Spy on serial data – Arduino

A serial protocol mostly uses a master/slave configuration with different addressing schemes:

• SPI: Serial Peripheral Interface; pins often labeled MOSI, MISO, SS or Select, and S_CLK or CLK.
  • Here is a library and user guide. Pins 10-13 are used.
    
    
• I²C / TWI: Inter-Integrated Circuit / Two Wire Interface; pins often labeled SDA and SCL.
  • The library is included, called Wire, and here is a user guide. Pins 4 and 5 are used with the Arduino Diecimila.
    
    
• 1-Wire: Actually needs two or more wires -- at least power/data and ground.
  • Here is a library and user guide or two. Uses any free GPIO.
    
    
• UNI/O: used with some EEPROM ICs from Microchip; will likely have to write your own library.
  
  
• U[S]ART: Universal [Synchronous/] Asynchronous Receiver/Transmitter; typically does not use master/slave configuration, only being used for point-to-point communication.
  • The library is built in, and there are extensions (two) and optimizations. It uses pins 0 and 1.
    
    
• USB: Universal Serial Bus; uses a host/device scheme akin to master/slave. Pins often labeled V_CC, DATA+, DATA-, and ground or GND.
  • Although possible without, usually requires extra hardware and interpreters for 8-bit micros like the Arduino to use. Ready-made Arduino platforms have USB-to-serial converter chips for IDE communication and extensions; there are also some shields (with libraries).
    
    
• PS/2: Personal System/2 connector, an old IBM PC std. connector; pins are often labeled V_CC, DATA, CLK, and ground or GND; often made interchangeable with USB sockets.
  • Here is a library and guide. Any GPIO can be used for DATA, but pin 3 must be CLK.

The library NewSoftSerial by Mikal Hart allows the user to implement an interrupt-driven software (as opposed to hardware, like UART peripherals) routine for serial communications. Use it if you need to, but be aware that debugging clashing interrupts is more challenging than debugging polling routines (which is apparently what the default libraries do).

As I understand, you connect your Arduino to two different target machines and on one it works and on the other it doesn't.

So it seems there is a difference between the initialization requirements of the two machines. On this page at the very bottom there is a listing of a possible initialization sequence. Start by comparing your initialization to that one.

It will be a lot easier by using a logic analyzer. I am using the Intronix Logicport, but there are both cheaper and better ones, though not at the same time.

Tapping into an open-collector bus is a bit cumbersome because you don't see which device is talking. However, if you put in a series resistor at the end where the pullup is not, you can tell by the voltage level which device is holding down the bus. Every open-collector bus (like PS/2) needs pullup resistors, usually they are built in in the PC. You can see the different voltage levels easily on a DSO. With only a LA you have to record twice with different threshold voltages.