Siemens SIMATIC FM 458-1 DP User Manual page 80

Configuring
1
Input SOK
0
Sampling time
Synczyklus
SYNC_command
Bus circulating
time
Fig. 4-7 Timing diagram, SYNC version 2
Configuring
version 3
4-22
FB-SYNPRO
SYNC_quit
Normally, the transmit blocks are always enabled (SOK=1). If, due to time
fluctuations, the SYNPRO function block is calculated before SYNC has
expired (to the right in the diagram), the transmit data are not updated,
but the values from the previous sampling time are transferred. The
Synccycle and the data consistency are not influenced.
Instructions to achieve good SYNC functionality:
In addition to a low Synccycle, it is also necessary to have the lowest
amount of jitter (time-based fluctuations) in the Synccycle. The following
measures support this:

Irregular data transfer along the DP bus should be prevented: Single-
master operation; stations must not be temporarily switched-in.

Alarm tasks should not be configured on the same FM 458-1 DP.
Sampling time overruns are not permissible; this would result in a
SYNC command failure or a shift by a complete sampling time.

Configure a high baud rate and short telegram lengths (the time to
poll a slave is included in the jitter.).

Configure the SYNPRO function block and all associated transmit
blocks in T1=T0 (basic sampling time). The SYNC command is
always initiated with the basic clock cycle interrupt. It is received with
more accuracy (timing accuracy) as an interrupt, initiated in the
system mode.
Configuring version 3 is for generally less frequently used applications of
FREEZE:

Generating SYNC and FREEZE or only FREEZE commands.

The data consistency over all slaves is guaranteed.

The Synccycle is at least 300 % longer than the sampling time
(CNX>1).
(FB-SYNPRO is calculating too early)
FB-SYNPRO
SYNC_quit
SYNC_command
Application Module FM 458-1 DP - User Manual
A5E01078222-02
SYNC_command
Edition 03.2009