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By RICK WALTERS Addressable card for driving a stepper motor This interface card allows you to drive a stepper motor using software control. It plugs into your PC’s parallel port and you can connect up to eight units in daisy-chain fashion. The interface card featured here is the first of two new cards that allow you to control stepper motors via the parallel port of a PC. It is capable of driving one stepper motor, while the second unit (to be described next 54  Silicon Chip month) is capable of driving two stepper mo­tors. In practice, you can connect up to eight cards (in daisy-chain fashion) to the printer port, so that you can control eight different motors. Each card is set with a unique address from 1-8, so that it can be individually selected. In addition, two or more cards can be coded with the same address in a master-slave setup, so that even more motors can be controlled. Of course, those cards that have the same address will identically control their motors. In operation, an address from 0-7 is placed on three pins of the PC port connector, then the strobe line is toggled. This latches the address in a decoder. If this address matches that selected by a jumper on the card, the logic levels present on the port’s normal data lines are latched (stored) and fed to the motor drivers. Fig.1 (right): the circuit is based on address decoder IC1 and 8-bit data latch IC1. When the correct address is fed to IC1, the data on the Port A lines is latched into IC1 and transferred to the Q outputs. These outputs then drive transistors Q1Q12 to control the stepper motor. August 1997  55 Note that the card is capable of driving the stepper motor in both the forward and reverse directions. When the motor is not stepping, the driver transistors are turned off to prevent the motor from overheating. Circuit details The circuit of the card is shown in Fig.1. It uses IC1, a one-of-eight active low decoder, as the address latch. Basically, this IC looks at the binary coded decimal (BCD) data on its A, B & C inputs and pulls the corresponding decimal output (Y0-Y7) low. In greater detail, this only occurs when the strobe line from inverter stage IC3b goes to a logic high (+5V). This momen­ tarily pulls the latch enable input (pin 4) of IC1 high via a .001µF capacitor. The decoded output then goes low (0V) to give a unique address. If this is the output selected by the address link, the decoded logic low is fed to pin 2 of IC3a. IC3d inverts the strobe signal and so pin 3 of IC3a will also be low. As a result, pin 1 of IC3a goes high and momentarily pulls the latch enable input (pin 11) of IC2 high via a second .001µF capacitor. IC2 is a 74HC573 8-bit data latch. When its LE input is taken high, the data present on its Data inputs (D0D7), as fed in from Port A of the parallel port, is latched and transferred to the Q outputs. The latch enable signal then goes low 47ms later (as set by the associated 47kΩ pull-down resistor), so that the data remains latched until the next strobe signal. Resistors (0.25W, 1%) 1 10MΩ 4 2.2kΩ 1 47kΩ 1 470Ω 9 10kΩ from the positive supply rail through Q1, coil MA and Q4 to ground. Conversely, when outputs Q1 & Q2 are high and Q0 & Q3 are low, transistors Q6, Q2 and Q3 are tuned on and the current flows through the coil in the opposite direction. Therefore, depending on the logic levels at the Q0-Q7 out­puts of IC2, we can control the direction of the current through the two coils and thus the stepping direction of the motor. If all outputs are low, all the transistors are off and no current flows through either coil (ie, the motor is stopped). To actually step the motor it is necessary to switch the current through the coils in a logical sequence. Table 3 lists the different modes for driving a stepper motor, along with the binary code required at IC2’s output which, of course, is identi­cal to that at CON1. The decimal value can be used in a Basic program to apply the correct bit pattern to the parallel port. Almost all motors can be powered from the 12V supply, in­cluding centre-tapped 5V motors (because we don’t use the CT). If you want more torque and a faster stepping speed, you can run the motor from a higher voltage, in which case a resistor must be added in series with each coil to keep the motor current within specification. It is the inductance of the motor windings which limits the current and hence the torque, so by applying a higher voltage we get a higher initial current. Miscellaneous Tinned copper wire for links Card selected indicator Parts List 1 PC board, code 07108971, 120 x 112mm 1 DB25 PC-mount male rightangle connector 1 stepper motor, Oatley Electronics M35 or equivalent 1 8-way