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Stepper motor driver with onboard buffer This new buffered design stores the instructions for up to 63 revolutions and can be jumpered for forward or bidirectional stepping. Design by RICK WALTERS While this new stepper board is similar in function to the designs featured in the August & September issues, it has the advantage of an on-board buffer to store data from the computer. This means that the computer could give an instruction to step the motor by, say, 50 steps. The computer can then move on to other tasks, for example, monitoring 60  Silicon Chip the I/O card (described in July 1997) while the motor is stepping. By contrast, the two previous designs required the computer to issue continuous instructions while the motors were being stepped; it could not perform any other function while a motor was stepping. As with the previous designs, this new buffered stepper driver can be daisy-chained with seven others, either buffered or unbuffered. For example, if you wanted to produce an XY plotter, you could have two of these buffered stepper drivers connected to the parallel port. The computer could then control both steppers for the XY plotter and still have time to perform other tasks. We have produced new BASIC listings to go with the buffered card and these are featured elsewhere in this article. The procedure for driving the buffered card is virtually the same as for the unbuffered card: an address from 1-8 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 the jumper on the card, the logic level present on the port’s normal data lines is latched into the buffers. Once that happens the card takes over and the motor is stepped to the required position. Jumper options This buffered card is capable of driving the stepper motor in either forward or reverse direction. A jumper on the card selects forward only or bidirectional stepping. In forward only mode, using a 7.5 degree per step motor, up to 63 revolutions can be stored, in bidirectional mode the maximum is 32. The motor begins stepping at a preset slow speed and accel­erates to the preset maximum speed for that particular motor and supply voltage. When the motor is not stepping all the drivers are turned off, thus preventing the motor from overheating. Another jumper selects full step or half step operation and provision is made via additional jumpers for the computer to interrogate the card(s) to determine whether it is still stepping or can accept another instruction. Circuit details Refer now to Fig.1 for the circuit details. While the over­all operation of the circuit is quite complex it can be broken down into a number of simple blocks. The first of these is the card select logic which is carried out by IC1 and IC2. IC1 is a 74HC137 three line to eight line active low latched decoder. This IC looks at the BCD address 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 IC2a goes low and momentarily pulls the latch enable (LE) input of IC1 low via the series .001µF capacitor. Step counter Once the desired card has been selected, the number of steps the motor has to make is taken care of. This information will have been loaded into PortA and is present on the preset inputs (P0-P3) of step counters IC3 & IC4. The data is loaded into IC3 & IC4 by the action of pin 5 of IC2c going high (+5V) which takes the PL (parallel load) inputs of these two ICs high. Once there is any data present in the ICs, the TC pins (terminal count, pin 7) which were low will go high. Parts List 1 PC board, code 07109971, 176 x 123mm 1 stepper motor, Oatley Electronics M25 or equivalent 1 25-pin PC mounting R/A “D” male connector 1 200kΩ PC mount trimpot (VR1) 1 500kΩ PC mount trimpot (VR2) Semiconductors 1 74HC137 octal latch (IC1) 1 4572 complex gate (IC2) 2 4029 presettable counters (IC3,4) 1 74HC4046 phase locked loop (IC5) 1 74HC4017 decade counter (IC6) 1 74HC02 quad NOR gate (IC7) 1 74HC32 quad OR gate (IC8) 4 74HC4066 quad analog switch (IC9,10,13,14) 1 74HC00 quad NAND gate (IC11) 1 74HC112 dual JK flipflop (IC12) 4 BD681 NPN power transistors (Q3,Q4,Q9,Q10) 4 BD682 PNP power transistors (Q1,Q2,Q7,Q8) This has two outcomes: the output of OR gate IC8b (pin 6) will go high and via D4, it will rapidly turn on the CMOS switch­es IC13 and IC14, allowing pulses to reach the stepper motor coils, MA & MB. We’ll come back to describe how MA & MB are driven later in this article. This high level from pin 6 of IC8b is inverted by IC2e and the inhibit pin of IC5 (pin 5) which was held high now goes low. This allows the VCO (voltage controlled oscillator) in this chip to start. The oscillator output at pin 4 is a square wave which begins clocking decade