Is a signal's STATE ON or is it OFF?

[Richards]

Is a signal's STATE ON or OFF? Is it HIGH or is it LOW?

I remember how confused I was when I started in electronics. The terminology was foreign and it seemed that there was no logic to "logic". George Balding, who had headed defense research at Kaiser, moved to Utah. He took me under his wing, and little by little electronics started to make sense.

SINKING or SOURCING?
Page 7 of the Acorn CNC Controller Specification Manual shows that the Input screw terminals are of Type "SOURCING", so do they SOURCE or SINK current when they are turned "ON"? The simple answer is that the Inputs provide a current source so that devices like switches or NPN transistors can provide a path to GROUND for current to flow. So, a device attached to a SOURCING Input must provide a current SINK - or path to ground. Page 5 of the manual shows that each Acorn Input has an LED and a resistor connected to it. It also shows that all of the resistors are connected to a common 24VDC voltage/current supply. When a switch or transistor allows current to flow, the Input's internal LED turns ON.

ON or OFF?
If current is flowing through the Input's LED, is the signal's STATE ON or is it OFF? That depends. The Wizard allows us to decide whether ON means that current is flowing through the device (NC) or whether ON means that current is not flowing through the device (NO). To know whether to assign NC or NO to that Input, we have to know how the signal works. On my ClearPath SDSK-2321S-RNP servos, if I connect the RED wire of the HLFB line (fault line) to GND and the GREEN wire to an Acorn Input, the ClearPath servo will conduct (SINK) current if the motor is NOT in a fault condition. If I select NC for the Acorn Input and if the ClearPath is ready to run, then the Acorn's STATE indicator will show GREEN or READY for that signal. (View the Acorn's internal I/O statuses.) However, the DMM DYN4 servo driver SINKS current when that drive is in a fault condition, so I would have to select NO in the Wizard to indicate that the DMM servo is not faulting.

The STATE of a signal depends on what is connected to that signal and whether that signal conducts or does not conduct current when it is "READY".

The beauty of the Acorn is that a DIY user can configure the Acorn so that the GREEN or READY STATE on the Acorn's pin illuminates when the signal is in its normal condition.

Knowing that the Acorn can be configured to use either current flowing or no current does not solve the problem of mixing different signals. In the case of using ClearPath motors and DMM DYN4 drivers/motors on the same machine, the user must know how to convert one type of sigal to the other type of signal. He can use relays, PLCs, transistors or microcontrollers. The choice is his and the responsibility is also his.

Here's the way that I handle mixed signals going into the Acorn. I condition the signal (change the way that the signal operates) so that I have a GREEN light in the Acorn's I/O panel when the signal is in its NORMAL condition. That means that whenever I see a RED light, I know that I have an error that needs attention. In the case of using both ClearPath servos and DYN4 drivers with DMM servos, I have to pre-condition the FAULT Input signal. I've done that by connecting those Ready/Fault signals to an Arduino or Raspberry Pi or (very soon) to a PLC. Then, inside that microcontroller or PLC, I can use computer logic to provide a READY STATE signal if ALL conditions are good/ready/normal or I can provide a FAULT or NOT READY STATE signal if ANY condition is not good/ready/normal. An added bonus of off-loading some of the logic from the Acorn to an external device is that I can capture signals (such as motor enable) so that the servo motor holds position from the moment that its position is sensed (by a limit switch) until the Acorn is turned off. Doing that means that I don't need to add an electric brake to an axis.

Learing about ON/OFF, HIGH/LOW, READY/NOT READY and machine STATES is part of being a DIY builder. Centroid has done a very good job to make things as easy as possible. Their hook-up schematics are clear, but when anyone enters the DIY world, the operative word is YOURSELF. I've lived in that world since the 1970's. I enjoy it. It's not hard but it can be complex, especially when acquiring the basic knowledge and a working vocabulary. That brings us to the other two words in DIY - DO IT.