Functional Blocks

External Power Supply

• 3-5A Switching PS
• Integrated or external battery backup
  • 12V sealed lead-acid

Power Input Protection

• In-line or crowbar diode
  • 6A Silicon rectifier type
  • 30V MOV (10mm)
    • Good for slow transients
    • High energy capability
    • Non-polarized
  • TVS Diode
    • Good for fast-moving transients
    • Quick rise time
• LC or capacitor filtering
  • Good for fast pulses, general ESD filtering

Internal Power Supply

• MC34063 Switching Power Supply
  • Adaptable for 5V or 3.3V
  • Cheap

Output Drivers and Protection

• Integrated Driver Chip
  • ULN2004 or ULN2803 for 7-8 outputs
  • Includes flyback diodes
• Suitable for relays or external
  • Current-limit external (LED/buzzer) outputs
• SMT Resistors

Relays

• 5A, Form C relays for external
  • No ground plane on output traces
  • Spacing for 250V+ voltage rating

Logic input protection

• Current-limiting resistor
  • MELF or other type with pulse-handling capability
• Pulse clamping
  • TVS Diode
  • Zener diode
  • 0.22uF decoupling capacitor
    • Provides ESD protection
    • May cause problems with data signals
• ESD Suppression chips

RS-485 Circuit

• Integrated driver chips
  • Dallas/Maxim
    • MAX-485
  • Texas Instruments
    • SN65HVD485EDR

Real-time Clock

• Dedicated chip with battery backup
  • Dallas/Maxim
    • DS-1307 (+/-20ppm, I2C)
    • DS3231 (+/-2ppm, I2C)
    • DS1302 (SPI, supercap backup)
  • NXP
    • PCF8563

External Power Rails

• Overvoltage protection
• Overcurrent protection
• ESD Protection
  • TVS diodes
  • Decoupling caps
  • High-side switching ICs with built-in protection
  • Polyfuses

Terminals

• 3.5mm screw terminals
  • Pluggable
  • Non-Pluggable