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buffer for your printer A 1 This easily built unit installs between your computer's Centronics printer port and your printer. By feeding your printout files to a buffer, you and your computer can proceed with other tasks while the printer operates in the background. Design by DON McKENZIE Waiting for your printer to do a printout can be a real pain, particularly if the file concerned occupies several hundred kilobytes of memory. With an average printer, printing out a file of this size could take several hours. That's a long time to have your computer tied up , if you are anxious to get on with other computing tasks. If you use your computer for CAD work, the files can be very large when fed to a plotter. Here's where this buffer really finds its niche. By having a printer buffer with a large memory, you can dump your files to it and it can then feed the printer at the pace it requires, leav- ing the computer free for other things. Printer buffers are not cheap though and those with large memory can set you back quite a bit. For example, a buffer with 256Kb of memory is likely to cost around $339 [eg, the BIT PEPbuffer) while a unit with one megabyte (1Mb) will set you back a lot more. Some printers can also be fitted with fair-sized buffers which achieves the same result but they're often even dearer for a given amount of memory. RAM chips have come down drastically in price in the last year or so, so you really can save quite a The printer buffer is built into a standard plastic instrument case. The two pushbutton switches are for Reset and Test, while the two toggle switches are for printer Pause and Copy control. 40 SILICON CHIP lot of money by building your own printer buffer to this design. You can also tailor it to suit your needs, building in as much or as little memory as you need. We expect that you could build the 1MB printer buffer described here for $300 or less. To build the printer buffer you will need a reasonably well equipped tool kit which will include a multimeter. A logic probe could also be a help with any hardware problems that you could run into. The logic probe suggested by the designer is a 3800A which is available at most electronic outlets for around $30.00. Don't buy a logic probe right now but if you do run into strife then it may be needed. While you can build the buffer into virtually any box, the prototype was housed in standard plastic instrument case available from most kitset suppliers. It is powered by a 12V DC plugpack which means there is no 240V AC mains wiring to mess about with. Main features On the front panel of the buffer there are two LED indicators, one of which flashes when the unit is handling data. There are two toggle switches: one to pause the printer and one as the copy control. There are also two pushbuttons: one to reset the unit and one for the test routine. As the name suggests, the Copy control lets you print out the contents of the buffer. Thus, you can print out more than once without having to go back to your computer. On the rear panel of the buffer, there is a standard female Centronics printer port and a DB25 female socket. We'll talk more about these later. Inside, there are two printed cir- ~ ,-,. CD CXl CD .... ~ t'I:I 6ttl 0 (") .0 C R 10 3.58M X1 R1 100 12 B +5V CB .01 I _ INPUT _ - IV AC lR DC J4 I 2 ~ C4 ,: : W! - 1 4 IN GND --"'i 7805 ,-0+12V .,c....._.... 01 ~13\ _ lc9• 74SLOO 1 2 IC9b 9 10 D7 r 4 GND OUT \w ==- sc1-01-11:: 16 TANT c~ OUT ; IN 3 D7 DO 01 02 03 04 05 06 8255/CS fW !- PORT D J1 PIN 18 sr 2 20 19 11 12 13 15 16 17 1f 10 9 8 7 6 5 4 3 2s 24 21 23 22 " CS K ->- 1M PRINTER BUFFER 2!090 - P~Si°E~ le~ A K ~ A +5V IC 13 j1° c"'?a 11 26 11 IC11 BK EPROM 2764 OR 27128 120 121 I T j,!!..., I 1~ 4 RAS 15 14 13 9 18 19 11 12 13 15 16 17 IC1 I 22 OE 1 27 WE IC14 6364 28 14 ' ~ S3 DO 01 02 03 D4 05 06 07 02 IC2 IC3 10 04 tl1e 06 IC12 8255 26 ::i 4 PC4 13 A14 IC5 l10 IC6 IC7 1k IC13 e 6 1-s· 3_ D. 2 D - D OUTPU 130+ 5V PAUSE 120GI _ ~B: 10-Al 9;; s-D Dt 7 O 6- 0 5 . 