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COMPUTER BITS By STEVE PAYOR Where computing meets electronics: some ideas on inter£acing Just how do you go about interfacing external hardware to an IBM PC or PC-compatible? It's really not all that difficult. Here we show you how the IBM parallel printer socket can be used as a general purpose interface. There comes a time in every computer enthusiast's life when he or she wishes it could do more than just sit on the desk looking pretty. If only it could actually do something, like turn on a light, measure a voltage, operate a machine tool, etc. But before you go looking for a commercial interface card, consider what you might be able to achieve with your existing hardware. The games port For example, the Apple II games port can input the analog value of four variable resistors, which don't necessarily have to be the pots in a joystick - they could be LDRs, phototransistors, thermistors etc. Also provided in the one socket are A 36-PIN SOCKET AND A HANDFUL of resistors are all you need to make a perfectly useable PC-driven function generator. The difficult part is writing good, fast software to run it. 68 SILICON CHIP three bits worth of LS TTL input, four LS TTL output bits with a - STROBE pulse, and a + 5V supply for powering external equipment. In the author's experience, this is sufficient to perform at least half of the laboratory/process control tasks that Apple Ils -are currently being used for - given some clever programming. Even a small amount of machine code can go a long way. For example, you can write an extended version of the inbuilt analog input timing routines to obtain more than the usual 8-bit resolution. The analog inputs use 555-type RC timing circuits and so the games port can be used for precision measurement of external resistors or capacitors. About 3 or 4 decimal digits appears to be the limit of accuracy, which is on par with most cheap digital multimeters. There are a wealth of low-cost commercial devices on the market, from graphics tablets to scanners, which interface solely through the Apple games socket. The IBM games port can also input the analog value of four variable resistors, although extending the precision beyond 8 bits is quite tricky. This is because any long machine code timing loops will be affected by a host of internal goings-on, the most obvious of which is the DOS real-time clock interrupt. The BIOS and other system programs in your machine may also do a number of other things which will affect timing loops - especially if they have been "enhanced" over the original IBM specification. The IBM games port can also input the digital value of four switches but no output bits are provided as such. The parallel printer port The IBM parallel printer port is a goldmine of interface bits, both input and output, and the remainder of this article is concerned with how to use this port as a general purpose interface. Since the "Centronics" parallel interface standard has been adopted by most printer manufacturers, you will find that all brands of computers will provide similar facilities to the IBM, though you may have to do some detective work to find out the actual machine addresses for the various I/O bits and bytes. Just to whet your appetite, we will conclude this article with a useful, practical example, namely an 8-bit programmable voltage source/signal generator which is so simple it will cost you less than the price of a suitable connector. Port addresses Most IBM-compatible parallel printer cards can be set to respond as either LPT1: or LPT2: . These devices appear within the microprocessor's 1/0 address space as three separate 8-bit ports, which we will subsequently refer to as PORT.A, PORT.Band PORT.C. The actual machine addresses of these ports are as follows: PORT.A PORT.B PORT.C LPTl: 378(Hex) 379(Hex) 37 A(Hex) LPT2: 278(Hex) 279(Hex) 27 A(Hex) In order to use these ports for general purpose 1/0, we need to know which bits of these three ports are connected to the outside world. Details may vary slightly from computer to computer, so obtaining a circuit diagram of your printer card is highly desirable. Another useful source of information is the manual which comes with your printer. This will contain an appendix describing the parallel interface signals. Table 1 was prepared by tracing through the circuit diagram of a PORT.A PORT.B BIT SIGNAL NAME INPUT OUTPUT D7 D6 D5 D4 D3 D2 D1 DO D7 D6 D5 D4 D3 D2 D1 DO These bits may be input but all you will get is the value currently residing in the output latch, unless the interface is genuinely bi-directional. (Check circuit diagram.) Latched TTL push-pull outputs with 2. 