counter IC6. Note that IC2 is an odd chip, as it contains four inverters, one 2-input NAND gate and one 2-input NOR gate. Phase counter Each time IC6 is clocked it will sequentially take each of its 10 outputs high. Depending on the voltage at the cathode of D2, it will be reset by IC8a 4 BC548 NPN transistors (Q5,Q6,Q11,Q12) 1 2N7000 N channel IGFET (Q13) 4 1N914 signal diodes (D1,D2,D3,D4) Capacitors 2 100µF 25VW electrolytic 2 0.1µF monolithic ceramic 4 0.1µF MKT polyester 2 .01µF MKT polyester 4 .001µF MKT polyester Resistors (0.25W, 1%) 1 1MΩ 13 10kΩ 5 100kΩ 4 2.2kΩ 2 47kΩ 1 1kΩ Miscellaneous 1 7-way terminal strip (5.08mm spacing) 1 8 x 2 pin strip 1 5 x 2 pin strip 1 2 x 2 pin strip 2 2-pin strips 5 jumpers for above 1 58 x 6 x 12mm aluminium bar 4 3mm x 16mm bolts 4 3mm nut 8 3mm flat washer 4 3mm star washer 8 TO-220 insulating washers when its output is stepped to pin 1 or pin 11. The resistor and capacitor on pin 15 are necessary to widen the reset pulse, as IC6 is able to be reset with a pulse which is too narrow to clock the step counters. (This is one of the problems of mixing HC and 4000 series devices.) The pulse which resets IC6 also clocks the step counters, IC3 & IC4, which are connected so that they count down (ie, pin 10 tied low). When they are empty (zero count) both TC pins will go low and pin 6 of IC8b will go low, inhibiting the oscillator in IC5 as pin 11 of IC2e will go high. Diode D4 is now reverse biased and the voltage at pin 13 of IC13a and IC14a will slowly fall to ground as the 100kΩ resistor discharges the .01µF capaci­tor. So to recap, the card is selected and the number of steps loaded into the down counters. After this number of steps has been counted, the VCO will December 1997  61 62  Silicon Chip Fig.1: presettable up/ down counters IC3 & IC4 form a buffer for data from the computer’s printer port. This lets the computer download steps and it can then perform other functions while the motor is stepping through. be inhibited and will stop driving the phase counter. The logic signals to the stepper motor transistors will also be turned off, preventing any current flow in phase windings MA and MB. Full step - half step If you have looked at the driver software for the previous stepper motor cards you may have observed that for a full step, four sub-steps are used, but for half steps eight are needed. The same situation applies in this case (refer Table 3). A jumper across J3 sets the full step condition. This pulls pins 1 & 2 of IC11a low which results in the cathode of diode D2 being pulled high. This resets the phase counter (IC6) and the step counters are now clocked by IC8a when pin 1 of IC6 goes high; ie, after four steps. For the half step mode, a jumper across J2 pulls pins 1 & 2 of IC11 high, which holds diode D2’s cathode low, preventing pin 1 from resetting the counter. IC6 will be reset and will also clock IC3 and IC4 when it reaches a count of nine; ie, when pin 11 goes high, after eight steps. Speed ramp up Before we look at all the gates connected to the outputs of IC6, we should discuss the operation of the VCO, in IC5. It starts the motor stepping at a slow speed, as set by VR2, and gradually increases the stepper rate to a value dictated by the fast control VR1. This is done because a stepper motor will ramp up to a higher speed than it will start from, due to the inertia of the rotor. We achieve this speed increase by December 1997  63 Fig.2: component overlay for the PC board. Note that the ICs are all oriented differently so be careful to insert them in the right way. The same point applies to the rest of the semiconduc­tors and the electrolytic capacitors. varying the VCO frequen­ cy, which depends on two factors, the voltage on pin 9 and the resistance from pins 11 & 12 to ground. When pin 9 is low, the output frequency is set by VR2 (set slow), and when pin 9 is taken to +5V, the output frequency is dictated by VR1. By charging the 0.1µF capacitor through the 1MΩ resistor, the voltage on pin 9 slowly increases from zero to 5V and conse­quently the motor speed increases from the slow control setting mode is selected. As we explained previously, the full step mode has four increments, while the half step has eight. By switching in the extra capacitor we hold the maximum motor speed the same in both modes. This allows a card to have its trimpots initially set for a particular type of motor, allow­ing it to run in either mode without any readjustment to the presets. to the fast control setting. When the MSD counter, IC3, is empty its TC output will swing low and rapidly pull pin 9 of IC5 low, by courtesy of diode D3. This will immediately reduce the motor speed to SLOW for any counts