Robotix Tutorial 8

31 st Jan to 1 st Feb 2008 TECHNOLOGY ROBOTICS SOCIETY Indian Institute of Technology, Kharagpur

Computer Control Parallel & Serial Port Communication

PC Parallel Port - Features
Commonly known as the printer port
25 pin D-Type connector
It has 12 digital output pins, 5 digital input pins
Pins operate at the TTL voltage level i.e. 0 – 5V
Port identified by a base address in the computer I/O memory space

PC Parallel Port – Pin Layout

PC Parallel Port – I/O Registers
Most PC’s have 378 Hex base address
8 output pins are accessed via the DATA port (Base address)
4 output pins are accessed via the CONTROL port (Base address + 2)
5 input pins are accessed via the STATUS port (Base address + 1)

PC Parallel Port – I/O Registers
- marked bits in register byte are unused

Accessing Parallel Port – VC ++ #include <conio.h> #define Data 0x378 // Base address #define Status 0x379 // Base address + 1 #define Control 0x37a // Base address + 2 int main () { unsigned char Bits; Bits = 0b11001010; _outp(Data, Bits); /* output data */ Bits = _inp(Status);/* input data */ return (0); }

PC Serial Port - Features
Most commonly 9 pin D-Type connector. 25 pin D-Type is rare
Pins operate at -25 to + 25 voltage levels
Data transmitted as a bit sequence
Known as the EIA RS232C port or simply RS232

EIA RS232C Specifications
Logic 0 is between +3 to +25 volts
Logic 1 is between -3 to -25 volts
-3 to +3 volts region is undefined
Maximum data rate of 19,600 bps

Serial Port vs. Parallel Port
Serial cables can be much longer that Parallel cables
50V swing compared to 5V swing
Null Modem Configuration just needs 3 core cable compared to 19 core parallel port cable
Serial suited for wireless transmission
Microcontrollers just need two pins (RXD & TXD) for communication

Null Modem Configuration
Pin 3 Transmit Data
Pin 2 Receive Data
Pin 5 Signal Ground
Pin 4, 6, 1 shorted together
Pin 7, 8 shorted together

RS-232 Waveforms
Asynchronous Communication i.e. separate clock at both ends
Frame synchronization done by the start / stop bit
Line is kept logic 1 when idle. Start bit is logic 0
Bit order from LSB to MSB

RS-232 Level Converters
For using the RS232 data, digital devices need TTL/CMOS voltage levels
Level conversion is done using IC MAX232 / DS232

RS-232 frame decoding
The decoding of the bit sequence frame is done by computer hardware
When communicating with robots, decoding done by onboard microcontroller
Microcontroller Serial communication interface is known as UART (Universal Asynchronous Receiver Transmitter)

Accessing Serial Port - VB
MSComm Control 6.0 is used for serial port communication

MSComm Control Properties

Com Port identification number
Settings: Baud Rate, Parity Check, Data Bits, Stop Bits
Hand Shaking Method : Xon/Xoff for null-modem configuration

InBufferSize : Input buffer size
OutBufferSize : Output buffer size
RThreshhold : Receive size for interrupt generation
SThreshhold : Min. transmit size
InputLen : Number of bytes to be picked up from the input buffer
EOFEnable : Search for EOF character

ParityReplace : Parity Error replacement character
NullDiscard : Null characters sent to the receive buffer or not
RTSEnable : Request to Send enable
DTREnable : Data Terminal Ready

Example Code
‘ Receive Data
Private Sub CommunicationTerminal_OnComm()
Dim InputBuffer As String
If CommunicationTerminal.CommEvent = comEvReceive Then
InputBuffer = CommunicationTerminal.Input
End If
‘ Use InputBuffer as per requirement
End Sub
‘ Transmit Data
If CommunicationTerminal.PortOpen = True Then
CommunicationTerminal.Output = “Hello”
End If

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Robotix Tutorial 8

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