2nF slow-down capacitors -D7 D6 D5 D4 D3 BUSY -ACK PE SLCT -ERROR D2 D1 DO -D3 D2 not available not available D7 D6 D5 D4 PORT.C TTL inputs, no pull-up resistor -SLCT IN -INIT -D1 -AUTO FEED -DO -STROBE not available not available May be used as inputs if corresponding output bits are set to "1 " Open-collector TTL output with 4. 7kn pull-up resistor Open-collector TTL output with 1Okn pull-up resistor Open-collector TTL output with 4. 7kn pull-up resistor Open-collector TTL output with 4. 7kn pull-up resistor and 2.2nF slow-down capacitor TABLE 1: THESE ARE ALL THE 1/0 bits available from a typical IBM parallel printer port. The "signal name" column lists the signals by the names used in most printer manuals. A " - " sign in front of the name means that the signal is inverted. Refer to Fig.1 or your printer manual for the pin connections to 25-pin or 36-pin connectors. typical IBM PC ·'clone" of Asian origin. As far as we know, this would be the most typical circuit you are likely to encounter. As you can see, the eight PORT .A bits are mostly intended as outputs, while five of the PORT.B bits are available as inputs. Four of the PORT.C bits can be used as inputs or outputs. Note that some of the bits are inverted between the internal data bus and the outside world. For example, if you input bit D7 of PORT.B, it will appear as a "1" when its input signal line is grounded (logic "0"). The open-collector PORT.C bits can be used as inputs by first setting the corresponding output bits to "1". This enables the inputs to be taken low as required. A sample program will clarify the use of these open-collector bits. Assume we wanted to input the status of a simple pushbutton switch connected between the - STROBE signal line and ground. The following statements in GWBASIC will do this: PORT.C = &H37 A BIT.0 = 1 OUT PORT.C,4 X = INP[PORT.C) AND BIT.0 The OUTput statement writes to the output bits of PORT.C, which appear as "1111" externally, since D3, Dl & DO are inverted. The DO signal line can "0100" FEBRUARY1990 69 DATA BITS - - - - - - - , SLCT PE BUSY -ACK 24 - - 07 23 06 22 - 05 21 - - 04 20 -:- 03 19 18 02 DO - STROBE 01 17 16 15 - -:- -SLCT IN SIGNAL GROUNDS - INIT - ERROR -AUTO FEED i DATA BITS -STROBE DO 01 02 03 04 ..,"" 05 06 07 < I /;; ::, "' w c.. ...=l ...< c:, ::, "' SIGNAL GND I FRAME GND NC 0 2 3 4 9 10 11 12 13 14 15 impedance of the TTL outputs. Unfortunately, the pull-up and pulldown internal resistances of the TTL outputs are not equal, resulting in a non-linearity error of - ½LSB or - 5mV at mid-scale. The minimum output voltage (for a digital input of 0) is almost exactly 0.1 V, as determined by the "low" level TTL output. The maximum output voltage can be adjusted by the trimpot so that an output of 2.65 volts is obtained for a digital input of 255. Thus each step will correspond to 10mV. The output impedance is almost exactly lkO, and a token amount of filtering is provided by a .0047µ.F capacitor. A more "serious" digital audio waveform generator would use more elaborate filtering. A 3 or 4-section LC filter would reduce the distortion content from 0.5% to about 0.1 %. 16 Performance -:' - - - - - - SIGNAL GROUNDS _ _ _ __ _ _, ~ I i1~~AL I FIG.1: THESE DIAGRAMS SHOW THE pin connections for the various IBM parallel port signals as they appear from the wiring side of a 25-pin "D" connector and a 36-pin "Centronics" connector. Beware some cheap printer cables may not include all the auxiliary signals. Luckily, however, all manual T-switches switch the entire 36 wires regardless of whether they are used or not. now be taken low by pressing the pushbutton switch. After the INPut statement, X will be 1 if the switch is closed, and O if the switch is open. This is a very easy way to interface a switch, or an opto-isolator, since a 4. 