1. Connecting input and output devices to electronics.
Practical Design Considerations
I/O Circuits
Connecting input and output devices to electronics.
prepared by Prof. George Slack (EE)
Copyright © 2006 Rochester Institute of Technology
All rights reserved.

2. Prerequisite: Schematic Guidelines
Start your schematic(s), if not already.
Sketch a drawing or visualization your project including the following:
Interface Devices
ALL inputs and outputs (don’t stop at the board’s edge!)
power On/Off, reset and safety switches, sample select switch
removable AC power cord
connectors
harnesses (& cables)
board(s)
test points
LEDs (more is better)
Jtag, USB, RS232, Ethernet (may not need the form factor)
accessory power connectors

3. Steps in Designing I/O Circuits
Step 1. Interface Specifications
Step 2. Specification Sheets & Application Notes
Step 3. Analysis I/O
Step 4a. Synthesis Output Devices
Step 4b. Synthesis Input Devices
Step 5. Analysis Noise immunity
Step 6 Analysis Harness Signal Response
Step 7 Connectors/ Pins

4. Step 1: Interface Specifications
Bandwidth, frequency response; rf to dc
Data rate
Analog versus digital (you should know by now)
Circuit protection
Power/ Current
Voltage Levels: Vdd/Vcc, multiple?
Devices (Vdd vary from 1.5 VDC to 18 VDC)
Output Control: Sink or source?
Data Controls: enables, select lines, etc.
EMI Noise
Heat

5. Step 2. Specification Sheets & Application Notes Save Hours of design time! Exploit these Design Notes!
Many Specification Sheets have recommended applications that may apply directly to your design! Use them!
Not sure what you need? …. or researching components?
Manufacturer’s Home Pages
Suppliers: DigiKey, Mouser and Allied offer good on-line spec sheets
Google (I must admit, I do this first….)
RIT Library services
If complex solution, don’t start from scratch!
Purchase device evaluation/ demo kits (USB, Microcontroller, Motor controllers)
Concept development phase is relatively cheap and quick in comparison with other phases.
Effective teams will generate hundreds of concepts
The main messages include the benefits of a structured process and the value of multiple perspectives in the process. Students should realize that although concept generation is a creative activity, it also requires structured exploration and investigation through many sources.

6. (continued Examples)
CMOS (i.e. MicroController Interfaces) devices may need protection electronics.
Responsive to signal processing spec’s.
Example 1. PIC18F2455 Interface considerations
See attached pdf.
Example 2. Freescale Semiconductor Inc - application schematic
Motor Controller MC3PHACVDW
If the spec sheet document is large, send me the link and I will print for you. Soon may be 400 pages but DO IT!

7. Step 3. Analysis I/O Impedance matching to your control electronics
Outputs Inputs
Low Voltage to High Current Higher Voltage to Low Voltage
Low Voltage to High Voltage Higher Current to Low Voltage
Low Current to High Current
Low to Current to High Voltage
Apply to Thevenin Equivalent Circuits
Source electronics
Device being driven
RLC loads (& potential for unwanted current and voltage spikes)

8. Step 4a. Synthesis Output Devices
Logic: Microcontroller to input of electronics drivers.
Driver: (voltage to voltage, voltage to current.)
Device:
Motors
Linear Actuators
Solenoids
Storage Capacitors
HVPS
LEDs
Heaters

9. cont. Types of Output Driver Components
Matching (Voltage, Impedance, Current):
Components
Transistor: Single stage and Darlington.
JFET
MOS FET
IGBT DC to AC inverters (hybrid cars, mass transit)
Devices:
Isolators: Optical Drivers
H bridge: bi-directional control for DC Motors (See later slide)

10. Step 4b. Synthesis Input Devices
Op Amp
Isolator: Optical and Magnetic
Level Shifter/ Translator
(other) Power
Regulate
DC to DC converters
voltage boasting/ ± polarity
current boasting

11. Step 5. Analysis Noise immunity
Circuit noise immunity is the ability of a device or component to operate in the presence of noise disturbance .
Electro Static Discharge is the sudden discharge (i.e. transients, surge). To the circuit, this is a rapid high voltage, low current situation.

12. 5 Continued Where does noise get into electronics?
through ground connections and loops
through power supply connections
through signal inputs
through inadvertent ESD
(human touch, lightning)
through Inductive devices (motors)

13. 5 Continued How does noise get into electronics?
Energy Coupling
(Conductive, Inductive, Capacitive)
EMI - Current surges
(ElectroMagnetic Interference) An electrical disturbance in a system due to natural phenomena, low-frequency waves from electromechanical devices or high-frequency waves (RFI) from chips and other electronic devices. Allowable limits are governed by the FCC.
RFI – high impedance devices requiring very limited current.
(Radio Frequency Interference) High-frequency electromagnetic waves that emanate from electronic devices such as chips.
If the source is sufficiently strong this can enter your circuit.

