Functional Blocks

External Power Supply

• 3-5A Switching PS
• Integrated or external battery backup
  • 12V sealed lead-acid
  • Connection point for monitoring DC/Battery voltage

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 and immunity

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

• Form C relays for external switching
  • Omron G5SB-1 DC12 (5A, rectangular)
  • Omron G5LE-14 DC12 (10A, square)
• No ground plane on output traces
• Trace separation 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
• Op-Amp on inputs
  • Provides impedence matching and logic buffering
  • Good choice for high-impedence and other non-standard devices

Logic pull-up

• Internal pull-up resistor (software addressable)
  • Provides ~50K impedence
  • Some logic pins may not have pull-ups avialable
  • Weak compared to external resistor/state easily changed
• External 10K resistor
  • Robust, simple
  • Not as flexible as internal pull-ups
  • Can be configurable via jumper

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
• Crystal
  • Maxim Crystal Selection Notes

External Power Rails

• Overvoltage protection
  • MOVs
  • TVS diodes

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

Terminals

• 3.5mm screw terminals
  • Pluggable, straight
    • Phoenix 8-position header, part #1945151
    • Phoenix 2-position pluggable terminal, part #1984015
    • Phoenix 3-position pluggable terminal, part #1984028
    • Phoenix 4-position pluggable terminal, part #1984057
    • Phoenix 6-position pluggable terminal, part #1984073
  • Pluggable, right-angle
  • Non-Pluggable

Circuit Board

• PCB Specs
  • FR4 PCB material
  • 0.62" substrate thickness
  • 1oz Copper
  • HASL finish or Gold
  • Double-sided with PTH and vias
  • Solder Mask both sides
  • Silk-screened legend on both sides

• Grounding
  • Continuous ground plane or mesh on bottom layer
  • Optional double ground or power plane on top layer
  • Exposed ground plane at case screw holes
  • Separate ground for differential signals as needed