e00a052.pdf

BEGINNERS COURSE

PC Peripheral Design

Part 2: Software

By B. Kainka

In this second part we

concentrate on software.

We take a closer look at the

IOtest program presented in

the ﬁrst article and use simple program examples

in Visual Basic to control a model traffic light and a clock generator.

In the ﬁrst article in this series we introduced

the IOtest program. The majority of readers

with some programming experience will be

interested in how we access the serial inter-

face using Visual Basic. Here we will use a

library of program subroutines called

PORT.DLL written by H.-J.Berndt. This DLL

ﬁle is available from the Free Downloads sec-

tion of the Elektor Electronics website at

http://www.elektor-electronics.co.uk. For

those with no access to the Internet, the ﬁle

is also found on the diskette containing the

course software (see Readers Services

pages).

In Visual Basic, all the procedures and

functions deﬁned in the DLL are declared and

must be in an external module. In our case

this is called PORTS.BAS ( Listing 1 ).

The experienced (Visual Basic) user will

probably recognise the most commonly used

routines for serial communications, e.g.,

OPENCOM which is used to open the inter-

face for communication and indicates that it

is available to the program. SENDBYTE and

READBYTE are used to transfer data over the

serial interface. For our purposes here, the

more important routines are those

that access the control and status

lines of the interface. The procedures

DTR, RTS and TXD control these out-

put signals and CTS, DSR, RI and

DCD read the state of these input

signals. One input line that you can-

not read directly is RXD. This is

because it would normally be used

to carry the received serial data.

Also the PORT.DLL ﬁle contains rou-

tines for time measurement that we

will be using later, as well as many

other functions for the other PC inter-

faces (Parallel port, Joystick, Sound

and Video).

DTR

RTS

TXD

I/Otest

GND

The application program I/Otest is

shown in Listing 2 .

The essential core of the program is

the timer procedure

Timer1 _ Timer , which is automati-

cally called at predetermined inter-

vals. The check boxes 1 to 4 are acti-

000074 - 2 - 11

Figure 1. The traffic light LEDs.

Elektor Electronics

10/2000

(2)

BEGINNERS COURSE

Listing 1

Declarations for PORT.DLL

Declare Function OPENCOM Lib “Port” (ByVal A$) As Integer

Declare Sub CLOSECOM Lib “Port” ()

Declare Sub SENDBYTE Lib “Port” (ByVal b%)

Declare Function READBYTE Lib “Port” () As Integer

Declare Sub DTR Lib “Port” (ByVal b%)

Declare Sub RTS Lib “Port” (ByVal b%)

Declare Sub TXD Lib “Port” (ByVal b%)

Declare Function CTS Lib “Port” () As Integer

Declare Function DSR Lib “Port” () As Integer

Declare Function RI Lib “Port” () As Integer

Declare Function DCD Lib “Port” () As Integer

Declare Sub DELAY Lib “Port” (ByVal b%)

Declare Sub TIMEINIT Lib “Port” ()

Declare Sub TIMEINITUS Lib “Port” ()

Declare Function TIMEREAD Lib “Port” () As Long

Declare Function TIMEREADUS Lib “Port” () As Long

Declare Sub DELAYUS Lib “Port” (ByVal l As Long)

Declare Sub REALTIME Lib “Port” (ByVal i As Boolean)

vated and will show a tick when the corre-

sponding input line is switched to a ‘1’. The

three output lines are switched when the

user activates the corresponding check box.

The other parts of the program are used

to select and open the interface and we will

look at them a little later. We can see that the

interface will be initialised with a communi-

cation rate of 1200 baud, no parity bit, 8 data

bits and 1 stop bit. For our purposes here,

these communication parameters are entirely

superfluous because we are only interested

in the control lines that form part of the inter-

face. Windows will however configure the

port as if it were to communicate with, for

example, a modem. But all we need to do

here is to read the input status lines and

write to the output control lines.

