Low-Cost, 24V Industrial Controller Is ESD-Protected
The circuit in Figure 1 incorporates PLC programmability with the low cost of relay control. These features are combined with a software program to form a robust 24V industrial controller that offers low cost, full surface-mount construction, low power, and ESD protection.
Figure 1. This low-cost, programmable industrial controller is an attractive alternative to larger, more expensive PLC controls.
With an 8-bit microcontroller used to provide the program control, the circuit's eight debounced and ESD-protected inputs can accept voltages up to 24V DC. The eight open-drain outputs can handle up to 45V DC and sink up to 250mA (or 1.5A of pulsed current). A 24V, low-power linear regulator drives the circuit, while a voltage monitor with a manual reset guards against power brownouts.
CMOS switch debouncer U2 ensures robust industrial-rated inputs and a wide input-voltage range. It offers eight fully debounced and ESD-protected inputs, eight tristate outputs, and a change-of-state output (CH-bar) for simplifying connections to the microcontroller. Pull-up resistor R3 provides a logic high that disables U1 and U2 during power-up until the microcontroller I/O has been configured.
An octal D-type latch (U3) supplies eight open-drain outputs, rated to 45V DC. These outputs can easily drive process indicator lamps, isolation relays, and other logic controls in a 24V control system. A built-in voltage clamp on each output enables direct connection of relays, solenoids, buzzers, and other inductive loads. Resistor R1 ensures that all outputs are off during power-up until the microcontroller I/O has been configured.
Microcontroller U1 includes 32 bytes of RAM for variables and 1232 bytes of EPROM for storing the process control program. (A low-cost, one-time- programmable version is also available.) The bidirectional pins of port A (PA0 to PA7) furnish an 8-bit data bus for I/O data to U2 and U3. Using software, the first three pins of bidirectional port B (PB0 through PB2) are configured as outputs. As a result, they can provide the control signals EN-bar, CLR-bar, and CLK for U2 and U3.
By issuing a logic-low reset during power-up, the U5 voltage monitor ensures automatic recovery during power brownouts. U5 also offers a debounced manual reset to U1 when the reset pushbutton is pressed. R2 prevents contention with the microcontroller's bidirectional RESET-bar pin.
Power to the controllers is provided by U4, a high-voltage, low-power linear regulator configured to deliver up to 30mA from a 5V output. U4 accepts input voltages to 28V DC. Therefore, it lets the controller function with 12/24V control systems and mixed-voltage control systems (i.e., 24V field devices and 12V control logic).
Since every U2 input pin (IN1 to IN8) has an internal, 63k pull-up resistor, each one requires an applied logic low to indicate a change of state. Input "8" shows a pushbutton-switch interface where R5 limits the 24V-to-ground current to 10mA. This enables the use of low-cost switch contacts and a logic-high level (24V) as the change-of-state signal. Input "1" demonstrates a limit-switch interface inlogic low (0V) is the change-of-state signal. U3's output "1" illustrates a lamp, solenoid, or other load connected to a 24V source.
The timer function internal to the microcontroller can replace expensive relay timers in the control system. Additional inputs are easily gained by adding another switch-debouncer IC, installing a pull-down resistor on the microcontroller's IRQ-bar pin, and adding steering diodes between each CH-bar pin and U1's IRQ-bar pin. One of U1's unused port-B pins can be configured as the EN-bar signal to the new IC. For analog functions, a serial digital-to-analog converter (DAC) or analog-to-digital converter (ADC) and the microcontroller's unused port-B pins (PB3 to PB5) can be used to implement 0 to 10V or 4-20mA process controls.
A similar version of this article appeared in the July 09, 2001 issue of Electronic Design magazine.