4 - D: ~i . ,. R5 = 16- D 15-~ 14 - S 17 = 24_ 8 22;; 0 21;; D 20 D 19;; 0 10;;0 25o 6 ND 23-A J2 INPUl 260 ➔ 5V r---------- A13 J ·RN2 4.7k 5 ICB 5 16 17 8 = +n - 1M: o----:f. CAS AO A3 AB o----~- CAS =- ~ 1M 256- 256 1M- 2:~- - AS M2 TO M5 A D D I T I O N A L ~ GND BUS FOR 1 MEG ,. M2 M3 M4 M5 l 1 l l 256 __ 1M- ......25__s,.,_._M.1,_....., u MS AND M7 ADDITIONAL +5V FOR S4 Kl~:K M 7 12"'+7 'i;718 07 ~ l RN4 4n RN 3 47 - k + I 13~-" - , ~ ) 6 I IC9c ~ D ' ' - - - - - + - ~ . +5V RN5 4.7k + 5V 5 6~ LED1 DATA'i; K R4 IC13d 3300 2 r> PC 12 5 PA 7 37 PAS 38 PA 39 5 PA 40 4 PA3 1 2 PA 2 1s PCS 11 PC 2 16 25 PB7 24 P86 PB 5 23 P84 22 21 PB3 20 2 PB 19 CS PB1 18 PBD RESET RD WR 35 5 36 PC7 - n ◄;i18 L L L - 05 '\ BxDYNAMIC RAMS 4164 ,41256,4&1024 IC4 ◄;718 03 'I 7 15DpFI C5 •~ ...Ji ~ 9 ~ AO __] --F°" 34 33 32 31 30 29 28 27 ,----1J! COPY\( RN6 4.7k L...1...._.,_,..__1..._..,_,,,__1..._..,_,,,__.__.,._,,,_,--.__.,._,,,__.L-_...,,, __,-..__...,,,,__,--'--..,,,_.,,.....,19 A2 A3 A4 A5 A7 A11 _ ~ 7 l2 •-=---',i 1 D1 'I 06 01 DO 01 02 03 04 05 9 8 7 6 5 4 3 25 24 21 2 2 20 CS1 26 CS 2 10 - '-------~--➔------< . ➔_,, A1 A2 A3 A4 A5 A6 A7 AB A9 A10 A11 A12 A14 A1 5 AO 4 iz71s 10 ';718 10 tl1e 10 3 WR DC ADDRESS BUS --.....,-::::::::~::: 'DATA BUS _ _ I AO A1 A2 A3 A4 A5 A6 A7 AB A9 A10 A11 A15 N?-,.L i ___J S ~ bH n _ , o lI 22 WR 21 Ml lORO 20 27 19 13 9 10 00 01 02 03 04 05 06 A2 A3 A4 A5 ., A6 A7 AB A9 A10 A11 A12 A13 A14 A15 AO A1 ------- '-----.l!.j-~)2..----___. 9 14 15 12 8 ~ I MREQ ' - -- ~.J;2!9_ _ - :1i1i 32 33 34 j 35 36 '7 38 39 1 40 1 2 3 4 5 i I 31 1 30 L-------~---•.---~--~ ~ - .,. ,------:'.:J AC TANT 1lv2wI 16 INT 17 NMI 6 CLOCK 111 25 lz4 VCC BSRQ WT 26 , _ RESET 4 ~ I l + 5V -. 7 _J; :I 5 I3 R6 4 .7k _j_ • 11e 14 • .,. -:- 555 IC1 5 4 +5V 4 m~ 1 7 14 7 ~.,. .,. 3"" RN 7 4.7k T~JT l R2 2700 E 1~~ RltETI . .- - - ~ ,____. 6 ....__] I , I 1 8 0 E C VIEWED FROM BELOW ~ •= C1 220pF' I c:, CS ,:= 680pF -------"+~-I 8~48 .,-.,,,.C 8£ r"'< The circuitry for the printer buffer is built onto two PC boards, one carrying the microprocessor and memory chips, and the other the two parallel interface sockets. Note the heatsink on the 3-terminal regulator. cuit boards, one to carry the microprocessor and memory chips while the other is for the two parallel interface sockets. Z80A microprocessor The printer buffer is designed around the good old Z80A which can be had for peanuts. The operating system and instructions for the Z80A are stored in a 2764 EPROM (Electrically Programmable Read Only Memory) while a 6264 static RAM (Random Access Memory)serves as a stack for the microprocessor's instructions. Input and output interfacing to the outside world is provided by the 8255 PPI (Programmable Peripheral Interface). The main concept of the design is to cut down the hardware required to produce a microprocessor controlled printer buffer with a large capacity and perform all the bells and whistles in the software. No address or data buffers are used or required. As already noted, the only port addressed device is the 8255 PPL This is the interface to the computer input and printer output bus. 42 SILICON CHIP The Z80A's IORQ line is connected to the chip select of the 8255. Any time an 1/0 instruction is executed the 8255 responds. Any port from zero to 255 is the 8255's port decode. The power up routines program the 8255's three ports to mode 1: Port A input, Port B output, and Port C handshaking for A and B. Capacitor C5 hanging on the chip select line to ground produces enough of a delay to satisfy the timing restrictions of Z80A to 8255 interfacing. The printer output strobe is produced by one inverter of the 74LS04 which inverts CPU signal A13. Any time the program sets A13 high, a printer strobe pulse is produced. The setting of A13 is actually done with a BC register pair port instruction, not a memory address. During a power up or reset of the CPU all address and data bus signals go to a high impedance state. The high to A13 appears as a printer output strobe every time the buffer is reset. This is where resistor R5 is useful. Without R5 installed some printers will hang up. They detect the output pr-inter strobe during reset, read the data and issue an acknowledge (ACK) to the buffer. The buffer doesn't pay much attention to this ACK because it has just had a reset and now wants to do initialisation. With R5 installed, instead of A13 tri-stating during a reset, it is held low by R5 to ground and this spurious strobe isn't produced. Pin 12 of the 8255 is the input BUSY to the computer. This signal is inverted by the 74LS04 to produce the ACK signal. This means that while the 8255 is holding the ACK active (low) it sends the BUSY signal to the computer. The ACK is also connected to R4 and the Data LED. The Data LED lights up when the BUSY is high