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By DON McKENZIE "Have your name up in lights". That is the ambition of every budding actor and actress. Well, we can't help you with any breakthroughs in the acting business but we can show you how to put your name up in lights. Your own lights, that is, with your own moving message display board. If you live in or around any city you can't fail to have seen these moving message displays. They are used by all sorts of commercial establishments to highlight their goods and services. Apart from television, they must be one of the most effective advertising devices 34 SILICON CHIP available. Wherever they are placed, they really do catch the eye. Watch them for any more than a few seconds or so and they can have a mesmerising effect. They can be programmed to move the words from left to right, up and down, to flash or pause - in fact, you name it, these new computer-driven message boards can do it. The message board presented in this article has all the features of expensive commercial moving message displays but at much lower cost. The main reason for the saving, of course, is that you have to do the work in putting it together. Many uses come to mind for this message board. If you are in business then you already have an application. If you are a member of a club or charitable organisation, a moving message board provides an admirable method of advertising, particularly at fetes, flea markets, conventions and so on. We should place a qualification here: LEDbased displays of any sort are up against it in bright sunlight. Under any other lighting conditions, this display works well. Features The message board itself is 740mm long and 100mm high and is mounted in a dark Perspex housing which effectively hides the individual light emitting diodes (LEDs) when they are not illuminated. In this shot, the Message Board has a static display but the real visual interest comes from the variety of moving displays possible: shot on, scrolling up and down, wiping up and down and sideways, flashing letters, or words and extra large letters. Messages can be either entered via a computer or direct from any IBM PC-compatible keyboard. I <, .. i .· [ ,,. Build .your own LED ··Message Board If yoµr computer has been languishing for several years for want of some exciting peripheral to drive, this is the one. It is a large and enticing moving message display board. It can be driven from the Centronics port on your computer or from any IBM PC compatible keyboard. The LED characters are 50mm high and up to 16 characters can be displayed simultaneously. With bright characters this high, the display can be read from a distance of 20 metres or more, the upper limit depending more on your keenness of eyesight. For even greater visual impact, the message board has a "large letter" mode which produces extra wide letters. The full ASCII character set is available - ie, every character available on an IBM PC keyboard or computer. This includes all upper and lower case characters. Each ASCII character is presented in a 5-wide by 7-high LED matrix with a column of LEDs bet- ween each character. This allows all upper case characters to be presented correctly but lower case characters with descenders (ie, g, j, p, q and y) are squashed with respect to others. This is not really a problem as most message board displays tend to use all upper case letters for greater impact. Interestingly, the considerable visual impact of a moving message display depends on the unwitting cooperation of the viewer. You can only comprehend a moving message written in moving LEDs because your eyes follow the moving characters. If you stare fixedly at the display, to stop any eye movement, you will only see a jumble of rapidly LEDs. The total number of characters that can be displayed at any one time is 16. Since each character is a matrix of 7 x 5 LEDs, each having the