Siemens SINUMERIK 840C Installation Instructions Manual page 847

12 Functional Descriptions
12.10.1 Rapid block change using FIFO function (up to SW 2 only)
The next part program blocks are temporarily stored in this FIFO memory in a prepared state
(predecoded). Only when the memory is full with such preprocessed blocks is the program
started or continued.
The NC can now access the prepared blocks in the FIFO memory and process them in quick
succession.
If the NC occasionally processes a block with a longer traversing path or with auxiliary
functions, there is time to replenish the FIFO (filling FIFO in background).
It can however happen during a long part program section with short traversing blocks that
there is no spare time available to replenish the FIFO memory. As soon as the supply of
predecoded blocks in the FIFO memory is exhausted a feedrate drop again occurs.
Filling the FIFO memory with G171
To avoid a drop in the feedrate at an important section of the program, it is possible to fill the
FIFO memory selectively before such a program section occurs. Selective filling is initiated by
programming G171 in the part program.
This ensures that the following program section from the FIFO memory can be executed with
short block change times.
By programming G171, the following occurs in this order:
• Processing stopped
• FIFO memory filled up
• Execution continued
Execution is stopped until the FIFO memory is full, up to a block with M02/30, up to @714 or
up to a block which generates a @714 (setting data block).
It only makes sense to program G171 in AUTOMATIC mode.
Behaviour on "Program hold"
If a part program is stopped and edited during execution, the blocks already in the FIFO
memory are no longer changed. On NC START these blocks are traversed in their original
condition.
Note:
If the tool offset and zero offset values are altered in the stop state, the changes are included
the blocks already in the FIFO memory.
FIFO memory size
The NC CPU has a FIFO memory which can hold up to 60 blocks. This total memory can be
divided amongst the individual channels with NC MD 914* bit 0.
The following then applies:
FIFO active in one channel:
FIFO active in two channels:
The FIFO function cannot be activated in more than 2 NC CPU channels. If a FIFO buffer is
activated for a channel which does not exist, it is ignored.
12–72
60 FIFO blocks available in the channel
30 FIFO blocks per channel available.
© Siemens AG 1992 All Rights Reserved
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SINUMERIK 840C (IA)