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Part.3 Build your own LED Message Board In this third episode on the LED Message Board, we give the full construction details for the message board controller. We also give the setting up procedure and troubleshooting. Designed by DON McKENZIE Building the LED Message Board is firstly an exercise in logistics. At present you cannot buy a complete kit, with everything .down to the last nut and bolt. You will have to go out and buy many of the bits separately. 62 SILICON CHIP As noted last month, a short form kit is available from the designer, Don McKenzie. Included in this kit is a number of bare, double-sided boards with plated through holes. One is the main CPU board, one is the interface board and four others are identical, being the LED display panel itself. Also included are assembly instructions and a programmed EPROM which is absolutely necessary if the unit is to work. To build the project you will need a fair degree of experience and a good tool kit which will include a digital multimeter. A logic probe is also desirable although not mandatory. We'll describe the construction of the message board controller this month and the LED display panel next month. The processor board, . BC1 C4 • BC5 IJ Qecs interface board and power transformer are all housed in a standard plastic instrument case available from Dick Smith Electronics (Cat. H-2507). The processor board is based on the designer's well debugged Z80-controlled printer buffer circuit. Hence the board is labelled "PBUFF". Since this board was intended as a printer buffer, it has provision for a number of RAM ICs which are not needed in this . Message Board application. Hence eight 16-pin IC positions at one end of the board will be vacant. IC sockets may be used for the remaining ICs, including the Z80, 8255 and EPROM. They are recommended by the designer. One point to watch is that you should make sure that all sockets fit into the board before you start soldering them into place. Because the ICs are closely packed together some chunky sockets will not fit. Two changes to the board pattern are necessary for the Message Board application. They are as follows: • Solder neat wire links from E12 pin 17 (8255) to E13 pin 13 and from ElO pin 17 (Z80) to E13 pin 12. This enables the 8255 port A to interrupt the CPU. • There is a sharp 45° angle track which juts out from the component side of J6 near pins 6 and 7. Cut this track and then solder a neat wire link between pin 6 of E9 (address C000H enable) and pin 11 of J2. This provides the clock signal for the 74LS373 on the interface board. Having made those changes, use your multimeter to check that the ground and + 5V lines are not shorted together. As you progress BC7 Fig.I: parts layout for the PBUFF processor board. IC sockets should be used for all the ICs so that the ICs can be installed in turn and tested as detailed in the text. Note the orientation of E12. Close-up view of the completed processor board. Do not remove the protective cover from the EPROM window - you could accidentally erase the contents if you do. A heatsink must be fitted to the 7805 regulator. along, do the same test for + 5V and ground shorts at regular intervals. Since these two lines go to every chip, and at least one of these lines goes to almost every other component, they are the most likely to short. They are also the hardest faults to find and isolate when all the sockets are installed. Examine the board on both sides for defects before assembly. Any fault on the solder side can be easily fixed but once you have your sockets in, the component side of the board is impossible to fix without removing these sockets. So pay particular attention to the component side artwork before assembly. Check any suspicious sections with a meter. MAY1989 63 Building the LED Message Board~:.-·c ~ ~~ • CD D1 "'· 7~~5 G• (le C30 BC1c::» •C eB eC 05 04 eB •E eE eB eC •C eB eC 03 02 01 •E eB •E •E e(]I]e• •+ RUN J5 D825 MALE OUTPUT J3 CENTRDNICS 36 PIN FEMALE INPUT Fig.2: parts layout for the interface board. Make sure that the Q1-Q7 driver transistors are correctly installed and that the pins of the Centronics and DB25 connectors are correctly aligned with the pads on the PC board. Now install the IC sockets on the board. Take note of the orientation of the ICs as they are not all oriented in the same direction. This has been done to minimise the size of the board. Once you have the sockets installed in position, they can be soldered in. Install the resistor network into the board as shown on the overlay, with pin one away from the 555 timer circuitry, and solder it in