variable format (with a variable data field length).
A general formula for the message cycle time:
t
MC
= (9+n1) x 11 + (9+n2) x 11 + 2 x 0 + 30 + 37 t
BIT
= (265 + 11n) t
BIT
where: n1 = number of action data bytes
n2 = number of reply data bytes
n = n1 + n2
The formula is based upon the following assumptions:
•
The action frame and response frame have a variable format.
•
The transmission delay times are negligible.
•
t
STATION-DELAY
= 30 t
BIT
(typical value for the ASIC SPC3)
•
t
IDLE
= 37 t
BIT
Live list check time
Each bus cycle the master requests the FDL (Fieldbus Data Link) status of
one of the stations connected to the network, except for the master stations
that have been entered in the LAS (List of Active Stations). The stations are
checked in sequence.
Each master is designated a certain range of slaves that it has to check. This
range is determined by the station addresses of the masters connected to the
network and the value for the parameter HSA (Highest Station Address).
A master checks the station addresses one higher than his own address up to
the next master address. If there is no master with a higher address, the
master checks until the value of HSA and starts again with checking from
station address 0.
If the station that is being checked is present and functions correctly, the
check time is approximately:
t
GAP_REQ
= t
REQ_FRAME
+ t
RES_FRAME
+ 2 x t
TRANSMIT_DELAY
+ t
STATION-DELAY
+ t
IDLE
= 6 x 11 + 6 x 11 + 2 x 0 + 30 + 37 t
BIT
= 200 t
BIT
Содержание C200HW-PRM21
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