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Addressable card for controlling two stepper motors Based closely on the design published last month, this new interface card allows you to control two stepper motors via your PC. It plugs into the PC’s parallel port and you can connect up to eight units in daisy-chain fashion. By RICK WALTERS We envisage that this new design will be suitable for those who wish two drive two stepper motors to achieve 2-axis control. The new card is capable of independently driving each stepper motor in either forward or reverse direction, or it can drive just one stepper motor at a time. When a motor is not stepping, its driver transistors can be turned off to prevent the motor from overheating. 80  Silicon Chip As with last month’s design, the card is set with a unique address from 1-8 so that it can be individually selected and two or more cards can be coded with the same address in a master-slave setup. 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 is the address selected by a jumper on the card, the logic level present on the port’s normal data lines is latched (stored) and fed to the motor driv­ers. Circuit details Refer now to Fig.1 for the circuit details. The decoding and latching circuitry is identical to that published last month but, for those who missed that article, we’ll recap the details. IC1, a 74HC137 one-of-eight active low decoder, is used as the address latch. This IC looks at the BCD address Fig.1 (right): when the correct address is fed into IC1, the data on the Port A lines is latched into IC2 and appears at its Q outputs. These outputs then drive transistors Q1-Q24 to control the stepper motors. September 1997  81 8 & 9 of IC3c are pulled high via a 10MΩ resistor and so pin 10 is low and LED1 is off. When a valid address is received, pins 8 & 9 of IC3c are pulled low via D1 (since the decoded output from IC1 goes low). As a result, pin 10 of IC3c switches high and LED1 lights to show that the card has been selected. The 0.1µF capacitor connected from pins 8 & 9 of IC3c to ground ensures that the LED remains on for at least one second. Motor drivers Transistors Q1-Q24 make up four H-bridge circuits which drive the stepper motor coils. These circuits are identical, so we will only describe the circuit based on tran­sistors Q1-Q6. This top circuit is driven from the Q0 & Q1 outputs of IC2. Let’s first consider the situation when Q0 is high and Q1 is low. In that case, transistor Q5 will turn on and this will also turn on transistors Q1 and Q4. As a result, current now flows from the positive supply rail and through transistor Q1, coil M1A and transistor Q4 to ground. Conversely, when output Q1 is high and Q0 is low, transis­tors Q6, Q2 and Q3 turn on and the current flows through coil M1A in the opposite direction. If both the Q0 and Q1 outputs are low, all transistors are off and no current flows. Therefore, depending on the logic levels on the Q0-Q7 out­puts, we can control the direction of the current pulses through the coils and thus the stepping direction of each motor. To actually step a motor, it is necessary to switch the current through its coils in a logical sequence. Table 3 lists the different driving modes and shows the binary code required at IC2’s output. This code is, of course, identical to that required at D0-D7 (Port A) of CON1. The decimal value is also shown in Table 3 and this can be used in a Basic program to apply Fig.2: exercise care when installing the power transistors on the PC board. You must use the correct type at each location and it must be correctly oriented. data on its A, B & C inputs and pulls the corresponding decimal output (Y0-Y7) low. However, this can only happen when the strobe line from inverter stage IC3b goes high and momentarily pulls the latch enable (LE) input of IC1 high via the series .001µF capacitor. As a result, the card will be addressed if the decoded output is selected by the address link. In that case, the decoded low will be fed to pin 2 of