Presentation for Desert Code Camp 2012.1

1. Implementing RS-232 Serial Ports
On An Arduino
DESERT CODE CAMP 2012.1
Presented by Don Doerres
Embedded Pro Guy
don@azlaborlaw.com

2. TODAY’S TAKEAWAYS
 The Arduino has a ―Low Level‖ serial
connection; many electronic devices have
higher voltage serial connections called RS-
232
 This class will explain
 The basics of the serial connection
 What the low and high level signals look like
 How to convert between the types of signals
 Some generalized RS-232 interfacing
suggestions

3. LET’S PICK A CHARACTER FOR AN EXAMPLE
 Let‘s use capital ‗C‘ for Code Camp
 Look this up on an ANSI Chart
 Dec: 67, Hex:43, Binary, 8 bits: 01000011

4. THE BASIC SERIAL PROTOCOL 1
 The bits of the characters come one at a
time, through a single wire.
 The wire ―idles‖ at a ‗1‘ level to show that it is
alive
 The bits come out low order bit first
 We will program for bytes of 8 bits each

5. THE BASIC SERIAL PROTOCOL 2
 We know the wire is alive because it idles at
‗1‘, the higher voltage level
 We know in advance how fast bits will be
coming out of the wire; this is called the
―baud rate‖
 So, our Arduino is sampling the incoming
wire for a transition from ‗1‘ to a ‗0‘

6. THE BASIC SERIAL PROTOCOL 3
 So now we have one start bit
 This will be followed by 8 bits of the byte
 And, to keep this straight, we will have 1
extra bit of 1 at the end of the string of bits to
guarantee that a ‗1‘ to ‗0‘ transition can take
place to indicate the start of a next byte

7. PUTTING IT ALL TOGETHER
 1 Start Bit, 8 data bits, 1 Stop bit
 The bits of the byte come out low order bit
first in time
 So we will have this string of 10 bits, with
gray showing the state of the wire before and
after the byte:
…1 1 1 1 0 1 1 0 0 0 0 1 0 1 1 1 1 1…
Time

8. ANOTHER VIEW OF THE SAME THING
 Here we show both the bit stream and the
Arduino voltage levels

9. NOW RS232
 On the Arduino, the serial line swings
between 0 and 5 volts (some are 0 to 3.3V)
 RS-232 is an older protocol
A ‗1‘ is -12V
 A ‗0‘ is +12V
 Don‘t even think about connecting the
Arduino directly to this…please

10. ANOTHER VIEW OF THE SAME THING
 This time RS232
 A ‗1‘ is -12V
 A ‗0‘ is +12V

11. SO HOW DO WE GET FROM LOW LEVEL TO
RS232?
 We need a hardware level translator
 This one is a Sparkfun PRT-00449

12. HOW TO HOOK TO ARDUINO
 Note the signal arrows between the adapter
and the Arduino

13. TEST CODE INCLUDES AND DEFINES
#include <SoftwareSerial.h>
#define INSOFT1 7
#define OUTSOFT1 8
int incomingByte = 0;
SoftwareSerial SWSerial(INSOFT1, OUTSOFT1);

14. TEST CODE SETUP
void setup()
{
Serial.begin(9600);
SWSerial.begin(9600);
}

15. TEST CODE LOOP
void loop()
{
while (Serial.available() > 0)
{
incomingByte = Serial.read();
SWSerial.print((char)incomingByte);
}
while (SWSerial.available() > 0)
{
incomingByte = SWSerial.read();
Serial.print((char)incomingByte);
}
}

16. SCREEN CAPTURES

17. SCREEN CAPTURES

18. MORE STUFF…
 Q: How do I hook up right?
 A: Use an RS232 Analyzer and adapters

19. HOOK TO THE MAIN COMPUTER
 The TD light should light
 Note TD and DTR, may light for a more
advanced connection

20. HOOK TO THE TARGET
 The RD light should light
 Sometimes Target powered from the host,
and there will be no lights—or the target is
off!
 Note possible:
 CD
 CTS
 DSR

21. HOOK TOGETHER
 The ideal outcome – all the lights!

22. FIXING IT
 Sometimes you need a NULL adapter on one
side

23. IF YOU DO THIS A BIT…
 Invest in some adapters
 These are a long term investment
 I bought the first of mine 30 years ago…they
wear well

24. SOURCE FOR RS232 GEAR
 These companies offer different adapters
over time
 Radio Shack (www.radioshack.com)
 Frys Electronics (www.frys.com)
 This company is more expensive, but has a
huge variety in stock at all times
 Black Box (www.blackbox.com)

25. RS232 TO LOW LEVEL ADAPTER
 Sparkfun has two different ones
 PRT-00133 ($6.95) is a through hole kit
 PRT-00449 ($13.95) is surface mount and is pre-
built
 www.sparkfun.com

26. QUESTIONS?
 Find more slides from Don at
www.slideshare.net/dondoerres
 The Arduino code shown came be
downloaded from
https://github.com/dondoerres/drdCodeCamp2012.
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