
B-63003EN-2/01 Chapter 3 Installing the Startup Ladder 3 - 3
3
The Ladder Diagram
If you look at Figure 3-1 on the previous page again, you will notice that the CNC cannot “talk”
directly with the machine tool. They must communicate to each other through the PMC. For
example, if a
VALVE PRESSURE LOW
signal were generated by the machine, the PMC must relay
this information to the CNC. How does the PMC do this?
The PMC must be programmed to “know” how to tell the CNC that the valve pressure has dropped.
This program is called the sequence program and the ladder diagram is just a visual representation of
this. As you can see in Figure 3-2 below, each relay contact symbol in the ladder diagram
corresponds to a PMC instruction in the sequence programming memory.
The processor in the PMC “sweeps” through the ladder at periodic intervals. This interval depends
upon the number of instructions the CPU must perform (i.e. the number of rungs in the ladder), but it
is typically 8ms or 16ms. That means that the PMC is being updated every 8ms with new information
from both the CNC and the machine tool.
Sequence programming
memory
RD X0.0
AND R10.1
OR X6.1
AND.NOT R20.3
WRT Y0.0
CPU
PMC (Programmable Machine Controller)
Ladder diagram
relay contact symbol corresponding PMC
instruction
Figure 3 - 2. Execution of sequence program by PMC