and ACK is low, and switches off when BUSY is low and ACK is high. This LED is used for the ROM diagnostic routines to aid kit builders, and also indicates data entry into the buffer. When the buffer is full, the LED lights up brightly to indicate a full BUSY condition. The value of R4 is a compromise between lighting the LED up brightly enough to see data activity, and not loading the ACK line to the computer excessively. The Reset signal to the 8255 is a positive going pulse, which is the inverse of that for the CPU. One gate of the 74LS04 is used to generate this pulse from the CPU reset. The dynamic RAMs are enabled when address lines A14 and A15 are high. This gives a starting address of C000H. A15 high disables the EPROM. The Z80A takes care of refreshing the DRAMs (D for dynamic) in the normal refresh register mode. The data in the refresh counter is sent on the lower portion of the address bus (A0-A6) along with a refresh control signal while the CPU is decoding and executing the fetched instruction. This mode is totally transparent to the programmer and does not slow down the CPU operation. The program must constantly service this refresh requirement. This involves calling a refresh routine approximately every 256 instructions. So the DRAMs aren't really part of a typical memory map; it's soft- ware deception. The true map of the DRAMs as far as the CPU addressing is concerned is C000H to C0FFH (only 256 addresses). H register set to C0H and L register set to LSB select each byte. The MSB (most significant bit) of the DRAMs is dependent on the contents of the R register during a read or write procedure. What we are left with is a lump of DRAM that is not much good for anything except storing great chunks of data. You can't run a machine language program in it. It's a complete wraparound buffer. When the storage pointer gets to FFFFH, its next location is zero, and round and round it goes. So what is the 6264 static RAM used for? It actually functions as an external stack for the microprocessor since the Z80's internal stack is not big enough for the program. Short form kit If you want to build this printer buffer, there's only one way to do it. The designer, Don McKenzie, owns the copyright on the printed circuit board patterns and the software in the EPROM. He sells a short form kit which comprises the double sided buffer printed board, an EPROM containing the software and the assembly instructions for $39 plus $3 for packing and postage. If you want the small DB25 to Centronics socket adaptor board, that will be an extra $9.00. All the rest of the components have to be obtained separately by you, the builder. If you are looking for a one stop shop, one good possibility is Rod Irving Electronics who have stores in Sydney and Melbourne (see their adverts elsewhere in this magazine). You can build the buffer in three memory sizes: 64Kb, 256Kb or 1Mb. The software in the EPROM will sense how much memory is installed and operate accordingly. No fuss, no bother. Assembly The first step in assembly is to check the double sided printed board for manufacturing faults . These are rare but it is highly desirable that you find any problems before components are in- PARTS LIST 1 plastic instrument case, 200 x 160 x 65mm, DSE Cat. H-2505 or equivalent (see t~xt) 1 9V 500-600mA DC plugpack, DSE Cat. M-9560 or equivalent 1 double sided plated through printed board, code PBUFF (available from Don McKenzie) 1 single sided printed board, code BPIO (available from Don McKenzie) 1 3 .58MHz crystal 1 DB25F socket 1 Centronics female socket 1 IBM printer cable 2 SPST toggle switches 2 momentary contact pushbutton switches 1 LED bezel 1 T0-220 clip heatsink (for 7805 regulator) 2 40-pin IC sockets 2 28-pin IC sockets 8 20-pin IC sockets 2 1 4-pin IC sockets 1 8-pin IC socket 1 26-way dual row male header socket (J2) 1 1 6-way dual row male header socket (J3) Semiconductors 1 Z80A microprocessor (IC10) 1 2764 EPROM (IC11), available from Don McKenzie 1 6264 8K static RAM (IC14) 1 8255 programmable peripheral interface (IC12) stalled. If a track fault is under an installed IC it can be impossible to find. So check it thoroughly, now. IC sockets are recommended by the designer for all the large chips and the memory. The memory chips are very close together so you can't use bulky sockets. Use the good quality slimline types. Take note of the orientation of pin 1 of all ICs, as