remaining in IC4. The filter network on pin 7 of IC4 is used, as one of the data books claims that glitches can be pres­ent at this output. Mosfet Q13 switches an additional capacitor in circuit when the full step Decoder The outputs of IC6 are fed to seven gates which are used to decode and direct the logic levels to the appropriate points. The explanation of how this is done is too involved to go into Table 1: Resistor Colour Codes ❏ No. ❏   1 ❏   5 ❏   2 ❏ 13 ❏   4 ❏   1 64  Silicon Chip Value 1MΩ 100kΩ 47kΩ 10kΩ 2.2kΩ 1kΩ 4-Band Code (1%) brown black green brown brown black yellow brown yellow violet orange brown brown black orange brown red red red brown brown black red brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown yellow violet black red brown brown black black red brown red red black brown brown brown black black brown brown Table 2: Capacitor Codes ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.1µF   100n   104 .01µF  10n  103 .001µF   1n0   102 in detail. Table 3 explains the logic sequence used to drive the stepper in each mode. By using this table you will be able to trace out the logic paths if you wish. Step control The quad analog switch package IC9 is labelled as the step control. It switches either IC7a & IC7c or IC7b & IC7d to the inputs of IC10, the Direction switch. If the jumper is placed on J3 (FULL) the signals MAF and MBF from pins 4 & 13 of IC7 are fed to IC10. If J2 is selected (HALF), then MAH and MBH from pins 1 & 10 of IC7 are the selected signals. Also IC11c and IC11d, which are disabled in the FULL mode, will be able to pass the MAINH and MBINH signals from pins 8 & 11 of IC8 to IC13 and IC14. When these ICs are turned off the zero current in Table 3 is achieved. The coil driver transistors (Q1-Q4 and Q7-Q10) are all bolted to a common aluminium heatsink to aid heat dissipation. Note that the transistors must all be isolated from the heatsink using insulating washers. Fig.3 drilling details for the aluminium bar heatsink. Motor direction If F/R (forward-reverse) is selected with jumper J1, then the logic level on A7 of PortA (pin 9) will control the direc­tion. If it is high, IC10 will be switched and the motor will step backwards. What this IC does is to swap the pairs of gates (from IC7 which are selected by IC9) to the inputs of IC12. IC2d is used as a power-on reset to ensure that both flip­flops of IC12 are reset at turn on. Each time an input of IC12 (pins 1 & 13) goes low the logic levels on the outputs change. The outputs of IC12a are fed through IC13 to drive motor coil MA and the outputs of IC12b are fed through IC14 to drive coil MB. Winding control The path through IC13 (and IC14) is actually two switches in series. As we have explained previously, when IC8b’s output is high one switch is on and this will allow the coils to be ener­gised. The outputs of IC8d & IC8c (MAINH and MBINH) will switch off the drive signals through IC13 and IC14 when a zero is needed in the half step table. In the full step mode, IC11c and IC11d will have one input low (J3) and their outputs will always be high, keeping that switch turned on. Coil driver Transistors Q1-Q12 make up two H-bridge circuits which drive the stepper motor coils, MA & MB. These circuits are iden­tical so we will only describe the circuit based on Q1-Q6 which drives MA. This top circuit is driven from the Q and Q-bar out­puts of IC12a, via switches IC13d and IC13c. Consider the situation when Q is high and Q-bar (of IC12a) is low. Q5 will turn on and this will also turn on Q1 & Q4. As a result, current flows through Q1, coil MA and Q4. Conversely, when Q-bar of IC12a is high, transistors Q6, Q2 & Q3 turn on, causing current to flow through coil MA in the opposite direc­tion. If IC13 is turned off, then both Q5 & Q6 will be off and no current will flow through coil MA. Almost all motors, including the centre-tapped 5V types (as we don’t use the CT) can be powered from the 12V supply. If you want more torque and a faster stepping speed you can run a motor from a higher voltage but you should include a series resistor in each coil to keep the motor current December 1997  65 Fig.4 this is the full-size etching pattern for the PC board. Check your board carefully before installing any of the parts. within specification. It is the inductance of the motor windings which limits the current and hence reduces the torque, so by applying a higher voltage we get a higher initial current. Building the board Before you begin the board assembly it is worthwhile checking the copper pattern against the artwork of Fig.4, J4-J8 J1 Jumper header pair J1 is used to select forward or forward/reverse (shown), while jumpers J4-J8 provide the card with a unique identification. 