Yaskawa Microtrac Gateway to Square D Sy/MAX Family of PLCs for use with Microtrac LAN - Manual (Page 11)

11
4/12/94
4 OPERATION
Power-up
General
Within the PAC program, the RDU and PLC initialization tables are used to
configure the multiple source parameter exchange. The PLC software must
help coordinate this multiple source exchange.
When the PLC gateway detects a change in data from the PLC, the drive
which has a PLC initialization command for that parameter will be updated
with the new data. This data transition initiates this exchange. This new value
is returned to the PLC from the drive as a confirmation of a successful update.
When the parameter is changed from a source other than the PLC, the PLC
gateway updates the PLC with the new value. The ladder logic program in the
PLC is then required to return this same value to the PLC gateway as
confirmation of the exchange.
Power up presents a special condition. There are two locations of nonvolatile
memory for the same parameter, the DSD drive and the PLC. The DSD drive
memory is chosen as the default on a power up condition.
The PLC gateway will inhibit transfers from the PLC until the DSD data
confirmation is returned from the PLC on power up.
A sample ladder diagram is shown in Figure 6. It illustrates one technique
which satisfies the above requirements. Table 4 explains each of the
designators used in the ladder diagram. Rung 1 seta a load latch (LL) on a
momentary transition of the LOAD_PLC bit. Rung 2 starts the two second
handshaker timer (TMR) when the Load latch (LL) is on and resets the timer
(TMR) when the load latch (LL) is off. Rung 3 unlatches the load latch (LL) if
one of two events occur: the leading edge of the timer (TMR) turning on, or
the handshake confirmation has occurred wherein the value of data read from
the DSD drive (DSD2PLC) is equal to the value sent to the DSD drive
(PLC2DSD). Rung 4 returns the data sent from the DSD drive (DSD2PLC)
back to the DSD drive (PLC2DSD), unless the load latch (LL) is true. Rung 5
executes the write to the drive system through the DSD gateway (DSD_INT).
Rung 6 transfers the PLC pending data (PLC_DATA) to the value that will be
sent to the DSD drive (PLC2DSD) when the LOAD_PLC bit is in the ON
state.
Normally, input and output numeric transfers will use one 16 bit word in a
PLC register. This word should be a in 2’s complement binary format
(sometimes called a signed integer). For example, if the physical input to the
PLC is from BCD thumbwheel switches, then the PLC ladder logic program
must convert the BCD number to 2’s complement format before putting it in
the appropriate register.
A BCD thumbwheel switch assembly may have a decimal point (i.e., two
digits may exist to the right of the decimal point, allowing numeric inputs to
0.01 precision).
RD 914-10