x 2-pin header strip (2.54mm pitch) 1 jumper for header strip 1 3-way terminal block (5.08mm pitch) 1 4-way terminal block (5.08mm pitch) Semiconductors 1 74HC137 decoder (IC1) 1 74HC573 8-bit latch (IC2) 1 74HC02 quad nor gate (IC3) 4 BD682 PNP Darlington transistors (Q1,Q2,Q7,Q8) 4 BD679, BD681 NPN Darlington transistors (Q3,Q4,Q9,Q10) 4 BC548 NPN transistors (Q5,Q6,Q11,Q12) 1 1N914 small signal diode (D1) 1 5mm red LED (LED1) Capacitors 2 100µF 25VW PC electrolytic 2 0.1µF monolithic ceramic 1 0.1µF MKT polycarbonate 2 .001µF MKT polycarbonate The only circuit function yet to be described is the card selected indicator. This is based on D1 and IC3c and lights LED1 whenever a valid address is received. This feature provides a convenient way of checking which card has been selected at any given time in a multi-card system. The way in which this works is quite straightforward. As shown, pins Motor drivers Transistors Q1-Q6 and Q7-Q12 form two bridge circuits which drive the stepper motor coils. One circuit is controlled by the Q0-Q3 outputs of IC2, while the other is controlled by the Q4-Q7 outputs. Because these two bridge circuits are identical, we shall only describe the circuit based on transistors Q1-Q6. We’ll begin by considering what happens when outputs Q0 & Q3 of IC2 are high and Q1 & Q2 are low. In this case, transistors Q5, Q1 & Q4 will all be turned on and so current flows Table 1: Resistor Colour Codes ❏ No. ❏  1 ❏  1 ❏  9 ❏  4 ❏  1 56  Silicon Chip Value 10MΩ 47kΩ 10kΩ 2.2kΩ 470Ω 4-Band Code (1%) brown black blue brown yellow violet orange brown brown black orange brown red red red brown yellow violet brown brown 5-Band Code (1%) brown black black green brown yellow violet black red brown brown black black red brown red red black brown brown yellow violet black black brown TRANSFORMERS •  TOROIDAL •  CONVENTIONAL •  POWER •  OUTPUT •  CURRENT •  INVERTER •  PLUGPACKS •  CHOKES STOCK RANGE TOROIDALS BEST PRICES APPROVED TO AS 3108-1994 SPECIALS DESIGNED & MADE 15VA to 7.5kVA Tortech Pty Ltd 24/31 Wentworth St, Greenacre 2190 Phone (02) 642 6003 Fax (02) 642 6127 Fig.2: install the parts on the PC board as shown here. Don’t forget to fit a jumper to the pin header to select the address of the card and take care when mounting the power transistors as they don't all face in the same direction. 8 & 9 of IC3c are normally pulled high via a 10MΩ resistor and so pin 10 is low and LED1 is off. However, when a valid address is received, the decoded output from IC1 goes low and so pins 8 & 9 of IC3c are pulled low via D1. This in turn switches pin 10 of IC3c high and so LED1 lights to indicate that the card has been selected. Because a card can be selected and deselected very quickly, a 0.1µF timing capacitor is included between the inputs of IC3c and ground. This ensures that the LED stays lit for one second after the card has been de­ selected. Building the card The circuit is easy to build, with all the parts mounted on a PC board coded 07108971 (120 x 112mm). Fig.2 shows the parts layout on the board, while Fig.3 shows the full-size etching pattern. Begin by checking your etched board for defects by compar­ ing it with Fig.3. In particular, check for undrilled holes and shorts between tracks, especially around the IC pads. This done, install the wire links (11), followed by the resistors and diodes. Table 1 shows the resistor colour codes but it is also a good idea to check the values using a digital multimeter, just to make sure. The capacitors can be installed next, followed by LED1 and the transistors. Be careful when fitting the transistors as two different TO-220 types are used. Note also that the metal faces of Q3, Q4, Q9 & Q10 (all BD679) face towards CON1 (the DB25 connector), while the metal faces of Q1, Q2, Q7 & Q8 (all BD682) face towards CON3. Take care to ensure that the LED is correctly oriented. Its anode lead will be the longer of the two, while the cathode lead will be adjacent to a flat section on the bevel at the bottom of the plastic body. Finally, complete the assembly by fitting the 8-way pin header and the P.C.B. Makers ! If you need: •  P.C.B. High Speed Drill •  P.C.B. Guillotine •  P.C.B. Material – Negative or Positive acting •  Light Box – Single or Double Sided – Large or Small •  Etch Tank – Bubble or Circulating – Large or Small •  U.V. Sensitive film for Negatives •  Electronic Components and •  •  Equipment for TAFEs, Colleges and Schools FREE ADVICE ON ANY OF OUR PRODUCTS FROM DEDICATED PEOPLE WITH HANDS-ON EXPERIENCE Prompt and Economical Delivery KALEX 40 Wallis Ave E. Ivanhoe 3079 Ph (03) 9497 3422 FAX (03) 9499 2381 •  ALL MAJOR CREDIT CARDS ACCEPTED August 1997  57 Listing 1 10 REM Step motor clockwise 20 PORTA = &H378 ‘This is for LPT1 Enter &H278 for LPT2 30 PORTC = PORTA + 2 ‘and card 1 selected 40 DATA 153, 150, 102, 105, 102, 150, 153, 105 50 FOR A = 1 TO 4: READ ROTCW(A): NEXT ‘Read data for clockwise steps 60 FOR A = 1 TO 4: READ ROTCCW(A): NEXT ‘Read data for anti-clock steps 70 OUT PORTA,105: OUT PORTC,11 ‘Set motor to known position 80 FOR A = 1 TO 12 ‘Go forward 12 steps of 30 degrees 90 FOR B = 1 TO 4: OUT PORTA,ROTCW(B) ‘Four steps of 7.5 degrees 100 OUT PORTC,11: OUT PORTC,10 ‘Select card one, then take strobe low 110 FOR C = 1 TO 350: NEXT ‘Delay to allow motor to step 120 NEXT B: NEXT A 130 OUT PORTA,0: OUT PORTC,11: OUT PORTC,10 ‘De-energise motor coils 140 FOR A = 1 TO 20000: NEXT ‘Pause for a while 150 REM Now step motor anti-clockwise 160 FOR A = 1 TO 12 ‘Go backwards 12 steps of 30 degrees 170 FOR B = 1 TO 4: OUT PORTA,ROTCCW(B) ‘Four steps of 7.5 de­grees 180 OUT PORTC,11: OUT PORTC,10 ‘Select card one, then take strobe low 190 FOR C = 1 TO 350: NEXT ‘Delay to allow motor to step 200 NEXT B: NEXT A 210 OUT PORTA,0: OUT PORTC,11: OUT PORTC,10 ‘De-energise motor coils three connectors. Make sure that the DB25 connector is sitting flat against the board before soldering its pins. Testing To test the board you will need a 25-way “D” male-to-female cable (ie, a printer cable) and a power supply capable of supplying 5V at a few milliamps and 12V at up to 1A. If you are careful, you can pick up the 5V supply from the games port on the computer. The connec­ tions on the 9-pin “D” connector are pin 5 for the 5V line and pins 4, 5 and 12 for ground. The 12V rail can come from a suit­able plugpack supply. Alternatively, you can wait and build the power supply to be described in next month’s issue. Before applying power, connect the card to your computer’s parallel (printer) port LPT1 using the extender cable. You will also have to install a jumper on the pin header to set the ad­dress of the card. If you only have one controller card, you can choose any address you like although it’s probably best to fit the jumper to the C1 position. That way, you won’t have to alter the program shown in Listing 1 in order to address the card. Now load Basic and enter the program shown in Listing 1. You can omit the line numbers if you use Q-Basic. You can also omit the remarks (after the ‘) as they are only there to give you an idea of what the software is doing. When you run this program, the motor should rotate clock­ wise one revolution, stop and then step anticlockwise to its original position. A pencil mark on the gear will let you see what is happening. Check that LED1 on the card lights to confirm that the card has been addressed. If you use LP2 as the parallel port, you will have to change line 20 (ie, Table 2 Fig.3: check your board against this full-size artwork before installing the parts. 58  Silicon Chip Card No. Address Card 1 11 Card 2   9 Card 3 15 Card 4 13 Card 5   3 Card 6   1 Card 7   7 Card 8   5 change &H378 to &H278). The address value for each card from 1-8 is given in Table 2. The illogical sequence of the numbers is due to the fact that both C1 and C3 on PortC are inverted logic; ie, if they are programmed high in Basic (or any other language), they will actually go low. If the stepper motor you use is different to that specified in the parts list, your results may not be the same as ours. If the motor runs in the wrong direction, just swap the wires to pins 1 and 2 of CON3. The motor we used has 7.5° steps and if the one you use is different (eg, if it has 1.8° steps), you will have to change the number 12 in lines 80 & 160 to some other value to get a complete revolution. For example, you would have to change 12 to 50 for a motor with 1.8° steps. A close examination of the program shown in Listing 1 will reveal how it all works and you can experiment with your own values. The values we have used are for a single full step with both windings energised. You Up to eight cards can be connected to the printer port, so that you can control eight different motors. Each card is given a unique address by fitting a jumper to an 8-way pin header. may wish to load the “one winding energised” values into the program and compare the torque difference. Fault finding If it doesn’t work, the first thing to do is to check that you have the jumper or link set for card 1. If this is OK, check that LED1 lights when you run the program. If LED1 doesn’t light, connect pins 4 & 16 of IC1 together and run the program again. If the LED now lights, then the prob­lem probably involves IC3b or the components connected to pin 4 (LE) of IC1. The same technique can be used to test the circuitry that drives the latch enable input (pin 11) of IC2 (ie, connect pin 11 to pin 20). SC August 1997  59