Points

The points are the inputs, outputs, internal coils and registers of the MatPLC. Each point has a name by which it is known throughout the MatPLC.

Points may be 1-bit, corresponding to discrete inputs, outputs, internal/memory coils, flags etc, or multi-bit (up to 32-bit), corresponding to analog inputs, outputs, integer or floating point registers etc.

Points are defined in the PLC section of the config.

Point ownership

For each point, there is exactly one module which is allowed to change it, to ensure that processing takes place in an orderly fashion.

The diagram on the right shows the situation for one point and three modules: the point is owned by module 1. It has a private copy of the point, and is allowed to write to it. At the bottom of the scan, the point is copied to the global area, from which it is copied to modules 2 and 3 at the top of their scans.

This means that while troubleshooting, if the point has an incorrect value, we know to check module 1.

Example

For instance, in the basic demo, there are the following points:

L1, L2, L3, L4: the four "lights", which are changed by the chaser program;
left, right: the left and right "buttons", which are changed by the kbd module and used by the chaser program to control the direction of movement; and
quit: the quit "button", which is also changed by the kbd module; the plcshutdown module reads it so that it knows when to shut everything down.

The declaration in the config file looks like this:

point L1        "light 1" Chaser
point L2        "light 2" Chaser
point L3        "light 3" Chaser
point L4        "light 4" Chaser
point left      "<- (key L)" Kbd
point right     "-> (key R)" Kbd
point quit      "quit (key Q)" Kbd

All of the points are also displayed on the screen, by the vitrine module. No special access configuration needs to be done to allow this - any module may read any point it wishes; it's only changing them that's restricted.

In the modbus demo, the L1, L2, L3 and L4 points are also sent to the Modbus slave.

FAQ

Q: I'm looking for a solution to share a point between 2 or more modules.

A: In the MatPLC, all modules are written by one module (the "owner") and read by all modules. There is no way to have a point that could be written by two modules which are both active at the same time.

It's probably better to have a separate point for each module that wants to write, anyway, and then have explicit logic deciding between those values. That way if the wrong value comes out, you know where to look for the problem...

For instance, you might have:

automatic logic writes to foo_auto
HMI writes to foo_manual

if automatic_mode
    then copy foo_auto to foo_cur
    else copy foo_manual to foo_cur

all modules read foo_cur

$Date: 2006/03/06 04:20:16 $