Siemens CPU 312C

Communication
3.2 Communication services
CPU 31xC and CPU 31x, Technical Data
3-10 Manual, 01/2006 Edition, A5E00105475-06
Send and receive conditions
Conditions which should be satisfied for GD communication:
• For the transmitter of a GD packet:
Reduction ratio
transmitter
x cycle time
transmitter
≥ 60 ms
• For the receiver of a GD packet:
Reduction ratio
receiver
x cycle time
receiver
< reduction ratio
transmitter
x cycle time
transmitter
A GD packet may be lost if you do not adhere to these conditions. The reasons being:
• the performance of the "smallest" CPU in the GD circuit
• asynchronous transmitting / receiving of global data at the stations
When setting in STEP 7: “Transmit after each CPU cycle“, and the CPU has a short scan
cycle time (< 60 ms), the operating system might overwrite a GD packet of the CPU before it
is transmitted. The loss of global data is indicated in the status box of a GD circuit, if you set
this function in your STEP 7 configuration.
GD resources of the CPUs
Table 3-4 GD resources of the CPUs
Parameters CPU 31xC, 312, 314 CPU 315-2 DP, 315-2 PN/DP,
317-2 DP, 317-2 PN/DP,
319-3 PN/DP
Number of GD circuits per CPU Max. 4 Max. 8
GD packets transmitted per GD circuit Max. 1 Max. 1
GD packets transmitted by all GD circuits Max. 4 Max. 8
GD packets received per GD circuit Max. 1 Max. 1
GD packets received by all GD circuits Max. 4 Max. 8
Data length per GD packet Max. 22 bytes Max. 22 bytes
Consistency Max. 22 bytes Max. 22 bytes
Min. reduction ratio (default) 1 (8) 1 (8)