place. Install all the resistors as shown on the overlay into their mounting holes. They can now be soldered in and the legs cut off flush with the solder joint. Install the BC (bypass) capacitors. Install the Cl, C5, C6, C7 and CB capacitors. Capacitors C2, C3 and C4 are polarised and must be soldered in the correct way around. Install diodes Dl-D4, then mount the 7805 with the tab facing away from the diodes. The heatsink can be fitted at this stage. Install transistor Ql and the crystal. Don't push the crystal -fully down onto the board, but leave enough length on the legs so that they don't short out the tracks underneath. J2 and J6 header sockets A 26-pin male header can be soldered into the J2 position. By using a 26-pin me type crimp header and flat ribbon cable, the connection can ultimately be made to the interface (MMBCNT) board. A 16-pin male header can be soldered into the J6 position. By us- Where to buy the parts A complete set of printed circuit boards plus the programmed 2764 EPROM and full instructions are available from the designer, Don McKenzie, for $150 . The specially made grey Perspex channel, suitable for all LED colours, is also available from Don McKenzie for $50 plus $15 for packing and postage . Write to Don McKenzie, 29 Ellesmere Crescent, Tullamarine, Victoria 3043. All the semiconductors and other parts are readily available from parts suppliers such as Jaycar Electronics, Dick Smith Electronics, Altronics, Geoff Wood Electronics and David Reid Electronics. 64 SILICON CHIP ing a 16-pin female me type crimp header and 22cm of flat ribbon cable, the connections can be made to the front panel controls. Don't cut off the spare wires . The pads marked GND and + 5V are used for test connections. PC pins can be soldered into these pads. Now you can make ready for checking out the PBUFF board. Connect two insulated wires about 22cm long from the Reset button to pins 9 and 1 of J6 (see Fig.3) The Data LED can be wired directly onto the board. Don't install the chips yet. Connect up your power pack or transformer. As the circuit has a diode bridge at the input, it doesn't matter which way around you connect the positive and negative terminals. With your multimeter check that ground and + 5V are not shorted together. Power up the unit and check that the + 5V is there before proceeding. All OK? Power down and install chips E12 (8255) and E13 (74LS04}. Power up again and check the + 5V. If the LED is not already on, press the reset button. The LED should light up brightly. No LED? Then you have one of th~ following faulty: E12, E13, Reset button, reset circuit, LED circuit, or more likely you have the LED wired in back to front. Power down and install E9 (74LS00}, ElO (ZB0A}, Ell (EPROM) and power up. The LED should be flashing at about 4 times per second. If the LED is flashing, it indicates that the Z80A is running and the software is doing its job but a RAM fault exists. With no RAM installed this is to be expected. If the LED doesn't flash as expected at any time during the construction of this kit then refer to the "It Doesn't Work Department". Power down and install E14 (6264) and power up. The LED should be on most of the time and blink off for a short period. This indicates a pass for the 6264 and a fail for the 555 timer. Well, once again this is to be expected if the 555 isn't installed. Power down, install the 555 and power up. The 555 timer is used to interrupt the ZB0A so that controll- , ed updates of the message board This view shows the completed interface board installed in the case but with the connector to Jl from the adjacent PBUFF (processor) board removed for clarity. Note the heatsink fitted to the 7805 regulator (bottom, left). can be achieved. For the technically minded, the output of the 555 timer (pin 3) has a duty cycle of 3µs low and 0.95ms high. During power up or reset, the LED lights up for 1 to 2 seconds and then goes off. It doesn't work department Yes, with all of the components installed, the 7805 will get warm enough to burn your finger, but no other component should. However, the ZBO does get quite warm, which may be a worry for those unaccustomed to micros. Let's outline the four major faults found in these kits: (1). Bad socket solder joints (open circuit). (2). Faulty socket connections (open circuit). (3). Solder shorts between tracks (short circuit). (4). Point not soldered (open circuit). The rear panel of the Message Board controller. The DIN socket is for connection of a standard IBM PC keyboard. Below the DIN socket is the 36-pin Centronics port and next to it the 25-pin DB25 socket for connection of the LED display panel. Faults 1 and 2 can usually be found with a logic probe. Any pin on any chip should be in one of three states: high, low or bobbing. Well almost every pin. If no LEDs light up, this is known as a high imMAY 1989 65 (n), 5-PIN DIN SOCKET TO KEYBOARD CORD CLAMP -- GROMMET 3 ~1 '(_:} J5 DB25 MALE OUTPUT TO DISPLAY BOARD J3 CENTRONICS 36 PIN - - FEMALE INPUT FROM COMPUTER •• AB J4 TO DIN SOCKET Hl J1 .