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Pt.2: adding the parallel interface board BY RICK WALTERS Computer controlled dual power supply Last month, we presented the standalone version of this power supply. By building & fitting the interface board de­scribed here, you will be able to control it from your computer. The power supply interface board connects via a 25-way cable to the parallel port of your computer. The interface allows your computer to perform two functions. The first is to set the required posi­ tive and negative output voltages and current limit which will be delivered by the power supply. The second is to display the actual voltage and current from the power supply on the comput­ er’s video monitor. 16  Silicon Chip Normally the voltages set by the computer will be the same as those displayed but if the power supply goes into current limit, the associated output voltage will be reduced. One good feature of the computer control is the ability to use the settings which were in use the last time the supply was turned off. These settings are stored in a file which is read each time the software is run. This gives you the option of using the same supply values and printer port as previously or selecting new values or a different printer port. Now let’s have a look at the circuit of Fig.1 and even the most dyed-in-thewool computer hardware enthusiast would have to admit that it’s not too inspiring. However, before you turn the page and give up, let’s note a few key points. First, if you refer to the circuit presented last month, you will note that there are four outputs labelled IN1, IN2, IN3 & IN4 and three inputs labelled D/A1, D/A2 & D/A3. The outputs from the power supply board become the four inputs for the A/D converter on the interface board. They are fed to IC7, a 74HC4051 1-of8 multiplexer. Depending on the BCD data at its ABC inputs, it feeds the selected IN value through to IC8, the ADC0804 analog-to-digital converter. The four IN values relate to the fol­ Fig.1: the circuit allows data from the computer’s printer port to set the sup­ ply’s voltage and current outputs. It also allows the voltage and current to be monitored on the computer screen. Octal latches IC1, IC2 and IC3 are used as D/A converters and are con­trolled by IC4, a 74HC137 latched one-of-eight decoder. February 1997  17 Fig.2: follow this parts layout diagram to assemble the interface board. The assembly is straightforward but take care to ensure that the ICs are all correctly oriented and that the correct IC is used at each location. lowing four power supply parameters: IN1 Positive output voltage (V+) IN2 Output current (I+) IN3 Negative output voltage (V-) IN4 12V supply All these IN values will be in the range of 0-5V and will be converted by the ADC0804 chip, IC8, to an 8-bit word (a number between 0 and 255) which is fed to the computer’s paral­ lel port, on pins 10-17. Note that the parallel port is bidirectional so it can accept data on these pins, as well as outputting data. Three of the IN values, IN1, IN2 and IN3, are displayed on the computer’s screen. As we just remarked, the parallel port also outputs data and in this case it delivers 8-bit data to control the volt­ age and current settings on the power supply. This 8-bit data is delivered on pins 2-9 (D0-D7) of the 25-pin socket. From there it is connected to the D0-D7 inputs of IC1, IC2 and IC3. These are used as three D/A (digital to analog) converters. Digital to analog converters IC1, IC2 and IC3 are octal (8-bit) latches under the con­ trol of IC4, a 74HC137 latched one-of-eight decod­ 18  Silicon Chip er. Depending on the data fed to its pins 1, 2 & 3, IC4 enables IC1, IC3 or IC3 (via their latch enable pin 11 and inverters IC5b, IC5e & IC5c) so that the data on their input pins 2-9 is latched onto their Q outputs, pins 19-12 (Q0 to Q7). Each 74HC573 has a ladder network consisting of 10kΩ and 20kΩ resistors connected to the eight outputs. These networks convert the 8-bit data at the output to a voltage with a value between zero and 5V. These analog voltages are D/A1, D/A2 & D/A3, corresponding to the positive volt­ age setting Vo+, current limit setting Io, and the negative voltage setting Vo-. We have just described the two main functions of the interface board: first, monitor the four IN values from the power supply board and provide the three control values for positive and negative voltage and the current limit setting. Apart from that, there is little point in going further with the circuit description since the interface board is entirely under software control. PC board assembly The interface board measures 178 x 100mm (code 04101972) and has a 25-pin D socket mounted at one end. Fig.2 shows the component layout on the board. The board assembly is reasonably straightforward. In es­sence, you have a few rows of equal value resistors