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These are PLC boards and units used in Centroid CNC controls from 1993 to present. Not all versions of all boards are pictured.

Centroid 15/15 PLC Board

Basic PLC with 15 inputs (top), 15 relay outputs (bottom), and optional 8-bit analog spindle speed output.

Used in M40 family controls 1993-1995, and in M39 controls 1997-2002

Inputs are 5VDC current-sourcing, powered by an external 5V supply.

Logic power is derived from a 24VAC supply.

Analog spindle speed output requires a separate "SPINOVER" board, mounted to the right of the main PLC in the above picture.

Some early controls used the partially-populated board shown at right, with 15 inputs and 8 relay outputs.

Note that the unit at right also lacks some of the components in the top right section of the daughterboard. They are part of the analog spindle speed output, and were not required for a non-inverter control.

Centroid RTK1 PLC Board

Very basic PLC with 1 input and 2 relay outputs, combined with AC fuses and distribution; limit switch connections; and Emergency Stop connections.

Used in M10 and M20 controls, 1993-1996.

Input is 5VDC current-sourcing, and is generally used for Emergency Stop.

Centroid RTK2 PLC Unit

The RTK2 combined the I/O of the 15/15 PLC and SPINOVER board with DC power supplies and AC power distribution in a single assembly, in order to simplify control cabinet wiring.

The unit consists of three boards in a stack, with a fourth short board joining the ends.

It was used in M40 family controls from 1994-1997, and in M400 controls from 1996-2002.

Inputs are 5VDC current-sourcing, powered by an on-board 5V supply.

Input power and logic power are derived from a 24VAC supply connected to the short board (RTK2D).

SPINOVER components for 8-bit (255 step) 0-10V analog spindle speed output were installed on the middle board (RTK2B). Some units, for non-inverter mill controls, were shipped with partially-populated boards that lacked the SPINOVER components.

To see if your RTK2 supports analog spindle speed control, look for the two blue trim pots at the top end of the top board.

Illustration of RTK2 units with and without spindle speed control

There are status LEDs at the top and bottom of the unit, indicating status of the fiber communications and of the power supplies.

How to interpret the RTK2 status LEDs

Although the RTK2 logically has the same 15 outputs as the 15/15 board, OUT2 is not externally available. It is wired on-board to reverse the polarity of the analog spindle speed output signal. This is useful when controlling some older spindle drives (notably Reliance and GE ValuTrol DC drives) which use a bipolar +/-10V reference to control both speed and direction.

In factory controls the RTK2 was generally mounted upright, with the short board at the bottom and the fiber optics and SPINOVER pots at the top.

Earlier editions had a cover plate and label over the top board. Later versions had the top board exposed, with plastic finger guards over the fuse holders.

In Revolution controls the RTK2 was generally mounted sideways, with the short board on the left and the fiber optics and spinover pots on the right.

Centroid PLCIO2 Unit

The PLCIO2 is a general-purpose PLC unit which expanded the I/O protocol to support 35 inputs, 39 outputs and a 12-bit analog spindle speed output.

Used in M400, T400, and some M39 and T39 controls, 2002-present.

Inputs may be 5V, 12V or 24V, either current-sourcing or current-sinking. They are configurable in banks of four: different banks on the same PLCIO2 may operate at different voltages and/or with differing current flow. Input SIP resistors must be installed to match the input voltage on each bank of four inputs.

Input power must be externally supplied.

Logic power is supplied by an external switching power supply, providing +5, +12 and -12 VDC to header H1.

Of the 39 outputs, 22 drive on-board relays rated up to 10A and 250V; the remaining 17 are 5V solid-state outputs which require external relays or SSRs.

The analog spindle speed output can be configured for either 0-5V or 0-10V.

There is no on-board provision for bipolar analog output. You must supply an external DPDT relay to switch the analog polarity if your spindle drive requires it.

Centroid RTK3 PLC Unit

Following the pattern of the RTK1 and RTK2, the RTK3 combines the inputs and outputs of the PLCIO2 with power distribution, fusing, and a layout that favors the wiring of typical mill applications.

The RTK3 is specifically directed at mills with the most common armless and swing-arm tool changer designs.

Inputs are all 24VDC, variously current-sourcing or current-sinking.

Input power and logic power are derived from a 24VAC supply connected to the middle board.

115VAC and 230VAC are connected for pass-through to various fused outputs.

The original RTK3 had on-board 3-pole relays for direct control of a flood coolant pump and an ATC carousel motor. However, experience showed these relays were not suitable for even very small motor loads. Since mid-2007, the coolant and carousel relays on the RTK3 have been used only to switch coil power (generally 24VAC) to external motor starters.

Koyo DL205 PLC and Centroid OPTIC232

For applications which require more input and output points than are available on Centroid's own PLC units, Centroid provides the OPTIC232 interface. This is fiber optic serial interface which communicates with Koyo DirectLogic DL205 and related PLCs, available from AutomationDirect (

The Centroid PLC software exchanges blocks of data bits with the Koyo PLC as needed. The protocol includes 32 inputs (passed from Koyo to Centroid) and 48 outputs (passed from Centroid to Koyo). 12 of the output bits hold the analog spindle speed request; the remaining 36 may be used for general purposes.

The OPTIC232/Koyo system is sometimes used in combination with a Centroid 3/3 PLC board, to provide fast-responding inputs for a touch probe and/or tool detector. In this configuration, the first 3 inputs and first 3 outputs are reserved for the Centroid 3/3 PLC.

The Koyo itself may have hundreds of inputs and outputs. On complex machines, the Koyo is generally programmed to handle the detailed sequencing of various tasks (such as a tool change), so that only a few bits need to be exchanged with the Centroid PLC software (for example, a tool number and start-tool-change request out, and a tool-change-complete signal back in).

The OPTIC232 interface was introduced in 1997. It has mostly been used for board-level dealer retrofits. It was also used for factory controls on ATC mills prior to the development of the PLCIO2.

Centroid 3/3 PLC Board

The 3/3 is a simple single-board PLC with 3 inputs and 3 relay outputs.

It was used in some M15 controls, and is also used in OPTIC232/Koyo systems to provide higher-speed inputs for a touch probe and/or tool detector, and also to provide a 5V-compatible input for the drive fault signal from a SERVO1-family drive or OPTIC1 interface.

The inputs are 5V current-sourcing, powered by an external supply.

Logic power is 5DVC, also externally supplied.

Centroid SPIN232 Interface

SPIN232 provides spindle on/off, direction, and analog speed control via a PC serial port output. It is a good way to add programmable spindle speed control to a system with an older PLC that does not support such features directly (e.g. RTK1, partially-populated 15/15 and RTK2 units, and early M15 controls).

The SPIN232 set consists of three components: the SPIN232 board itself; a switching power supply for logic power (+5, +12 and -12 VDC); and a fiber optic transmitter on a female DB9 plug, to plug in to one of the PC serial ports.

The analog output is 12-bit (4096 steps), and can be configured for 0-5V, 0-10V, +/-5V, or +/-10V output.

The SPIN232 requires at least version 7.00 of the DOS-based CNC7 control software. It will work with any version of the Linux-based CNC10 control software.

Combined PLC and Servo Drive Units

See the Servo Drives page for descriptions of the SERVO3IO, DC3IO and All-In-One DC units used in M39S and M400S controls.

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Copyright © 2020 Marc Leonard
Last updated 21-Jan-2020 MBL