IC3a and to the cathode of D1. At the same time, the high strobe signal is inverted by IC3d and so pin 1 of IC3a goes high and momentarily pulls the LE 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, it latches the data present on its D0D7 inputs as fed in via Port A of the parallel port. This data is transferred through to IC2’s Q outputs and is used to control the stepper motors via transistor H-bridge driver circuits. The LE signal then goes low 47ms later (as set by the 47kΩ pull-down resistor), so that the data remains latched until the arrival of the next strobe signal. D1, IC3c and LED1 form the card selected indicator. Normally, pins Table 1: Resistor Colour Codes ❏ No. ❏  1 ❏  1 ❏  9 ❏  8 ❏  1 82  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 Parts List 1 PC board, code 07208971, 120 x 112mm 1 D25 PC-mount male rightangle connector 2 stepper motors, 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) 8 PC stakes This view clearly shows how the power transistors are fitted to the heatsink. Note that each transistor must be isolated from the heatsink using a TO-220 insulating washer. Semiconductors 1 74HC137 decoder (IC1) 1 74HC573 8-bit latch (IC2) 1 74HC02 quad NOR gate (IC3) 8 BD682 PNP Darlington transistors (Q1,Q2,Q11Q14,Q23,Q24) 8 BD679, BD681 NPN Darlington transistors (Q3,Q4, Q9,Q10,Q15,Q16,Q21,Q22) 8 BC548 NPN transistors (Q5,Q6,Q7,Q8,Q17-Q20) 1 5mm red LED (LED1) 1 1N914 small signal diode (D1) Capacitors 2 100µF 25WV PC electrolytic 2 0.1µF monolithic ceramic 1 0.1µF MKT 2 .001µF MKT Resistors (0.25W, 1%) 1 10MΩ 8 2.2kΩ 1 47kΩ 1 470Ω 9 10kΩ Heatsink parts (optional) 1 aluminium bar, 110 x 6 x 3mm 16 TO-220 insulating washers 8 3mm x 15mm bolts 8 3mm nuts 16 3mm flat washers Fig.3: this diagram shows the drilling details for the aluminium heatsink. the correct bit pattern to the parallel port. Almost all motors can be powered from the 12V supply, including centre-tapped 5V motors (as we don’t use the CT). If you want more torque and a faster stepping speed, you can run the motors from a higher voltage but you should add a resistor in series with each coil to keep the motor current within specifica­tion. PC board assembly Fig.2 shows the parts layout on the PC board (code 07208971). As usual, check your etched PC board against the full-size pattern shown in Fig.4 before installing any of the parts. Once this has been done, begin the assembly be installing PC stakes at the eight external wiring pints, then install the wire links (11), the resistors and the diode (D1). The ICs (or IC sockets if you use them) can go in next, followed by the capacitors, address jumper, the LED and the D connector. Take care with the LED polarity – its anode lead will Miscellaneous Tinned copper wire for links be the longer of the two. In addition, the cathode lead is adjacent to a flat section on the bevel at the bottom of the plastic body. The eight BC548 transistors can now be installed, followed by the 16 power transistors. Note that it is advisable to bolt the power transistors to a common heatsink if you intend driving high-current stepper motors for long periods. The heatsink September 1997  83 Listing 1 10 REM Step both motors clockwise 20 PORTA = &H378 ‘This is for LPT1 Use &H278 for LPT2 30 PORTC = PORTA + 2 ‘and card 1 selected 40 DATA 85, 102, 170, 153, 170, 102, 85, 153 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 anticlock steps 70 OUT PORTA,85: 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 anticlockwise 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 fitted to the prototype was cut from square-section (6 x 12mm) aluminium rod and is 110mm long. Fig.3 shows the drilling details for the heatsink. The best procedure is to first loosely attach the transistors to the heatsink and then mount the entire assembly on the PC board. Be sure to use insulating washers to isolate the metal faces of the transistors from the heatsink. The BD682 PNP transistors are all mounted on one side of the heatsink, while the BD679 NPN