additional column of 7 LEDs just mentioned, the total number of LEDs employed is 672. No computer required As noted above, the message board can be controlled from the Centronics port of a computer or from an IBM PC XT type keyboard. In the latter mode, no computer is necessary. So if you presently don't have a computer but have a yen for a message board, then it will only be necessary to purchase an IBM XT type keyboard. These are widely available at modest cost. For example, you can purchase an 84-key model from Electronic Solutions at around $125. (Phone (02) 427 4422). Large and varied messages can be entered and stored in the message board (or actually in its control unit). The unit has 10 message buffers, the first two of 2K bytes and the remaining 8 of 256 bytes each. So the facility is there to store or display a wealth of information MARCH 1989 35 The LED display panel is covered and protected by a dark Perspex channel which makes it look smart and professional. on the message board. This could include a whole catalog of prices, news and coming events and so on. Display modes Most message boards used in shops just have the words running across from right to left, so you can read them in the normal way, from left to right. But there is much more to this message board than that. It has quite a few display modes. Some examples are as follows. The Flash mode enables single letters, words or sentences to be flashed on and off as they pass across the display. The display can also be paused for as long as you want and the flash mode can be operating at this time. A message can be built up on the display by individually shooting the letters from right to left. The display can then be pushed off to the left (exit left) or to the right (exit right). The display can also be scrolled down to reveal the next line of text, overwriting the previous line (wipe down). Alternatively, the new line of text can be scrolled down, pushing the previous line out of sight (roll down). The same two modes can also be made to scroll up rather than down 36 SILICON CHIP (wipe up and roll up). Then there are two more "wipe" modes, whereby a new line of text is displayed from left to right, or from right to left, wiping over the old line of text (wipe-forward and wipe-back). Finally, there is the large letter mode mentioned previously, which can be combined with the modes listed above. In fact, when all the combinations of display are used, the message board can give a very interesting and intriguing display. Mesmerising, did we say? Writing and editing Writing a message onto the display is easy. Having done it both ways, we are inclined to the view that it is easier to use the keyboard to enter messages directly rather than use a computer to create the message and then dump it to the printer port (ie, the Centronics port). Perhaps using a computer is easier if very long messages are to be composed. Writing the message entails no special software although there are single or two-key control codes (entered using the Ctrl key) which are necessary to engage the display modes described above. Messages can be stored for as long as you want, even if the unit is turned off, provided the optional RAM battery backup feature is included. We'll talk more about writing and editing the messages in a future episode of this article. System description In addition to the keyboard and the Perspex LED display already briefly mentioned, the message board also includes a Controller. This is housed in a standard plastic instrument case and has a number of sockets on the back. One is a 5-pin DIN socket for the keyboard while another is a 25-pin (male DB25) socket for the multiway cable to the display unit. The third socket is for the Centronics cable from a computer. On the front panel of the case are two LEDs, a toggle switch and a pushbutton. The two LEDs are: one Fig.1: this is the heart of the Message ► Board and is essentially a Z80A microprocessor controller with a 2764 EPROM, 6264 8K x 8 static RAM and 8255 programmable peripheral interface (PPI). The Z80A runs at the relatively high speed of 4.9MHz. ..:i ~ .... ::r:: co 0:) co n ::0 > ~ 10k~ R3: +5V ! 