Lets begin straightaway with an argument….The world is moving towards RS-232, USB, Fire-wire and other serial bus standards, so why do I still bother using the parallel port?
For starters we are talking about controlling motors here. When it comes to controlling such devices, it’s always the legendary parallel port that hobbyists have been resorting to, since minimizing the required hardware, facilitating simple operation and a faster development cycle are some of the advantages that the parallel port has to offer.
So, without wasting anytime, lets start our session…your development time starts here…..
The Hardware:
Here is how the standard DB25 connector (parallel port connector) at the back-side of your CPU cabinet is configured:
Fig: Parallel port pin configuration
As the name refers, your data is transferred over data lines (bidirectional), Control lines are used to control the peripherals and of course, the peripherals return status signals back to computer through Status lines. These lines are connected to Data, Control and Status registers internally.
The details of parallel port signal lines are given below:
| Pin No (DB25) | Register – bit | Direction | Inverted |
1 | Control-0 | Out | Yes |
2 | Data-0 | In/Out | No |
3 | Data-1 | In/Out | No |
4 | Data-2 | In/Out | No |
5 | Data-3 | In/Out | No |
6 | Data-4 | In/Out | No |
7 | Data-5 | In/Out | No |
8 | Data-6 | In/Out | No |
9 | Data-7 | In/Out | No |
10 | Status-6 | In | No |
11 | Status-7 | In | Yes |
12 | Status-5 | In | No |
13 | Status-4 | In | No |
14 | Control-1 | Out | Yes |
15 | Status-3 | In | No |
16 | Control-2 | Out | No |
17 | Control-3 | Out | Yes |
18-25 | - | - | - |
You don’t generally need to bother about the control and status lines unless you are writing a parallel port driver.
And there is no great point in ‘reinventing the wheel’, so I guess you won’t bother to write the driver all over again unless it’s a device driver assignment given by your prof…
The data line is what will be of utmost importance to us, since we will either need to send out some navigational command (OUT) to our motor, or acquire some sensor input (IN).
Programming:
Before we start talking about programming, the register addresses are what we need to be aware of. Also downloading a port monitoring software would be a good idea to confirm our results when we are done.
Here is a link to download one such application: Click Here
Fig: Screenshot of parmon (parallel port monitor)
The numbers ‘378’, ‘379’ and ‘37A’ seen out here in the screen shot are parallel port register addresses. For a typical PC, the base address of LPT1 is 0×378 and of LPT2 is 0×278. The data register resides at this base address, status register at base address + 1 and the control register is at base address + 2. So once we have the base address, we can calculate the address of each registers in this manner. The table below shows the register addresses of LPT1 and LPT2
Register | LPT1 | LPT2 |
data register (base address + 0) | 0×378 | 0×278 |
status register (base address + 1) | 0×379 | 0×279 |
control register (base address + 2) | 0×37a | 0×27a |
Programming Concepts
Almost all programming languages allow programmers to access parallel port using some library functions. We will be discussing about the conventionally used ‘C’ language as well as MATLAB.
Parallel port access in ‘C’:
In C, there are following functions used for accessing the port:
outportb( PORTID, data);
data = inportb( PORTID);
outport( PORTID, data);
data = inport( PORTID);
outport() function sends a word to port, inport() reads a word from the port. outportb() sends a byte to port and inportb() reads a byte from the port. If you include DOS.H header, these functions will be considered as macro, otherwise as functions. Function inport() will return a word having lower byte as data at PORTID and higher byte as data at PORTID+2. So, we can use this function to read status and control registers together. inportb() function returns byte at PORTID. outport() writes the lower byte to PORTID and higher byte to PORTID+1. So this can be used to write data and control together. outportb() function write the data to PORTID. outport() and outportb() returns nothing.
A sample program for blinking LEDs connected at the parallel port at a constant frequency is as shown below:
#include”conio.h”
#include”dos.h”
#define PORT 0×378
void main()
{
while(!kbhit())
{
outportb(PORT, ~inportb(PORT) );
delay(1000);
}
}
NOTE: Don’t forget to start ‘parmon’ so that you can comfortably monitor your port’s real time status.
Also remember that the parallel port output is internally latched and hence it retains the same status till a new data is sent onto the parallel port. So, for a motor control application if you have a byte dedicated for movement, make sure you have a byte dedicated to halt the motor. This byte should be sent at the parallel port whenever you want to halt the motor, otherwise the latched output will make the motor run continuously.
Parallel port access in MATLAB:
Initially, we create a parallel port object as shown below,
parport=digitalio(’parallel’,’LPT1′);
Post this, we add the actual hardware lines to this newly created parallel port object as also define the direction of data flow as done below:
line=addline(parport,0:7,0,’out’);
We declare an array containing the data that we want to snd at the port pins,
pval = [1 0 1 0 0 0 0 0];
Finally, using ‘putvalue’, we send the data at the port pins.
putvalue(parport,pval);
Use ‘getvalue’ the read the status of the port.
Points to note:
In case you are using the parallel port to drive motors or any such load, beware!! There is a very high amount of risk involved in this as the parallel port might get loaded and cause damage to the motherboard.
Hence, it is always a good idea to connect an opto-coupler between the parallel port and the motor driving circuit. This ensures isolation and allows us to use the parallel port without bothering about the safety of the motherboard.
A schematic for motor control using parallel port can be found below:
It uses MCT2E, an opto-isolator along with L293D for driving 2 DC motors.
You can download the data sheets from: 424848_DS and 1330
So, go on and start programming. Your development time ends here!!!
No comments:
Post a Comment