Moeller ps4-201-mm1, Hardware And Engineering

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71
04/
99 AW
B
27-
1184 G
B
Appendix
Optimizing the
exchange of send and
receive data The 128 byte communication buffer (COB) of the
PS4-200 master is used to alternately send and
receive data to and from each of the stations in turn.
After the master has sent the data (send data) to a
station, this now free area of the COB memory plus
any unused COB memory is available to receive data
(receive data) from the slave. As long as there is
sufficient free memory available in the COB each
time the master receives data from a slave in this
way, the 128 byte COB can be used alternately for
128 bytes of send data and 128 bytes of receive
data.
If there is not sufficient free memory when the master
receives data from a slave, valid data in the COB may
be overwritten and the PS4-201-MM1 may switch to
the “Not Ready” status (error messages “ERT” and
“EPM”) after transferring the program.
The reason for this behaviour and its remedy is
illustrated in the following examples.
Example
A PS4-201-MM1 (master) needs to exchange data
with three slaves A, B, C (also PS4-201-MM1) as
shown in the following figure. The number of bytes
received from each slave also includes the
diagnostics bytes from the slave and from any local
expansion modules which are connected to it.