8 m} 8 r8 19 i ELJc VIEWED FROM BELOW " " ~ RN7f +5V 16VW RESE T ~TANT 4.7k 1 3 R6 4.7k ~"" E15 +5V ,,,. J C7.• .,. Cl 220pF "T" .o,T RB lk R7 285k 4.91MHI x1i:;'° INT ~9 1-7' NMI , RrnET I 26 16 ElD ZBOA 19 IORQ 20 22 jijj 21 wii Ml 27 MREQ A A Al Al Al Al ., -- +5V N/C N/C : DO 01 02 D3 D4 D5 D6 D7 A2 A3 A4 A5 A6 A7 AB A9 AlD All A15 C A14 DATA BUS 20 CS 18 19 17 11 12 13 15 16 9 8 7 6' 5: 4 3 25 24 21 23 22 " A RN2 RN3 RN4 RN5 RN6 RN7 N/C l. 2o 3o 4o 5o 6o 7o 8o Huuncoo ouo AnnDCC'(' D11<' .,.14 Ell EPROM A1021 A924 _U 12 11 " f """I J Q4 __j§_ D5 17 D6 181 !!1 19 WE 27 E14 6264 I GND OUT ~ IN 5 t t -¥4 22 oe 201cs1 26 CS2 fil_JJi ,DO !!1 D2 :A14 A15 A122 1\1123 I C5 5 3 A825 A7 ~A....! '7ii'a1 "A37 J AO 3 9V AC OR DC INPUT Al ~ CSRESETPC3, 5 7 E12 8255 [ RN5 4.7k f -- 't +5V 2fii10.J: 16VW"'f!_ .,. GNO .1.t 16VW"'f!_ TANT f3 2 J4 +12V 1oA .!osre D7 D6 D5 D4 D3 D2 • 40 +5V r.lo+5V 5~ -r-- PAUSE (CUT HERE) BC1-BC111 llx.Olr R5 lk • RN2 4.7k .,__.,_...1N"'-I 7805 1ouT • +5V RN4 4.7k ================~~• =-------=:.:J~ PAl!l PA2::f11,:4 PAD PC41i1I3 rt 6 . WR ...-3liiii ifa---!!j A1 ~ - t 7GND Jl fil E D6 D4 D1 iii 03 DO \' PC5t:1jj2 ◄--------~___j:i,, 8SY PA71_37 PA6 38 D7 PAS 39 D6 PA4 4D D5 1 PA 2 Rl ~ ~ - = - r ~ ~ ~ - - + 5 LEDl eV DATA }. 100k RN6 J2 RN3 _ 128 127 126 ,, 24 26 11 25 4.7k ll 4.7k R4 INPUT BSRO WT Hi 3300 260+5V AD 10 25 ~ PC7 2 S2 l'ii,"'g 23 0GND ACK COM/KBD RB 2.211 10W JJ CENTRONICS 36 PtN FEMALE INPUT . - - - -6 a.N.DU16, 19-30,33 J2 26 +5V 25 GND I JA 12V INO--'IIM-~P-0""-0-➔-----, r-------------o+9V 9VINo---- 32 J1 13 12 25 + BC1-BC5I 5x.01 24~11 COM MONO--+-----➔---__. 23~10 22~9 POWER TO PBUFF 21~8 +5V 2 ~ 7 16 19~6 IBM PC/XT KEYBOARD CONNECTIONS 5-PtN DIN SOCKET 2 18~5 17~4 16 ~ 3 0 15~2 3 14~1 PINS 14,15,17,18, 31,34,35,36 N/C +5V013 I GND 012 BSY·-11 6 - 1 CLOCK 2 DATA KEYBOARD 4 GNO 5 +5V 74LS02 4 E4b 5 ACK 010 D7 -9 11, 8 17 14 D4= e I 6 13 B D3.,. 5 4 02::; 3 01::; El 74LS373 ENABLE 13 5 S1 14 2 so 15 I oo;:; 2 ,,. +5V 47011 E2 7445 7 4 4 MMB STB 3 1 ROW 1 2 ROW 2 3 ROW 3 4 ROW 4 5 ROW 5 6 ROW 6 7 ROW 7 1k BI/ : \..._~c Q1-Q7 7x8D646 +5V J5 D825 MAL J4 OUTPUT + 5 V ~ 1 3,26 _ -12,25 ROW 1 2,25 - 11 10 07 12 06 13 D5 14 D4 15 03 1 ROW 3 ~ 1 0,23 ROW 4 ~ 9,22 47011 Q7 1k ROW5~ 2 D1 3 DO 4 - -~ 2_:r E4a 3 R1 100k 7 ':' 8 4 E3 74LS151 1 5 +5V ·< : ES 555 ~ 12 * LEDf€ RUN R7 150f! ~ fii::- ; 8D646 .8r== 16VW ,,. -- DATA_ - _4,17 - CLOCK- -3 16 -- - 4,17 R3 1k ;- 8 K>-'-~ ~ 1 5,18 74LS04 .........,2 R2 100k - +5V t!L--+5v 7 +5V ~ 8,21 ROW 6 ~ 7,20 _6,19 ROW 7 6,19 BI/; 9 • D2 -, - ROW 2 ~ 11,24 .I! ,,. ..J,!O 15 OPTIONAL COMP/KBD DECODE CIRCUIT ~c -1:! STBol- .,. J2/14 +5V 12 6 S2 2 TO RN6 SIDE OF COMP/KBO SWITCH - J2/22 74LS157 KBO CLK DIN/1 l,6 9 COM STB J3/1 J2/14 STB J2/22 D7 J2/25 GNO MMBCNT J2/26 +5V 120 7 MMB D7 KBO DATA DIN/2 1 18 05- 7 D6= e COMP 07 JJ/9 BCE 7805 •;oo, - 3,16 R4 1k .,. +sv---o--oLlL-o-:- 2,15 ~ 1,14 GNO Fig.2: this is the interface board circuitry. It takes data lines from the PPI (programmable peripheral interface) and decodes it to obtain the 7 row driver lines. It also produces the serial data to drive the 8-bit shift registers in the LED display panel. to monnor Data input from the keyboard or computer while the other is a Run indicator when a message is being displayed. The toggle switch is used to select input 38 SILICON CHIP from the keyboard or computer while the pushbutton switch is a system reset in case you goof and latch it up. We used this button quite a lot while we learnt to program the message board. Inside the case of the controller is a power transformer and two printed circuit boards. One is a Jl + 5 V ~ 1 3,26 ROW 1- 12,25 ROW 2- 11,24 ROW 3 10,23 ROW 