they do not all face the same way. This has been done to minimise the size of the board. Regardless of what size memory you intend using it is a good idea to install all the memory IC sockets and their accompanying .01µ,F ceramic bypass capacitors. Install the 8-pin resistor network 8 41256 256K bit dynamic RAMs (IC1 -IC8) for 256Kb version or 8 4C1024 1024K bit dynamic RAMs (IC1 -IC8) for 1 Mb version 1 555 timer (IC15) 1 7 4LSOO TTL hex buffer (IC13) 1 7 4LS04 TTL quad 2-input NANO gate (IC9) 1 BC548 NPN transistor (01) 1 7805 3-terminal +5V regulator 1 W02 or W04 bridge rectifier (DSE Cat. Z-3304) Capacitors 1 2200µ,F 16VW PC electrolytic (C4) 1 22µ,F 16VW tantalum electrolytic (C2) 1 1 Oµ,F 16VW tantalum electrolytic (C3) 11 0.1 µ,F ceramic (BC1-BC11) 2 .01 µ,F ceramics (C7 ,C8) 1 680pF ceramic (C6) 1 220pF ceramic (C1) 1 150pF ceramic (C5) Resistors (¼W, 5%) 2 100k0 (R1 ,R7) 1 10k0 (R3) 1 4.7k0 (R6) 2 1 kO (R5,R8) 1 3300 (R4) 1 2700 (R2) 1 2200 (R9) 1 SIP resistor network package with 7 x 4 .7 kO resistors with one common pin into the board as shown on the overlay; ie, with pin 1 towards the dynamic RAMs. This done, install all the resistors as shown on the overlay. They can now be soldered in and the legs cut off flush with the solder joint. Watch the polarity of the three electrolytic capacitors C2, C3 and C4. They must be soldered in the correct way around. The same goes for the diode bridge Dl. Take notice of the markings on the bridge and make sure that these line up with the overlay. Solder in the 7805 regulator. The words " 7805 " must be facing towards the diode bridge. Fit the heatsink to the 7805. OCT0BER1989 43 DB-25 connector tracks so that the DB-25 connector will push hard onto the board without damage. Looking at the overlay side of the board, the two female connectors should be soldered with the widest part facing down. A 1k0 and a 4.7k0 resistor can be soldered into position as shown on the overlay, followed by the 26-pin dual row male header. The BPIO board is now finished. 9V AC OR DC INPUT Mounting the hardware 2 COPY 3 TEST 4 LED15 PAUSE 6 LED2- 10 COPY 11 TEST 12 LE01+ 13 PAUSE 14 LE02+ r~~~~:?-c~· Fig.2: this wiring diagram is for the 1Mb version of the printer buffer. The PBUFF board is connected to the PBIO board via a 26-way cable fitted with IDC female headers at either end. Position transistor Q1 into the board. If you use a DS548, the CBE connections are marked on the case and line up with the overlay as far as the flat side is concerned. The BC548 and the others, although not marked, should mount into the board the same way. Now solder in the 3.58MHz crystal. Don't push it fully into the board but leave enough length on the legs so that the crystal body will not short out the tracks immediately underneath it. BPIO hoard This is an adapter board to allow you to use an IBM to Centronics printer which is the cheapest you can buy. The BPIO board makes use of the IBM cable and standard connectors to save cost and simplify cable interfacing. Flat ribbon cable with standard 26-pin crimp type connectors is used to connect this board to the PBUFF board. 44 SILICON CHIP The BPIO board overlay is drawn showing the component side of the board. A standard DB-25F (female) solder type connector is soldered onto the edge of the board in the position shown. The female Centronics solder tail connector (or chassis type mount) will not be hard to obtain, but there are several types around. Some have spring clips that lock onto the male connector. The clips are nice but the standard (non-clip) connectors do lock hard onto the male connector quite firmly. The point to watch out for with the Centronics female connector is the distance between the two rows of solder tails. Look for a connector that has the two rows close enough together to be able to solder to the edge of a printed circuit board. The pin numbers of the two connectors are marked on the BPIO board. You may need to use a file to bevel a slight 45° angle along the The BPIO adaptor board can be mounted using the two outer mounting holes of the DB25 and Centronics connectors only. Board inter-connections can be done with a 200mm-long 26-wire flat ribbon cable fitted with a 26-pin dual row IDC type female crimp header at each end. When drilling holes in the front of the case to mount the switches, make sure that they don't interfere with the components on your PBUFF board. All the connections to the front panel hardware are made via a cable from the 16-pin dual row header J3 on the PBUFF board. This provides wires to the Reset button, Copy switch, Test button, Data LED, Power LED and the optional Pause switch. Powering up When all your assembly work is complete, you are ready to install the chips. But before doing that, connect up the plugpack transformer. The power pack connection points are shown on the overlay. 