and eight ICs to install, and not much else. As usual, before starting assembly, check the copper pat­ tern for open circuit or shorted tracks or undrilled holes. Make any repairs required and then fit and solder the 21 links and 10 PC stakes. Next, fit the resistors and diodes, followed by the IC sockets, capacitors and finally, the D connector. Check your soldering when you are finished to make certain that no IC pads are bridged. Interconnecting wiring Most of the interconnecting wires should have been taped up when you built the power supply. If you followed the colour code that we suggested last month, the brown wire will go to D/ A1, the red to D/A2 the orange to D/ A3 and the black to ground. There should be four other loose wires: the blue goes to IN1, grey to IN2, brown to IN3 and white to IN4. Two leads need to be run from the anode of D3 to TP14 and from the remaining PC stake to TP4 on the power supply board. Fig.3: the parallel port interface board is mounted at one end of the chassis, with the DB25 connector protruding through the rear panel. Use this chassis wiring diagram and the wiring table from last month’s issue to make the offboard connections. Because only low currents are involved, you can run the connections to the interface board using rainbow cable February 1997  19 to the positive output voltage. If you set the front panel voltmeter to read the positive supply voltage and short the positive output, the current reading on the computer should read .05 and the digit colour should change to red. Also the positive voltage should read 0 or .1 and again should be red, indicating that it is not the selected value. The current limit changes colour as the limit setting is reached to let you know that the power supply is in current limit mode. Voltage calibration The interface board mounts vertically on one side of the case and is attached to the rear panel via the rightangle 25-pin D connector. Mount the PC board to the back pan­ el using the hex head bolts to secure it, with the components facing the power trans­former. We stuck a mounting foot on the metal chassis to keep the board parallel to the case, and another on the plastic cover to keep the board firmly in place. Testing You will need a 25-way D female to 25-way D male cable to connect the computer to the power supply. This done, load GW Basic and SCREG.BAS and follow the on-screen instructions (see Fig.5). As there are no previous values saved, you should enter 10V for the positive voltage, 15V for the negative voltage, .05 for the current limit, and 1 or 2 for whichever printer port you plan to use. It is probably wise at this stage to use LPT1, the paral­lel port you have been using to drive your printer, as you know that this port works. When you switch the power supply to remote, the voltages you have set (or values very close) should be displayed as in Fig.5. Pressing the plus key should in­ crease the positive voltage and the minus key should reduce it. If you press the “T” key the negative volt­ age should reduce to the same value as the positive and follow it. This is the “tracking” condition whereby the negative output voltage is always equal Software Features  Positive and negative voltage setting in 100mV steps from 0-25.5V   Individual output voltages or negative supply tracking positive supply   Current limit setting in 10mA steps from 0-2.55A for both supplies  simultaneously  Computer screen readout of positive and negative output voltages   Voltage reading changes from yellow to red for out of tolerance voltage   Computer screen readout of positive supply current   Current reading changes from yellow to red at current limit   Selection of printer port 1 or 2   All settings are saved and can be restored at program start  20  Silicon Chip In spite of the fact that the power supply will have alrea­dy been cal­ ibrated for standalone operation, it needs to be recalibrated for computer control. The procedure is similar to that outlined last month. Set both supply rails for 24.5V out­ put and with your DMM across the negative output and ground, adjust VR4 until the voltage reads exactly -24.5V. Now set VR6 so that the posi­ tive output voltage is identical. If you can measure current with your DMM, find a 10Ω 5W or 10W resistor and connect it in series with your ammeter across the positive sup­ ply. Set the voltage to 22V and set the current limit for 1.95A. Disconnect your DMM, switch it back to volts and with just the 10Ω resistor for the load and using the front panel meter to check that the current is around 1.95A, adjust VR5 so that the voltage on TP8 is 3.82V (1.96 x 1.95). If your meter can’t measure current, wire the 10Ω resistor across the pos­ itive terminal and earth, then set the positive voltage to 22V and the current limit to 1.9A. Measure the voltage across the 0.1Ω resistor in the emitter of Q2, multiply