Traffic Light controller

Listing 2

User program IOtest

Private Sub Form_Load()

i = OPENCOM(“COM2,1200,N,8,1”)

If i = 0 Then

i = OPENCOM(“COM1,1200,N,8,1”)

Option1.Value = True

End If

If i = 0 Then MsgBox (“COM Interface Error”)

TXD 1

RTS 1

DTR 1

TIMEINIT

End Sub

If you are new to Visual Basic it is a good idea

to choose some hardware that you can actu-

ally see working. For this reason a model traf-

fic light with three LEDs was chosen ( Fig-

ure 1 ). The effects of current limiting and

reverse voltage on the LEDs were discussed in

the ﬁrst article of this series.

The model lamp is well suited to experi-

menting for our ﬁrst excursion into program-

ming with Visual Basic. For the program

development you will need to start with a

new blank form, and using the mouse, select

and drag graphic control elements from the

list of tools onto the form ( Figure 2 ). The size

of each element and its position can be eas-

ily altered. Each element has a complete

range of properties that can be attached to it,

including size, colour, text and much more.

Those that you are not sure of yet can be sim-

ply left as they are. For our application we use

the following elements:

Private Sub Form_Unload(Cancel As Integer)

CLOSECOM

End Sub

Private Sub Option1_Click()

i = OPENCOM(“COM1,1200,N,8,1”)

If i = 0 Then MsgBox (“COM1 not available”)

TXD 1

RTS 1

DTR 1

End Sub

- Two labels with the properties (the text)

Caption = “fast” and “slow” respectively.

Private Sub Option2_Click()

i = OPENCOM(“COM2,1200,N,8,1”)

If i = 0 Then MsgBox (“COM2 not available”)

TXD 1

RTS 1

DTR 1

End Sub

- A horizontal slider or scroll bar (HScrollBar)

with the properties Min = 50, Max = 500

and Position =100

- A timer (Timer) with the property Interval =

100, i.e. 100 ms

Private Sub Timer1_Timer()

Check1.Value = CTS()

Check2.Value = DSR()

Check3.Value = DCD()

Check4.Value = RI()

If Check5.Value Then TXD 1 Else TXD 0

If Check6.Value Then DTR 1 Else DTR 0

If Check7.Value Then RTS 1 Else RTS 0

End Sub

- Two Option Buttons with the Caption =

“COM1” and “COM2” respectively, the but-

ton for COM2 is “true”

- The form itself contains the Caption = “Traf-

ﬁc Light”

Figure 3 gives an overview of the project. The

aforementioned module PORTS.BAS is also

included in the project and contains all the

10/2000

Elektor Electronics

BEGINNERS COURSE

Listing 3

Traffic lights program

Dim Time As Integer

TXD 1

RTS 1

DTR 1

End Sub

Private Sub Form_Load()

i = OPENCOM(“COM2,1200,N,8,1”)

If i = 0 Then

i = OPENCOM(“COM1,1200,N,8,1”)

Option1.Value = True

End If

If i = 0 Then MsgBox (“COM Interface Error”)

TXD 0

RTS 0

DTR 0

Time = 0

End Sub

Private Sub Timer1_Timer()

Time = Time + 1

If Time = 1 Then red

If Time = 40 Then redyellow

If Time = 50 Then green

If Time = 90 Then yellow

If Time = 100 Then Time = 0

End Sub

Sub red()

RTS 1

DTR 0

TXD 0

End Sub

Private Sub Form_Unload(Cancel As Integer)

CLOSECOM

End Sub

Sub redyellow()

RTS 1

DTR 1

TXD 0

End Sub

Private Sub HScroll1_Change()

Timer1.Interval = HScroll1.Value

End Sub

Private Sub Option1_Click()

i = OPENCOM(“COM1,1200,N,8,1”)

If i = 0 Then MsgBox (“COM1 not available”)

TXD 1

RTS 1

DTR 1

End Sub

Sub yellow()

RTS 0

DTR 1

TXD 0

End Sub

Sub green()

RTS 0

DTR 0

TXD 1

End Sub

Private Sub Option2_Click()

i = OPENCOM(“COM2,1200,N,8,1”)

If i = 0 Then MsgBox (“COM2 not available”)

necessary declarations for PORT.DLL. It

needs to linked with the software in each of

the projects so that they can all have access

to the procedures and functions that are

needed to control the serial interface.