14. Cont 5 ESD and unwanted signals
Fatal to Electronics:
Inadvertent user misuse.
Extreme cases of user abuse and solutions:
Bridge to block reverse polarity,
Schottky diode: very fast switching times and low forward voltage drop. As low as 0.15 volts for low ma applications.
Zener diode across the input.
Circuit breaker – GFI

15. 5. Synthesis CMOS gate Reduce Transients
Protect from?
Misuse during debug and testing.
ESD and reverse polarity
Solution: Diode circuits to protect against
reduce transients
Component Specification Sheets
Solution: Low pass filter

16. 5. Isolating Noise
Analog and Digital Optocoupler /Optoisolators Somewhat expensive ($1/ channel) but good isolation.
an electronic device that uses optics to transfer a signal while keeping the receiving and transmitting circuits electrically isolated .
Analog Devices:
(Analog Devices)
OPTO:

17. 5 Example of Driver Circuit
Driver Circuit – Opto 22 ODC5
See diagram below for application.
Output Device – 2.5a, 50 vdc Inductive Load

18. 5. Polarity Protection + (+) Input Port – 1N5822 or 1N5817
Schottky diode
(–) +
Input Port –
1N4001
see page 238, MOSFET solution

19. 5. Over-voltage Protection
Fuse +
input port –
(+)
(–)
Zener diode
(also MOV)
1N5339 (5.6V for a 5.0V input)

20. 5. Over-voltage Protection for Digital Inputs

21. 5. Minimize Bandwith One solution: Lowpass Filter

22. 5. CMOS and Noise characteristics Print out the specification sheets.
1 uA input? Not sure? Get the spec sheet!...
Draw the equivalent circuit for each internal pin. That is, CMOS versus TTL input impedance, output is pull-up or pull-down circuit, current limiting resistor value.
CMOS inputs have very high input impedance which is good for low power consumption for well protected electronics but susceptible to misuse when connecting to the outside world.

23. 5. Decoupling Capacitors
Know your performance response needs. Try to eliminate frequencies outside your performance needs (i.e. filtering, smooth out spikes in DC power of IC’s).
Common practice is to isolate IC’s with capacitors. See manufacturer’s Spec Sheet for capacitor values.
Filters current transients when transistors switch with a digital logic gate.

24. Step 6 Analysis Harness Signal Response
Design Considerations: data rate, distance, noise, parasitic capacitance, reflections.
Frequency Response/ Rise Time needs
DC to 100 khz Open Wire
DC to 40 MB/s Ribbon Cable (less than 3’)
SCSI, SPI-3 applications
DC to 300 mhz Twisted Pair - unshielded, ribbon
DC to 100/1000 MB/s 10/ 100/ 1000 BaseT ethernet Cat 5 minimum spec. RJ45 connector.
Coax, VHF 3000 megahz (scope probe, cable TV) attenuation, reflectance, Cable TV, Rf
DC to 4 gigahz Fiber Optics
If you are using ribbon cable, run the lines in pairs. That is, signal and next line is its return. Every other line will be a signal separated by its return line.

25. Step 7 Synthesis Harness -Twisted Pair
When purchasing harness cable:
Unshielded Twisted Pair (UTP)
Shielded Twisted Pair (STP)
Study the manufacturer’s Specification to match to your needs.
Practical Design Considerations

26. Cont 7 Harness -Twisted Pair
What is does: Cancels out crosstalk from neighboring wires and electromagnetic interference from external sources

27. 7 Practical Harness Considerations
Current, Voltage Needs
Gauge of wire, insulation
Use and Abuse
Connect/ disconnect needs
Routing, protecting
Mounting, vibration and stability
Gold versus Tin; environment, corrosion, current rating
Instrumentation quality for long life
Color Coding for ease in Debug and future use.
As an example;
Red Vcc/Vdd
Black Grd/ Vss
Org for signal
Fabricating: Harness board and nails

28. 8 Connectors/ Pins Add all needed I/O connectors to your schematic(s).
Connect/ disconnect needs – screw terminators, push type
Major Manufactures: Amp, Molex
Location
between assemblies
interface to other projects (collaborate with other teams)
instrumentation
Get crimp compatibility to pin manufacturers (style and gauge).
Pins: crimp versus solder
Current rating rule of thumb is 10x. i.e. 100 ma purchase 1 amp pins
Types; ribbon, D shell, PCB mount

29. Misc: (H Bridge Considerations)
Motors or whenever you need to direct current.
Introduction to H bridge operation:
Designing an H-Bridge and PWM Circuits and Code
Debugging: Initial reset may close a short circuit or stress on the H bridge. Symptom: Motor shaft may pulse or flinch at power on. May cause immediate failure.
Design Needs: Inductive loads and protection diode.

30. Summary: Start your schematic(s), if not already.
Sketch a drawing or visualization your project including the following:
Define I/O Design Needs
Specification Sheets & Application Notes
Analysis I/O
Design Output Devices
Design Input Devices
Consider Noise immunity
Harness; Signal Response
Connectors/ Pins
Next Steps:
Define your core electronics and software development