Real-World Control via LPT

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Real-World Control via LPT

Introduction

For simple, byte-world or analogue I/O, PC expansion slots offer a far from cost-effective solution. But there is a very low-cost and easy to implement alternative path to simple i/o in the guise of the standard Centronics printer port, as we will explain.

Printer Port Structure

On PC compatibles, the printer port is complies with the Centronics standard, which involves transfer of eight parallel bits of date at a time. To control data flow, a number of handshaking bits are needed, bringing the total number of lines to 17. These comprise 12 outputs and 5 inputs. All the lines are TTL compatible, simplifying interfacing. Table 1 summarises connections at the PC end.

For controlling printer data flow, there are three bytes in the PC I/O map. Addresses of these bytes depend on which of the four LPT printer ports is being accessed. Generally, LPT1 address is at 378₁₆ and LPT2 is at 278₁₆but these addressees may vary, Table 2.

Table 1. Pin-out of the almost universal Centronics printer port.
Pin	I/o	Centronics signal	Pin	I/O	Centronics signal
1	O	Data strobe	14	O	Auto line feed
2	O	Data bit 0	15	I	Error status
3	O	Data bit 1	16	O	Initialise
4	O	Data bit 2	17	O	Select
5	O	Data bit 3	18	-	0V line
6	O	Data bit 4	19	-	0V line
7	O	Data bit 5	20	-	0V line
8	O	Data bit 6	21	-	0V line
9	O	Data bit 7	22	-	0V line
10	I	Data acknowledge	23	-	0V line
11	I	Busy	24	-	0V line
12	I	Out of paper	25	-	0V line
13	I	Selected

Table 2. Within the PC I/O map, these three bytes are available for controlling the printer port. Their position on the map depend on whether they relate to LPT1,2,3 or 4.
Bit	Base	Base+1	Base+2
0	Data bit 0	Unused	Data strobe
1	Data bit 1	Unused	Auto linefeed
2	Data bit 2	Unused	Initialise
3	Data bit 3	Error status	Select
4	Data bit 4	Selected	Interrupt enable
5	Data bit 5	Out of paper	Not used
6	Data bit 6	Data acknowledge	Not used
7	Data bit 7	Busy	Not used
Notes: Bit 0 is the least significant bit. Base is 378₁₆ for LPT1, 278₁₆ for LPT2. Base and base+2 are write only, base+1 is read only. Bit 4 of base+2 is not available on the port but is an internal line used to enable printer port interrupts.

Note: analogue i/o with eight-bit resolution and 1KHz sampling rate via the printer port is straightforward. This bandwidth is obviously not sufficient for audio or image processing, but it is more than enough for measuring room temperature, light level, battery voltage, etc. Cost of the interface is also very low, but the main difficulties lie in matching the connection requirements of analogue i/o devices with the pins available on the parallel port.

In summery the pines available are:

Address	Port	Lines
Base	0	8 outputs
Base+1	1	5 inputs (1 interrupt)
Base+2	2	5 outputs

In our project the interface is simpler than that, the device we are doing the interface for accepts digital output, so no need for D-to-A and A-to-D conversion and sampling.

Our part in the project was to write the software for an already built hardware.

The hardware is a machine that is built in the LAB and used to make a Printed Circuit Board. It consist of three stepper motors one for each X, Y, Z directions, each motor needs four inputs to operate, and they are all pulses in specific sequence that will move the motor one step forward or backward. So we need four bits output for the X motor and four bits output for Y motor and four bits output for Z motor. The interface was just driver consists of two 74374 TTL chips ( ) as in the figure 2-1.

For the system to be practical it must read an "NC Drill File", which is generated after the design of a circuit in a standard PCB software such as "PSpice". So software must understand the format of the NC Drill File and convert it to commands to the PCB machine. As expected the software have to be written in the Intel assembly, but because of the power of C++ and it’s low level language capabilities I wrote it in C++.

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