31st Jan to 1st Feb 2008

TECHNOLOGY ROBOTICS SOCIETYIndian 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

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

MSComm Control Properties

MSComm Control Properties 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

MSComm Control Properties

MSComm Control Properties 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