49,22 ROW 5_ 8,21 HOWL _ 7,20 ROW 7- 6,19 J2 13,260-0---+5V 12,25 - -ROW 1 11,24 10,23 9,22 --- 5,18 ·, -- I' ROW 4 ' , -ROW 5 --- -ROW 6 ROW 7 ~ +5V Lu -v 4,17 _ 9 7 6 2 1 9 13 + ~ 11 10 8 5 3 114 - 2 5 12 11 10 6 5 4 3 +~ E6 74LS164 -¥- -- 6 1 L 13 +5V 9 7 6 1 2 13 11 E3 75492 3 14 12 11 12-F 5,18 Li 7 9 6 8 5 3 6 5 4 3 14 12 -r 1 13 El 75492 4 10 2 10 8 5 3 14 12 10 6 5 4 3 "~' -DATA -CLOCK .,F¥ 24x33i! 0.5W +5V E5 75492 2 - 7 E7 75492 13 3,16 ---- ----- --- ROW 2 ROW 3 8,21 7,20 6,19 7x24 LED DISPLAY r 10 13 lJ 12 11 .l- E4 74LS164 9 +5V____r; -¥ r 2 QATA 13 2 li.J E2 74LS164 9 +5V_r:; -¥ r - -- 4,17 CLOCK- - - 3,16 +5V---o--<)2,15 2,150-0---+5V +5V 1,1 4 + + 1 ,14 + BCHC61 6x.01 COLUMN COLUMN Fig.3: the circuit of a 7 x 24 LED panel. Four of these multiplexed panels are employed in the Message Board. The 74LS164s are 8-bit shift registers which take the incoming serial data line and convert it to parallel data, to drive the columns of the display. Note that the data is shown coming in on the righthand side of the circuit and passing across to the left. The 75492s are hex inverting buffers. To turn on a particular LED, the row driver line goes high while the column line goes low. complete microprocessor controller which is the basis of the designer's (Don McKenzie) printer buffer. It employs a Z80A microprocessor, a 2764 EPROM, a 6264 BK x 8 static RAM and an 8255 I/O (input/output) driver, also known as a " programmable peripheral interface" (PPI). The PPI takes care of communications with an external computer via the Centronics 8-bit parallel port or with the keyboard (if connected). The PPI also distributes the data to the LED display panel. The second printed board inside the case functions as the moving message board controller and is actually a parallel and serial interface board. It decodes 4 lines of parallel data from the 8255 PPI (programmable peripheral inter- face) to produce 7 row drivers (for the 7 rows in the LED display). It also converts 8-bit data from the PPI into serial data which is later used to drive the 96 columns of the LED display panel. In addition, this board carries the 36-pin connector for the Centronics parallel port. This latter board drives a 25-way cable (from the DB25 connector on the rear panel) to the LED display panel. The ea ble carries the 7 row driver lines, doubled up to occupy 14 of the cable lines. It also carries the serial data line for column signals. This serial data is also doubled up and is carried on lines 4 and 17 from the DB25 connector. As well, there is a clock signal (lines 3 and 16), + 5V supply (lines 2 and 15) and GND return (OV, on lines 1, 5, 14 and 18). LED display panel The LED display panel is made up of four panels, each having 7 rows and 24 columns of LEDs (168 in all). It is actually possible to build up the display with 1, 2, 3 or 4 of these 24 column panels but our description will concentrate on the 4-panel version. Each panel incorporates 8-bit shift registers to convert the serial data line mentioned earlier to column drive signals. Naturally, the display is fully multiplexed. We'll describe how it works in more detail later in this article. Hardware operation Since the software which drives this moving message display is proprietary and subject to copyright, it is not possible to give an exact MARCH 1989 39 A rear view of the Message Board controller, showing the three sockets: 5-pin DIN for the IBM PC keyboard, 36-pin for Centronics cable and 25-pin for the display drive lines. description of how the microprocessor circuitry [the hardware) functions to produce the moving characters. The description which follows can only be generalised. With that proviso, let us describe how the circuit works. The microprocessor controller, containing the Z80 processor, 2764 EPROM and 8325 PPI, is depicted in Fig.1. The interface board, containing the 7 row drivers and Centronics parallel port, is shown in