__MAINS CORD 1 \ J6 i1 MMBCNT C PBUFF POWER TRANSFORMER J6 ~ LED2 '~ LE01 Fig.3: how it all goes together. J2 on the PBUFF hoard is connected to Jl on the MMBCNT board via short 26-way flat ribbon cable. Be careful to note the orientation of pin 1 on the two connectors when installing the cable. pedance state. Check over all chips for ground, + 5V and other signals. Make sure that there are no high impedance states anywhere. Exceptions to the rules The input and output ports of the 8255 could show anything random as this may not get programmed correctly on power up. The data bus may also show high impedance states under certain conditions. Pressing the Reset button will bring some life into the data bus. If a high impedance state is found where it shouldn't belong, then check with your meter for zero ohms between the solder pad under the board and the pin of the chip. If you find no connection then pull out the chip and check between 66 SILICON CHIP the solder pad and the socket connector itself. Still no connection? Try soldering this point again. If this doesn't work then you could be up for a complete socket replacement (a nasty job for a newcomer). Sometimes, bending the legs of the chips at a slightly different angle, or just pulling and reinserting a chip, can remove socket problems. Faults 3 and 4 are ones that you have created. A close visual inspection and perhaps a check around with an ohmmeter will usually overcome these faults. A magnifying glass may help you. Clock checks With a logic probe check E10/6. This should be bobbing up and down, as this is the clock signal. No clock? Remove E10 and check E13 /4 for activity. Still no clock? Check E13/3. If no dock is found here, try removing E13 and test the socket contact of E13/3. No clock? It has to be one of the following: the crystal, Cl, C6, R1, R2 or Q1. Is Q1 connected the right way around? If you are getting the clock to pin 6 of the CPU socket without the Z80A in and no clock with the ZBOA in, then it may be the ZBOA itself. Clock working but still no results? Remove E14 and check that the LED flashes at about 4Hz. The software checks the RAM and if none is present the LED is flashed at this rate. If this checks out OK, then it should mean that the system is working up to the point short of a RAM check. OK, where do we go from here? Press the reset and check the MREQ line, Z80A pin 19. If it is bobbing then the CPU is attempting to read the ROM. Press the reset and check the IORQ line, Z80A pin 20. If this is active then the CPU is attempting to access the 8255 ports. Check all ROM and RAM signals. All pins except ground and + 5V should be active. The Z80 should show activity on most pins. Reset can be checked using the Reset button. Pin 26 should be low when the Reset button is pressed and high when the Reset button is released. E13/8 should show the inverse levels to the CPU/26G (see the reset circuit diagram). The 8255/35 should have the same levels as E13/8. As mentioned before, the Data LED should light up when the Reset button is pressed and then go out when the button is released. This is an indication that the reset circuitry is working correctly. WAIT (24) should be high; BUSREQ (25) high; and INT (16) high. A13 (3) bobbing after reset means that the CPU is trying to send information to the message board controller. NMI (17) should be bobbing as this is connected to the output of the 555 timer. Quinella test What would happen if you had a short between Address 7 and Data bit 3, or A13 and ground, or etc. We don't know, but I dare say the beast won't work at all. How do you find these shorts? Do what I call a " quinella test". Using your meter, check for zero ohms between pin 1 and pin 2 of the ZB0A. Then check pins 1 and 3, 1 and 4, right up to pins 1 and 40. Step on to pins 2 and 3, 2 and 4 etc, up to pins 2 and 40. Get the idea? You end up checking pins 39 and 40. Zero ohms will be found at some points, as these points are tied together. Check the circuit to verify these shorts. If a short is found where it shouldn't be, you have found your problem; well at least you know which tracks to look at. This quinella test can be done on all PARTS LIST Other hardware 1 plastic instrument case, 250 x 190 x 80mm (Dick Smith Cat. H-2507 or equivalent) 