66  Silicon Chip espe­cially where there are three tracks through the centre of an IC or where there is a track between two IC pads. The first task is to fit and solder the 72 links, counting as you go, for a couple are underneath ICs and may be difficult to install later on. Next fit and solder the resistors and diodes, then the ICs. Continue with the trim­pots, jumper strips and capacitors. It is advisable to bolt the eight power transistors to a common heatsink if you intend driving high current stepper motors for long periods. The heatsink fitted to the prototype was a piece of aluminium bar 12 x 6 x 58mm long. Fig.3 shows the drilling details for the heatsink. The best procedure is to loosely attach all the transistors to the heat­sink bar 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 mount on the other side. Table 3 Full Step (Both Windings Energised) Step 1 2 3 4 Step 1 2 3 4 5 6 7 8 MA L-R R-L R-L L-R Half Step MA L-R 0 R-L R-L R-L 0 L-R L-R MB L-R L-R R-L R-L MB L-R L-R L-R 0 R-L R-L R-L 0 Once the heatsink assembly is in position, solder one lead at either end and then tighten all the mounting bolts. The assem­bly can then be adjusted to sit parallel to the PC board and the remaining transistor leads soldered. After you have finished, check the copper side of the PC board for any Listing 1 10 PORTA = &H378 ‘this is LPT1 use &H278 for LPT2 20 PORTB = PORTA + 1: PORTC = PORTA + 2 30 OUT PORTA,20: OUT PORTC,11 ‘set 20 steps and card 1 40 OUT PORTC,10 ‘reset strobe The answers! to 260,000 questions, ALL in one book! The following code will allow you to identify which cards are busy. You must run it after the previous code or redefine the ports (lines 10 & 20) 100 OUT PORTC,11 ‘select ANY active card 110 OUT PORTB,120 ‘set PORT B lines high 120 B = 127 - INP(PORTB) ‘read PORT B lines 130 IF B AND -128 THEN J7$ = “J7 busy “ 140 IF B AND 64 THEN J6$ = “J6 busy “ 150 IF B AND 32 THEN J5$ = “J5 busy “ 160 IF B AND 16 THEN J4$ = “J4 busy “ 170 PRINT J7$ + J6$ + J5$ + J4$ 180 WHILE B > 0 OR B < 0: B = 127 - INP(PORTB): WEND ‘wait for all cards 190 OUT PORTC,10 ‘reset strobe 200 PRINT “All motors stopped.” 210 END Table 4 Jumper J4 J5 J6 J7 Code 16 32 64 128 unsoldered pads which can mean missing components or links. Finally, complete the assembly by fitting the 8-pin header, the DB25 connector and the 7-way terminal block. Testing the board Before you apply power to the card, turn both trimpots anticlockwise, fit the jumper to select card 1 (C1), fit J3 and fit the two F/R links so that they are parallel to Con1. You will need a 25-way D male to female cable to connect the card to the computer’s parallel printer port. You will also need a power supply capable of supplying 5V at a few milliamps and 12V at probably around 1A, to supply the stepper motor. The first four lines of Basic code in Listing 1 will allow you to test the card. PortB jumpers The major advantage of this card is that the computer can send the number of steps for the motor to make, then do something else while the card is driving the stepper. We now need some way of letting the computer know when the job is completed. Two different methods are available on this card. If one or several of them are being used in a system, jumpers J4-J7 can be used. The STOP line on each card is low while the motor is running and goes high when the motor stops. If each card uses a different jumper the computer can read PortB and determine the status of the cards (see Table 4). If only one card is in use, J8 can be used but only if the card is left selected. In this case the line is high while the motor is stepping and goes low when the motor stops. As this input line is inverted the program will see SC the inverse of this logic. The largest range of replacement semiconductors in the industry! Call now to get your new NTE cross reference book for just $25. Stewart Electronic Components P/L P.O. Box 281 Oakleigh 3166 phone (03)9543-3733 fax (03)9543-7238 Silicon Chip Binders REAL VALUE AT $11.95 PLUS P &P How To Get The Software ★  Heavy board covers with 2-tone green vinyl covering ★  Each binder holds up to 14 issues ★ SILICON CHIP logo printed on spine & cover Price: $A11.95 plus $A3 p&p each All the software for this series of stepper cards and the I/O card described in the July 1997 issue is now available on a 3.5-inch floppy disc for $7 plus $3 postage and packing. Payment may be made by cheque, postal money order or credit card (Bank­card, Visa or Mastercard) to Silicon Chip, PO Box 139, Collaroy, NSW 2097 or via fax (02) 9979 6503. Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Note: prices rise next month Aust. only. Not available elsewhere December 1997  67