7k0 pull-up resistor is already supplied internally (how convenient)! A simple waveform generator Suppose you wish to use your computer to synthesise arbitrary waveforms, from DC levels to complex audio tone bursts? No problem. Analog voltages can always be obtained using a D-A converter IC but, just for demonstration pur70 S ILICON CHIP poses, we chose to "show off" just a little by making our own from a handful of resistors. Fig.2 shows how a binaryweighted resistor network can be hung directly off the TTL data bits to produce an analog output voltage. The exact values of the resistors are fairly critical, but if you check the resistors with a digital multimeter and put the ones closest to the ideal values towards the top of the· ladder, the output voltage steps should be within ½ LSB without any further trimming. Note that the resistors do not form an exact binary sequence, as we have subtracted 400 from all of them to compensate for the internal We wrote a demonstration program in TURBO BASIC to test the performance of this simple AID converter. As you can see from the oscilloscope photograph, the output is indeed quite linear. The program is too long to be listed in its entirety here, but the basic technique is to set up an integer array of data bytes, then output them repeatedly to PORT.A as fast as possible. Note that it is necessary to disable the DOS realtime clock interrupt whilst the waveform is being output, otherwise short pauses will occur throughout the waveform at a rate of 18.2Hz. The waveform pictured has one complete cycle of a sine wave, made up of 256 samples, followed by a linear ramp from O to 255. A sample rate of 20k per second was achieved by the compiled TURBO BASIC program on a bog-standard 4.77MHz PC-XT. The simplicity of this function generator is little short of amazing but as always, there is -a hidden price. You really need to be a competent programmer to get useful performance out of it. If anything, this simple project is a classic example of the modern trend that microprocessors have brought to electronics in general - CENTRONICS PIN NO. (9) HIGH QUALITY JAPANESE 500mA Hr (2.773k) 2.7k 68!l * OUTPUT 1k!l 07 (5.585k) 5.6k * ( 8) 06 .0047 5.6k (7) 3.3k (11.21k) 0.1V TO 2.65V IN 10mV STEPS PENLIGHT NICAD BATTERIES FOR UNDER $2.00 EACH??? A 7.2V multipurpose "Six Pack" that's easily reduced in size, or dismantled. Individually insulated cells. Bulk buyer enquiries welcome . UNREPEATABLE PRICE! 5.fik $10.90 per pack 2k 05 (22.46k) ( 6) 04 (5) 03 22k 470!l 27k 18k 180k ( 4) 02 (3) 01 .,. * MAY NEED SOME ADJUSTMENT ALL RESISTORS ¼W 1% (2) UNBELIEVABLY LOW PRICE $5.90 ea DO YOU WANT A REALLY LOUD SIREN FOR YOUR ALARM? 180k 180k Comes with circuit and instructions. Then have a look at this READY MADE and TESTED UNIT. All you need is a 6-14V supply and add a horn speaker to obtain an output of 110-125dB. NOW THAT'S LOUD! 180k 00 As used in a well-known, expensive commercial siren that costs over $70.00. (19) TO (30) GNO INCLUSIVE High power bridge design. Comes with circuit and instructions. ---i_ -:- FIG.2: THIS IS THE CIRCUIT DIAGRAM for the simple 8-bit D-A converter described in the text. Exact resistor values are shown in parentheses. Standard 1 % resistors will do fine if you check them with a digital multimeter first. Try to get the resistors for the two most significant bits within ± 0.5% or better. ONLY $11.80 ea Metal horn speaker to suit $13.50 Grab some while you can at this glveaway price. SOLID STATE RELAY BARGAIN fhese units are from BRAND NEW but unused equipment. Zero voltage switching. More than 2.f.KV isolation. Can switch inductive loads. 3-32V DC input from logic. 10A Max rating. 24-240V load rating. $21.90 ea 10+ $19.90 Priced at about '/, of their real value. You had better be quick! !SINGLE CHANNEL UHF REMOTE CONTROL! Don't waste your time ano money. This is a proven, reliable and GUARANTEED design. 5A switch and indicator relays. Very slim and rugged transmitter. ONLY $57.90 COMPLETE TRANSMITTER AND RECEIVER KIT HOW'S THAT FOR VALUE? MASTHEAD AMPLIFIER FOR TV See Silicon Chip May '88 High performance protected unit with 1 BdB gain. Easily covers VHF, UHi' and FM. Very easy to construct. Prewound inductor supplied. ONLY $29.90 for the amplifier and the power and signal combiner unit. THIS IS HOW OUR PROTOTYPE digital function generator performed when driven by a compiled TURBO BASIC program running on a 4.77MHz PC-XT. The sample rate here is approximately 20k samples/sec. Total non-linearity is within ½LSB, which translates to roughly 0.5% distortion. hardware is being replaced by software. Thoughts typed on a keyboard can replace the soldering iron (well, almost). All that is need- ed is a little interfacing and a lot of imagination. We'll have more on this subject [lti in future issue. NOW THAT'S CHEAP OATLEY ELECTRONICS PO BOX 89, OATLEY, NSW 2223. PHONE (02) 579 4985 MAJOR CREDIT CARDS ACCEPTED P + P $2·$4 (Aust); NZ add $2.00. Distributors: Slightly higher prices may apply. MELBOURNE: ELECTRONICS WORLD. (03) 723 3860 BRISBANE: KINGSWAY ELECTRONICS. (07) 390 2399