1 Altronics 21 56 multitap 2-amp transformer 1 3-core mains cord and 3-pin plug 1 cordgrip grommet 2 LED bezels 1 5-pin DIN socket CPU Board 1 double-sided, plated through board, code PBUFF (available from Don McKenzie) 2 40-pin IC sockets 2 28-pin IC sockets 2 14-pin IC sockets 1 8-pin IC sockets 1 26-way dual row male header strip 1 16-way dual row male header strip 1 heatsink to suit 7 805 regulator 1 SPST miniature toggle switch 1 momentary contact pushbutton switch Semiconductors 1 Z80A microprocessor (E10) 1 8255 programmable peripheral interface (E1 2) 1 6264 static RAM (E14) 1 EPROM (E11) - available from Don McKenzie 1 7 4LS04 hex inverter (E13) 1 7 4LSOO quad 2-input NANO gate (E9) 1 555 timer (E15) 1 BC548 NPN transistor (Q1) 1 7805 3-terminal +5V regulator 4 1N4002 diodes (D1-04) 1 red LED (LED 1 ) 1 4.9152 MHz crystal (X1) in a small case Capacitors 1 2500µ,F 16VW PC electrolytic (C4) 1 22µF 16VW tantalum electrolytic (C2) 1 1 OµF 16VW tantalum electrolytic (C3) 13 .01 µF ceramic (BC1-BC11, C7, C8) 1 680pF ceramic (C6) 1 220pF ceramic (C1) 1 150pF ceramic (C5) Resistors (0.25W, 5%) 1 100k0 1 1k0 1 3300 1 10k0 1 2700 1 4.7k0 1 SIP resistor network package with 7 x 4. 7k0 resistors with one common pin 1 50kn trimpot and 270k0 resistor (to replace R7) Interface board 1 double-sided plated through PC board, code MMBCNT (available from Don McKenzie) 1 20-pin IC socket 1 1 6-pin IC socket 2 1 4-pin IC sockets 1 8-pin IC socket 1 2 amp 2AG fuse 2 PC fuse clips 1 26-way dual row male header strip 2 26-way IDC (crimp type) female headers fitted to a 22cm ribbon cable for interconnection with the PBUFF board 1 36-pin female Centronics connector 1 25-pin male D825 connector 1 heatsink to suit regulator Semiconductors 1 7 4LS373 octal latch (E1) 1 7 445 decoder (E2) 1 7 4LS 151 8-input multiplexer (E3) 1 7 4LS02 quad 2-input NOR gate (E4) 7 8D646, 8D648, 8D650 or SE9400 PNP transistors (01 to 07) 1 6A bridge rectifier (D1 ) 1 7805 3-terminal +5V regulator 1 red LED (LED 2) Capacitors 1 2500µ,F 16VW PC electrolytic (C4) 1 1OµF 1 6VW tantalum electrolytic (C3) 1 3.3µF 16VW tantalum electrolytic (C1) 6 .01 µF ceramic (BC1-BC5, C2) Resistors (0.25W, 5%) 2 100k0 1 1500 10 1k0 1 2 .20 10W 7 4700 wirewound MAY1989 67 The two PC boards and the power transformer are mounted on 6mm pillars in a standard plastic instrument case and secured using machine screws and nuts. The interface board is also secured to the rear panel using the Centronics and DB25 connectors. chips if no fault is found at the Z80A. Try the 8255 next. Assembling the interface board The initial procedure is the same here as for the PBUFF board. Check the board very thoroughly for faults before any assembly work is done. Then install the IC sockets, the resistors (except for RB), the bypass capacitors BC1-BC5, and capacitor C2. Capacitors Cl, C3 and C4 are polarised and must be soldered in the correct way around. Install the diode bridge Dl, the 2AG PCB mount fuseholder and 2-amp fuse, and the 7805 regulator. The 7805 must face away from C4. The heatsink can be fitted to the 7805. Output sockets Before fitting the DB-25 male con68 SILICON CHIP nector and Centronics female connector it is necessary to bevel the edge of the circuit board with a file or a sharp knife so that the connector solder lugs will slip over the solder pads more easily. Carefully position each connector, making sure that the numbers on the lugs correspond with the numbers printed on the pads of the circuit board. Resistor RB is a 2.20 10 watt wirewound type which has to dissipate a fair amount of power. It should be stood up about 5mm from the surface to provide more effective cooling. When the board is fully assembled, you can remove the power pack from the PBUFF circuit board and connect it to the MMBCNT board as shown on the overlay. Check that you have + 5V on the sockets and that it is the correct polarity. If all is OK then power down and connect power to the PBUFF circuit by taking it from the pads as shown on the overlay. Reapply the power and once again check that there is + 5V on both the PBUFF board and the MMBCNT board. If all is OK, then power down again. Now install the ICs, making sure that they all have the correct orientation. Power up again for one last check of the power supply on the MMBCNT circuit board and if all is OK then power down again and link the two circuit boards together via a short header cable by using PBUFF connector J2 and the MMBCNT connector Jl. Be careful to note the orientation of pin 1 on the two connectors. When power is re-applied the RUN led should be on, indicating that data is being sent to the message board display. ~