types are all mounted on the opposite side. Once the assembly is in position, solder one lead at either end, then tighten all the mounting bolts. The assembly can then be adjusted so that it sits parallel to the PC board and the remaining leads soldered. Finally, complete the assembly by fitting the 8-way pin header, the DB25 connector and the 3-way terminal block. Testing the board To test the board, first connect it to the computer via a standard printer cable. You will also need a power supply capable of supplying 5V at a few milliamps plus a 12V supply capable of powering the two stepper motors (probably around 2A capacity). If necessary, you can obtain the 5V supply from the games port on the computer (provided it has one). Pin 5 on the 9-pin “D” connector is the +5V rail, while pins 4, 5 & 12 are ground. If you only have one card, the address jumper should be fitted 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. The line numbers can be omitted if you are using Qbasic. You can also omit the remarks (after the ‘), as they are only Table 2 Fig.4: here is the full-size etching pattern for the PC board. 84  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 Table 3: Stepper Motor Sequences Full Step - One Winding Energised Step No. Polarity Q0 Q1 Polarity Q2 Q3 Polarity Q4 Q5 Polarity Q6 Q7 Step 1 M1A+ 1 0 M1B0 0 0 M2A+ 1 0 M2B0 0 0 Decimal 17 Step 2 M1A0 0 0 M1B+ 1 0 M2A0 0 0 M2B+ 1 0 68 Step 3 M1A- 0 1 M1B0 0 0 M2A- 0 1 M2B0 0 0 34 Step 4 M1A0 0 0 M1B- 0 1 M2A0 0 0 M2B- 0 1 136 Q0 Q1 Polarity Decimal Full Step - Both Windings Energised Step No. Polarity Q2 Q3 Polarity Q4 Q5 Polarity Q6 Q7 Step 1 M1A+ 1 0 M1B+ 1 0 M2A+ 1 0 M2B+ 1 0 85 Step 2 M1A- 0 1 M1B+ 1 0 M2A- 0 1 M2B+ 1 0 102 Step 3 M1A- 0 1 M1B- 0 1 M2A- 0 1 M2B- 0 1 170 Step 4 M1A+ 1 0 M1B- 0 1 M2A+ 1 0 M2B- 0 1 153 Q0 1 Q1 0 Polarity M1B0 Q2 0 Q3 0 Polarity M2A+ Q4 1 Q5 0 Polarity M2B0 Q6 0 Q7 0 Decimal 17 1 0 M1B+ 1 0 M2A+ 1 0 M2B+ 1 0 85 Half Step - Windings Turned On & Off Step No. Polarity Step 1 M1A+ Step 2 M1A+ Step 3 M1A0 0 0 M1B+ 1 0 M2A0 0 0 M2B+ 1 0 68 Step 4 M1A- 0 1 M1B+ 1 0 M2A- 0 1 M2B+ 1 0 102 Step 5 M1A- 0 1 M1B0 0 0 M2A- 0 1 M2B0 0 0 34 Step 6 M1A- 0 1 M1B- 0 1 M2A- 0 1 M2B- 0 1 170 Step 7 M1A0 0 0 M1B- 0 1 M2A0 0 0 M2B- 0 1 136 Step 8 M1A+ 1 0 M1B- 0 1 M2A+ 1 0 M2B- 0 1 153 there to give you an idea of what the software is doing and play no part in the program operation. When you run this program, the motors should both rotate clockwise one revolution, stop briefly and then step anticlockwise to their original positions. In addition, the “selected” LED should light to confirm that the card has been addressed. Note that the values shown in Listing 1 are for a single full step with both stepper windings energised. As an experiment, try loading the “one winding energised” values into the program and check the torque difference. If you use LPT2 as the parallel port (instead of LPT1), you will have to change line 20 (ie, 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 motors you use are different to those speci­fied in the parts list, your results may not be the same as ours. If the motor runs in the wrong direction, just swap one pair of motor leads on the PC stakes. The stepper motors we used have 7.5° steps and if yours are different (eg, if they have 1.8° steps), you will have to change the number 12 in lines 80 and 160 to get a complete revolution. For example, if the motor has 1.8° steps, you would have to change the number 12 to 50. Fault finding The stepper motors used with the prototype card were M35s from Oatley Electronics. If you strike problems, first check that the address jumper is set for card 1 (C1). If so, check that LED1 lights when you run the program. If the LED doesn’t light, connects pins 4 & 16 of IC1 together and rerun the program. If the LED now lights, check IC3b and the components between IC3b and pin 4 of IC1. The same technique can be used to test the circuitry that drives the LE input of IC2 (ie, connect pin SC 11 to pin 20). September 1997  85