it by 1.96 and adjust VR5 until you can measure this voltage at TP8. Be careful as the resistor will get very hot. This will not be quite as accurate as the previous method as it assumes that the resistor value is exactly 0.1Ω. The linearity of the power supply output voltage versus the computer setting is excellent, with the DMM reading precisely tracking the reading on the computer screen. The voltage fed back to the computer is not quite as line­ar. There are slight errors in the converted voltages due Fig.4: actual size artwork for the PC board. Check your etched board carefully against this artwork before installing any of the parts. to A/D linearity around half scale, resistor tolerances, etc. We have made provision in the soft­ ware to apply five cor­rection factors to these readings. The first is for values bet­ween 0 and 5.5V, the next between 5.6V and 11V, the third between 11.1V and 16.5V, the fourth between 16.6V and 22V and the last between 22.1V and 25.5V. This will be explained later in the software description. Parallel port Before we start discussing the soft­ ware we should give a quick rundown on the parallel printer port and its peculiarities. It was originally designed to drive an 8-bit parallel printer, with suffi­ cient additional lines to provide data transfer in both directions, such as a BUSY line to prevent the computer feeding data to the printer faster than it can process it and a PAPER OUT line to allow an intelligent message to be shown on the compu­ter’s screen if this should occur. Because the original interface was for a Centronics print­er, some bits are true high, others are true low. These signal lines are split over three addresses on the IBM interface. For LPT1 which is the normal (and often only) printer port supplied, the addresses are 378H (hexadecimal, 888 in decimal) for the eight data lines, 379H for the next five lines and 380H for the remaining four lines. These are often called ports A, B and C. The data lines of port A are unidi­ rectional, capable only of sending data to the printer. The other nine lines can be used as inputs and those of port C can be used as outputs. This gives us the capability of sending and receiving 8-bit data from an external device to the computer. Port B has the highest bit inverted and port C only has one of its four bits true high. A subroutine in the software (at line 3000) untwists the input value Parts List 1 PC board, code 04101972, 178 x 105mm 1 rightangle 25 pin D male connector (COON1) 4 20-pin IC sockets 3 16-pin IC sockets 1 14-pin IC socket 10 PC stakes 2 3mm x 15mm machine screws 4 3mm x 10mm machine screws 6 3mm nuts 8 3mm flat washers 6 3mm spring washers tinned copper wire hookup wire Semiconductors 3 74HC573 octal latch (IC1-3) 1 74HC137 latched 1-of-8 decoder (IC4) 1 74HC14 hex Schmitt trigger (IC5) 1 74HC147 decimal-to-BCD encoder (IC6) 1 74HC4051 analog multiplexer (IC7) 1 ADC0804 analog-to-digital converter (IC8) 4 1N914 diodes (D1-D4) Capacitors 1 100µF 16VW electrolytic 4 0.1µF MKT polyester 1 .022µF MKT polyester 1 .001µF MKT polyester 1 150pF ceramic Resistors (0.25W, 1%) 4 1MΩ 27 20kΩ 2 47kΩ 23 10kΩ February 1997  21 Only a few connections need to be made from the interface board to the power supply board. The wiring diagram (Fig.2) has the details. which is the sum of port B and port C and gives a true value (TIN) for any data placed on these lines. Software The control program has been written in GW Basic, using screen 9, the highest resolution (640 x 350 pixel) colour screen. Contrary to the statement in last month’s issue, the software will work with EGA and VGA monitors, not just VGA types. The software code is quite conven­ tional and will run in QuickBasic if lines 1-14 are removed. You will also have to create a separate program containing just lines 5100-5199 and run it to create the file before you run the main program. Space does not allows us to present the full software list­ing in this article but we have included the main section from lines 20-999. Lines 20-70, as you can see from the comments, define the functions to be 22  Silicon Chip used, paint the introductory screen, read the previous settings from the hard disc and give you the option of reusing them or entering new values. Should you wish to retain an exist­ ing value, just press ENTER. The value will be accepted and the program will step to the next item. This is useful should you just wish to change the printer port for example, but retain the previous voltage settings. The values you selected are now written to the screen (line 60) then sent to the power supply in line 70. By structuring your program in this way you can write and debug each subroutine individually. Then if you decide to include an additional fea­ ture, it is only a matter of writing the rou­tine, debugging it, then adding a gosub in the appropriate place. Main program The main program, after the initial­ isation and preliminary housekeeping (lines 20-70), consists of lines 80-160 which, while there is no keyboard key pressed, will run lines 100 and 110 continuously. That is, read the data from the power supply and write these values to the computer screen. As the standard 8x14 text numerals look quite insignificant on the screen, we produced some larger, chunkier numerals using a rectangular block (CHR$219), defined on line 1260 and drawn by subroutine 4000. Keyboard input When a key is pressed the program branches to line 10000 which is the keyboard service subroutine. If a key which it recognises is pressed it will carry out the command and send a new value to the power supply. If a non-programmed key is pressed, it will be ignored and the program will return to run­ning lines 100 and 110. If you read the comments at lines 10021 to 10028 you will understand which keys do what. We used both E and V for the positive voltage and A and I for current, accepting both upper and lower case characters, just so you don’t have to try to remember which keys to use. When you are typing the program there is no need to include the comments but if you come back to study it at a later date, they will help your understanding. As described previously we have two functions, read from and write (send) to the power supply. We read the power supply voltages and current and write values to the D/A converters. Writing to D/A converters The write function is carried out by first placing the value we wish to write to a particular D/A converter on PORTA, then writing its address to PORTC. These addresses are listed in lines 1330-1400. The address for the first D/A converter ODA1 is 9. We can’t call it DA1, as we have defined D as a string in line 1030. You will notice that all the addresses are odd numbers, which indicates that the strobe line will be low (as we have explained previously, the logic for this line is inverted). When the address is written to PORTC, pin 4 of IC4 (latch enable) will be pulled low but after a short delay will go high as the .001µF capacitor charges through the 10kΩ resistor. The strobe is then taken high again to prevent the A/D converter being enabled (see “reading power supply values”) and placing data on the POR­ TA bus. ODA1 is now deselected, as any changes to PORTA data would be transferred to IC1’s output. Now the data which was present on PORTA has been latched by IC1 and is available as an analog voltage at D/ A1 output. The other converters are loaded in a similar manner. When we write to the D/As we al­ ways update the three of them and this is done in subroutine 8000. Listing 1 20 GOSUB 1000 ‘Initialise 30 GOSUB 2000 ‘Write screen heading 40 GOSUB 5000 ‘Get previous saved values from file 50 GOSUB 6000 ‘Write old settings to screen with option to change 60 GOSUB 7000 ‘Write selected data to screen 70 GOSUB 8000 ‘Output data to power supply 75 ‘MAIN PROGRAM loop 80 - 160 starts here. Monitor power supply & keyboard 80 K$ = INKEY$ 90 WHILE K$ = “”: K$ = INKEY$ ‘While no key is pressed 100 GOSUB 9000 ‘Read data from PSU 110 GOSUB 7000 ‘Write data to screen 120 WEND ‘A key has been pressed 130 GOSUB 10000 ‘Service keyboard 140 GOSUB 6360 ‘Update preset values 150 GOSUB 8000 ‘Write new values to power supply 160 GOTO 80 ‘Loop again 900 GOSUB 5100 ‘Save power supply settings 999 CLS: SYSTEM Fig.5: the positive and negative output voltages are displayed on screen, along with the output current. Also shown are the instructions for varying the output voltages and for setting the current limit and tracking. Reading power supply values To read a value from the power supply we latch its address into IC4. This time we don’t take the strobe line high as we did previously, as we want to turn on the A/D converter, IC8. After the delay introduced by the resistor and capacitor between the output of IC5a and the input of IC5f, this will be the case as its chip select (CS) will go low. This connects its tri-stated output to the PORTB and PORTC bus. For the PORTB and PORTC lines to be used as inputs they must all be set high. Then they will either stay high or be pulled low by the A/D. This procedure is carried out by subrou­ tine 9000. The last area to cover is the line­ arisation of the readings returned by the power supply. Lines 1420-1440 list the correction factors we found satisfactory for our supply. These are imple­mented in subroutine 9000 on lines 9140, 9210 and 9280. These have been REMmed out and values of 1.0 substituted in lines 1411-1413. The procedure is to make a table of the output voltage at the terminals ver­ sus the voltage shown on the screen. You then cal­ culate the adjustment factor to give the correct reading for each range. Once you have the values, delete lines 1411-1413, remove the REMs from lines 1420-1440 and enter SC your values. February 1997  23