Fig.2. And the 7 x 24 LED display panel (4 required) is shown in Fig.3 . When power is first applied, the Z80 microprocessor gets its operating instructions from the 2764 EPROM. It gathers, processes and stores data from the 6264 static RAM and picks up and sends data to the 8255 PPL The PPI handles 8 lines of parallel data and distributes (outputs) or picks up (inputs) data to and from 24 data lines which are arranged as three 8-bit parallel ports - port A, port B and port C. In this design, all 8 lines from ports A and B are used but not all lines from port C. You can tell which lines are from which ports from the labelling on the circuit. For example, data line DO on the J2 header socket (righthand side of Fig .1) is the O line from port A [PAO). 40 SILICON CHIP Whether or not the line from PC7 on the PPI is connected to OV determines whether data is accepted from an external computer via the Centronics port or from the keyboard on line PA7. This line goes to pin 2 of the 5-pin DIN socket for the keyboard. 21 lines from the PPI go via the J2 header socket on the controller board (righthand side of Fig.1) to the Jl header socket on the interface board [lefthand side of Fig.2). 13 of those lines go to J3, the 36-pin Centronics connector. The 8 data lines from port B [PBO to PB7) are used to drive the LED display panel. The 8 data lines feed a 74LS373 octal latch. At the appropriate timing, 8 bits are latched [ie, stored) in the 74LS373 by a positive-to-negative transition of the enable line, pin 11. Only four of the output lines of the 74LS373 are used and, of these, only three are used as data lines [pins 2, 5 and 6). These drive a 7445 which is a BCD decoder. It produces the 7 row driver lines: Each of these row driver lines is buffered by a BD646 PNP switching transistor which handles the heavy currents to the LEDs. The fourth line to the 7445, acts as the enable line for this chip. The three data lines from the 74LS373 also feed a 74LS151 which is an 8-to-1 multiplexer. Effectively, what it does is convert the 8 data lines to a single 8-bit serial data line, from pin 5. This line is buffered by a 74LS04 before being fed to pins 4 and 17 on the DB25 plug. Now refer to Fig.3. At the top of the diagram can be seen the seven row driver lines to the LEDs. Each of the 96 columns (only 24 shown on this diagram) is driven by one output of a 75492. This is simply a high current inverting buffer device so the real players on this circuit are the 74LS164 8-bit shift registers. Three are shown on this circuit but 12 are employed in total, to give the 96 column lines. Now consider the action as the display works. Each row is turned on in sequence and when each row is turned on, the appropriate columns are turned on so that the LEDs connected to the energised rows and columns are alight. For example, if we want the character " I" to be lit in the leftmost position, the column line 6 from E7 must be low when each row line goes high. Row lines are 'active high' while column lines are 'active low'. So what must happen is that each time a row driver goes high, the data in the 8-bit shift registers must be changed. In effect, all 12 8-bit shift registers are in series, making a 96-bit register. So before each row driver goes high, all 96 bits of data in the shift registers must be changed. The sequence goes something like this. First, 96 bits of data are latched into the registers and row 1 goes high. Row 1 then goes low, 96 new bits of data are shuffled through the registers and latched, and then row 2 goes high to turn on its LEDs. Then row 2 goes off (low), another 96 bits of data are shuffled in and latched, and then row 3 goes high and so on. It becomes a bit mind boggling when you think about moving characters, but data is all the same to the circuitry, whether the displayed characters are moving or static. Next month we will continue the description of the moving message display. ~