Yaskawa PLC-5 - Manual (Page 20)

GENERAL OPERATION
17 PLC Block Transfer
12/22/94- RD 3196-10
PLC Block
Transfer
The PLC INITIALIZATION table can be described as follows:
1. The PLC INITIALIZATION channel number establishes the rack and
word of the block transfer. The channel designation follows the same
conventions as the NUMI and NUMO channel conventions described above.
The "PLC IN" and "PLC OUT" headings on the PLC Table are from the PLC
Viewpoint. The "PLC OUT" column corresponds to the BTW and the "PLC
IN" corresponds to the BTR.
2. The PLC INITIALIZATION item number designates the position of
the parameter in the PLC block transfer data file.
3. The PLC INITIALIZATION table requires both input and output
channel and item numbers. Sensible PLC programming technique dictates that
the input and output channel and item numbers be grouped consecutively, and
that the input and output item numbers be equal. The programmer can then
take full advantage of the high level data manipulation techniques.
4. When the PLC gateway detects a change in data from the PLC, the
appropriate drive is updated with the new data. The data transition initiates
this exchange. This new value is returned to the PLC as confirmation of a
successful update. When this parameter is changed from the drive, the
gateway updates the PLC with the new value and expects the new value to be
returned as confirmation of the exchange.
Power-up presents a special condition. There are two locations of nonvolatile
memory for the same parameter, the drive and the PLC. The drive memory is
chosen as the default on a power-up condition. To insure proper initialization
of the RDCI or GETN blocks during power-up, the PLC gateway will inhibit
transfers from the PLC until the drive data confirmation is returned from the
PLC on power-up.
A sample ladder diagram is shown in Figure 10. It illustrates the technique
used to satisfy the above requirements. The first rung moves data from the
PLC into the write transfer buffer. The second rung moves data from the read
transfer buffer into the write transfer buffer. This is conditioned by a transition
of data from the last PLC scan. The third rung executes the write to the PLC