In a Visual Basic program, events are

produced by procedures that are isolated

from each other. The overall flow of the pro-

gram is controlled by Windows. The proce-

dure ‘Private Sub Form_Load()’ will be called

right at the beginning of the program ( List-

ing 3 ) This procedure contains all the instruc-

tions needed to initialise the serial interface.

In this case, the serial interface will be open

and all the outputs of the interface will be

switched off. Lastly, the global variable

‘Time’ is reset to zero.

The function OPENCOM in

PORT.DLL returns a value indicating

if the interface has been successfully

opened. It will not be possible to

open it if it is already in use by

another program. Initially the pro-

gram will use this function to open

COM2. If the return value from this

function is 0 i.e. it is busy then

COM1 will be opened. This will be

displayed on the screen with the

value of option button 1 (COM1)

‘True’. If both COM1 and COM2 are

busy then a failure message will pop

up in a MessageBox.

In the normal course of events

the program will open COM2. If

however you want to use COM1

then you can point to the corre-

sponding button and click. This will

cause Windows to call the proce-

dure Option1.Click which will open

COM1 and check for a successful

returned status. This automatic

selection and manual override has

already been used in the IOtest pro-

gram and will also used in the

upcoming program examples. It

would be a simple matter to add

extra buttons to control the COM3

and COM4 interface.

Elektor Electronics

10/2000

BEGINNERS COURSE

Listing 4

Timerprocedure for blink program

Figure 2. The traffic light controller

form.

Figure 4. The traffic light program

during run time.

Private Sub Timer1_Timer()

Time = Time + 1

If Time = 1 Then

RTS 1

DTR 0

End If

If Time = 2 Then

RTS 0

DTR 1

End If

If Time = 2 Then Time = 0

End Sub

Figure 3. Project overview.

Figure 5. The Blinker Program.

Blinker/Clock generator

This next application is a blinker or clock gen-

erator with adjustable frequency ( Figure 5 ).

The circuit is the same as for the traffic light

experiment. This time, the TXD output is per-

manently switched on while DTR and RTS are

switched in anti-phase, i.e., when one is on the

other is off and vice versa. It is not too difficult

to see how this program can be expanded to

turn it into a mini running light display.

The software from the traffic light pro-

gram is used again for this exercise. The

timer procedure is, however, new ( Listing 4 ).

One difference here is that no additional pro-

cedures are called to control the output lines,

they are switched directly from inside ‘timer-

procedure’.

For the traffic light controller we

will need some timer function to

introduce a delay between chang-

ing the lights. In the DLL there is,

for example, a procedure called

DELAY. However, unlike program-

ming in DOS, individual Windows

programs do not have the entire

processing time devoted to them. It

would be pointless to write a pro-

cedure containing a simple timing

loop to give us the delays that we

need for the traffic light. Instead,

the traffic light timer will be event

controlled. For this we will use a

Windows timer. The timer period is

set to 100 ms. The procedure will be

called every 100 ms.

In the timer procedure there is

a variable called “Time” which is

incremented and gives the time in

tenths of a second, so the proce-

dure can be called to find out if a

specific period has elapsed before,

for example, the traffic light LEDs

are changed. The speed of switch-

ing has been chosen arbitrarily and

can be easily altered. The IF state-

ments are used to compare the

time values and switch the corre-

sponding LED.

The form ( Figure 4 ) also con-

tains a horizontal slider or scroll

bar. This slider is used to alter the

speed of the traffic light changing.

As soon as the slider is moved, the

procedure Hscroll1.Change is

called. This procedure will change

the interval in the timer corre-

sponding to the actual position

(Value) of the slider control. The

slider is calibrated from 50 to 500;

the timer can therefore be altered in

the range from 50 ms to 500 ms.

The LEDs on the DTR and RTS lines should

now blink alternately. The speed of the blink-

ing can be increased or decreased. As you

increase the speed you may notice that the

timing for each blink is not precisely regular.

The reason for this is that Windows cannot

maintain a time interval of exactly 50 mil-

liseconds because it is a multitasking oper-

ating system many other processes will be

running at the same time. For this reason

Windows is generally considered to be not

capable of ‘real time’ operation. However,

there are some tricks and techniques that we

can use and we shall be investigating them

in the coming articles.

(00074-2e)

10/2000

Elektor Electronics

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: