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The LPT Simulator will take you next to no time to build. Note that the final version differs slightly from this prototype. Ideal for troubleshooting Lets you manipulate the data & control lines Has 6 LEDs for status monitoring Low cost & easy to assemble Printer por t harrdware simula ha imulattor Do you need to test printers or other items of equipment that connect to a PC’s parallel printer port? This low-cost, easy-to-build circuit will let you test them quickly, with­out the need for a PC or test software. By JIM ROWE B ASICALLY, THIS DEVICE is a simple hardware simulator. It allows you to manipulate the port’s data and control lines, monitor the status lines and even send the printer (or other equipment) a ‘strobe’ pulse. The idea for the Printer Port Simulator came about while we was developing our Windows-based EPROM Programmer. We struck a rather tricky timing fault and subsequently wasted a fair bit of time trying to work out whether it was due to a problem with the hardware or a bug in the software. The same sort of problem can occur when you’re trying to track down a 80  Silicon Chip fault in other kinds of PC-driven equipment, of course. It can even happen when you’re getting weird problems with a printer. We ended up resolving our particular problem by lashing up this Printer Port Simulator. This allowed us to send basic con­trol signals to the EPROM programmer and monitor its status lines, without having to worry about software debugging until later. It proved to be very effective and enabled us to track down the cause of the timing error. Later on, we realised that our Printer Port Simulator could also be used as a general troubleshooting tool to solve similar problems. So here it is and there’s really very little in it – just two cheap ICs, a +5V regulator, a couple of DIP switches to set up the data and control bit lines, six LEDs for status indi­cation, a pushbutton to produce strobe pulses and a handful of other components. It all fits on a small PC board measuring 113 x 61mm and runs from a 9V DC plugpack. The maximum current drain with all LEDs on is just 58mA. How it works Refer now to Fig.1 for the circuit details. The simulated “port interface” is provided via CON1, which duplicates the DB25 female connector used to provide the standard printer port on a PC. Pins 2-9 are used for the main data bus (DATA 0-7) to the printer. These pins are connected to a very simple data input circuit which uses eight 10kΩ pullup resistors and an 8-way DIP switch (S3). Each pole of S3 is simply connected between one of the data lines and ground – when a switch siliconchip.com.au Fig.1: the circuit is straightforward – just some DIP switches to set the data bits and control pins, a flipflop to generate the strobe pulse and some indicator LEDs to monitor the status lines. siliconchip.com.au May 2003  81 Fig.2: install the parts on the PC board as shown here, taking particular care to orientate the DIP switches correctly. In addition, switch S1 must be installed with its flat body surface to the left. Fig.3: this is the full-size etching pattern for the PC board. Check your board carefully before installing any of the parts. This means that pin 3 is normally low and so pin 11 (the strobe-bar output) is normally held high. Because pin 3 is low, D1 is forward biased and holds the voltage at the inputs of IC1b low as well. As a result, the output of IC1b (pin 6) is held high, as is the pin 13 input of IC1d. Now when S1 is pressed, the 100nF capacitor is discharged and so a logic low is applied to pin 1 of IC1a. As a result, the flipflop is triggered into switching states – ie, pin 3 goes high and pin 11 goes low. This marks the start of the strobe-bar pulse. When pin 3 goes high, it removes the forward bias on D1 and so it can no longer pull pins 4 & 5 low. As a result, the associated 390pF capacitor begins charging via a 10kΩ resistor. After about 2µs, the voltage on pins 4 & 5 rises high enough to switch IC1b. When that happens, pin 6 of IC1b goes low and because this pin is connected to pin 13 of IC1d, this triggers the flipflop into switching state again. As a result, pin 3 switches low and pin 11 switches high, bringing the strobe-bar pulse to an end. Note that this all takes place only if S2d is open. That’s because if S2d is closed, it holds both inputs of IC1b low permanently and so prevents IC1b from reset­ting the flipflop. Basically, S2d allows you either to produce strobe-bar pulses using S1 (when S2d is open) or to hold the strobe line down continuously after pressing S1. This second mode is handy for troubleshooting. Status LEDs is closed, that line is pulled to ground. Conversely, when a switch is open­ ed, that data line is pulled to logic high (ie, +5V) by the pullup resistor. As a result, the DIP switch can be used to feed any desired extended-ASCII data bit combination to the printer (or other device) – ie, from 00 to FF hex. Similarly, 4-way DIP switch S2 is used to set any desired combination of bits on three of the four control lines of the port: ie, pin 14 (Auto LF), pin 16 (Reset) and pin 17 (Select Out). Note that, in this case, the pullup resistors have a value of 4.7kΩ rather than 10kΩ. The remaining printer control line connects to pin 1 of the DB25 connector. This line is normally used to send the negative-going “strobe” pulse 82  Silicon Chip to the printer, to begin printing each character. For correct printer operation, each strobe pulse should be a single clean pulse about 1-2µs long. In the simulator, we generate this pulse each time switch S1 is pressed. This is done by using a simple oneshot circuit formed from three gates in IC1, a 74HC132 quad Schmitt NAND device. NAND gates IC1a & IC1d are connected as an RS (reset/set) flipflop which is triggered by pressing S1. The associated 2.2kΩ pullup resistor and 100nF shunt capacitor form­a simple “debounce” circuit. Diode D1 and NAND gate IC1b are used to convert the flipflop into a one-shot multivibrator. This works as follows: normal­ly, pin 1 of IC1a is held high by the 2.2kΩ pullup resistor. Most of the remaining circuitry in the simulator is used to drive LEDs 1-5. These are used to monitor the “printer status” lines of the parallel port – Acknowledge (pin 10), Busy/ Ready-bar (pin 11), Paper Out (pin 12), Select In (pin 13) and Error (pin 15). As shown in Fig.1, the LEDs are driven by inverters IC2a, IC2b, IC2c, IC2e & IC2f, all part of a 74HC04 hex inverter. Five of the 10kΩ resistors in SIL1 are used as pullups on the input lines, to prevent them from “floating” at an intermediate level when the simulator is not connected to a printer or other equip­ment. The series 10kΩ resistors are used for additional protec­tion against electrostatic charge damage to the gate inputs. IC1c and IC2d are used to drive siliconchip.com.au LED6, which indicates the status of the strobe-bar line. This LED is illuminated when the line is low (because this line is nominally active low) and is off when it’s high. Of course, the narrow nature of the strobe-bar pulse means that in pulse mode (S2d open), the LED glows so briefly it’s not easy to see. LED6 is therefore used mainly to verify the quiescent level on the line and of course, the level in non-pulse mode (S2d closed). Power supply The only part of the circuit we haven’t talked about yet is the power supply. This is very simple, consisting purely of a 7805 regulator (REG1) to produce a stable +5V rail from an unregu­lated 9V DC plugpack. Series diode D2 provides reverse polarity protection, while the 470µF and 100µF electrolytic capacitors provide filtering and stability. Construction Everything fits on a single-sided PC board measuring 113 x 61mm and coded 07105031. This is possible because we’ve used board-mounting components for DB25 socket CON1, DC input connec­tor CON2 and pushbutton switch S1. In fact, the board is designed to be freestanding, supported by four small screw-on rubber feet (one on each corner). Fig.2 shows the parts layout on the PC board. As can be seen, the display LEDs, DIP switches and pushbutton switch S1 are all arranged along the front of the board, for ease of use. Conversely, the two connectors are at the rear, to allow convenient cable connections. The assembly should take you next to no time. Begin by fitting the two connectors, then the three wire links, the DIP switches and pushbutton switch S1. Note that the DIP switches must all be fitted with their “ON” side towards the front of the board – they may look upside down but this gives the correct switching sense. Take particular care when installing switch S1. It must be installed with its flat body surface to the left –ie, one paral­lel pair of pins to the front and the other parallel pair to the back. If it’s installed incorrectly, you’ll get a permanent short across the 100nF capacitor and the switch won’t work. Next, install the resistors and the SIL resistor array. That done, you can fit the small capacitors and the electrolyt­ ics. Be sure to fit the latter with the correct polarity, as shown on Fig.2. The semiconductors can now all be installed. These include the diodes, LEDs, regulator and ICs. As usual, take care with the polarity of each of these. Note that all six LEDs are fitted with their cathode “flat” side towards the rear of the PC board. Regulator REG1 is mounted horizontally on the top of the board, with its three leads bent downwards at 90°, 5mm away from the body. Its metal tab is then secured to the board using an M3 x 6mm machine screw and a nut underneath. This also provides a small amount of heatsinking, as there’s a rectangle of copper underneath as well (there’s no need for a separate heatsink). Your simulator board should now be complete, apart from fitting the four rubber feet. These are fitted using M3 x 9mm machine screws passing up from underneath and fitted with nuts on the top. Parts List 1 PC board, code 07105031, 113 x 61mm 1 PC-mount pushbutton switch (S1) 1 4-way DIP switch (S2) 1 8-way DIP switch (S3) 1 DB25 female connector, PCmount (CON1) 1 2.5mm DC socket, PC-mount (CON2) 1 9V 150mA DC plugpack 4 small rubber feet 4 M3 x 9mm machine screws with hex nuts 1 M3 x 6mm machine screw with hex nut Semiconductors 1 74HC132 quad Schmitt NAND gate (IC1) 1 74HC04 hex inverter (IC2) 1 7805 +5V regulator (REG1) 6 3mm red LEDs 1 1N4148 silicon diode (D1) 1 1N4004 1A silicon diode (D2) Capacitors 1 470µF 16V PC-mount electrolytic 1 100µF 16V PC-mount electrolytic 3 100nF monolithic or ceramic 1 390pF ceramic Resistors (0.25W, 1%) 14 10kΩ 1 2.2kΩ 3 4.7kΩ 6 330Ω 1 8 x 10kΩ SIL array Check-out time It’s very easy to give the completed simulator a quick check-out. First, set DIP switches S2a-S2d to their OFF positions (ie, towards the rear) and connect a 9V DC plugpack to CON2. That done, apply power and check that the first five LEDs light. If they do, use your DMM to check the supply voltage at pin 14 of either IC1 & IC2 – it should be close to 5.00V. At this stage, LED6 should be off. Now set S2d (the leftmost DIP switch in S2, nearest the pushbutton) to ON and press S1. LED6 should now light and stay that way, unless S2d is turned OFF again. If all of the above happens as expected, your simulator is working correctly and ready for use. If not, turn off the power and look for faulty solder joints and components fitted with reversed polarity. These are the only likely causes of problems with SC such a simple project. Table 1: Resistor Colour Codes o No. o 14 o   3 o    1 o   6 siliconchip.com.au Value 10kΩ 4.7kΩ 2.2kΩ 330Ω 4-Band Code (1%) brown black orange brown yellow violet red brown red red red brown orange orange brown brown 5-Band Code (1%) brown black black red brown yellow violet black brown brown red red black brown brown orange orange black black brown May 2003  83