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 IC12 (8255) and IC13 (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. If the LED is not flashing, then you have one of the following faulty: IC12, IC13, the reset circuit or, more likely, you have the data LED wired in back to front. With that sorted out, you can install IC9 (74LSOO), IC10 (ZBOA) and The rear panel of the printer buffer carries a DB25 female socket (left) and a standard female Centronics printer port. This allows you to use a standard IBM printer cable which is cheaper than other types. Fig.3: here are the linking details for the 64Kb and 256Kb versions. Note particularly the different arrangement for Mt. ICl 1 (EPROM), and power up. The Data LED should now be flashing at about once per second. If the LED is flashing, it indicates that the Z80A is running and the software is doing its job but that a RAM fault exists. With no RAM installed this is to be expected. Now it's time to insert one RAM chip. Power down and insert a RAM chip into socket IC8 (Data bit 7). Watch out for pin 1 (look at the overlay). Switch to Copy ON, power up and check your + 5V again. At this stage, the LED should be flashing at 3 cycles per second, which indicates that RAM is present and can be written to and read from. Switch to Copy OFF. The LED should go out. Press the Reset button. The Data LED should light up brightly. Release the Reset button and the LED should go out. This indicates that the software is working. All looking OK'? Power down, install additional RAM chips to suit your memory requirements, and power up again. Repeat the above test procedures using the Copy and Reset switches. A simple RAM test is performed before the LED will flash. This test may not show up a shorted track, or a faulty RAM in some cases. If you have a suspect RAM, it can be further tested on its own in the Data bit 7 position by setting Copy ON and power up. The LED should flash at approximately 3Hz. If at any time the LED flashes at lHz, then a RAM fault has been detected. Loopback test Now power down, connect the output port pins 1 (STROBE), 10 (ACK) and 11 (BUSY) to the input port pins 1, 10 and 11. This can be done by simply plugging the input and output of your IBM printer cable into the BPIO board. If your input and output can't readily be connected together, then three jumper leads can be used to connect pin 1 to pin 1 etc. When this has been done, set Copy OFF, power up and press Test. This Test function outputs a sign on message to the printer. In this case, it outputs a string of characters to its own input port that keeps cycling through the buffer until the Reset button is press- ed. The Test function will light up the Data LED to about half brilliance. Now power down, connect a printer, set Copy OFF, power up and press Test. If all is well you should get the sign on message complete with memory size information. When the test facility is used, a ROM check is also done. If any byte in the ROM is incorrect then an error will be reported on the printout. Power down, have a quick read through the operating instructions and connect your computer and printer. Power up, and try printing a file. It either works OK or doesn't work at all. If it works then you can slap on the lid and forget it. If it doesn't work then Don McKenzie's instructions include an extensive troubleshooting procedure which should get you out of trouble. Thousands of this printer buffer have already been built so it's been well debugged. At the price you can build it yourself, it will make a very good add-on for your computer. ~ Where to buy the parts A short form kit of parts for this project is available from the designer, Don McKenzie, for $39 plus $3 for packaging and postage. This kit comprises the double-sided PBUFF board , an EPROM containing the software, and the assembly instructions. If you want the small 0B25 to Centronics socket adaptor board, that will be an extra $9.00. Write to Don McKenzie, 29 Ellesmere Crescent, Tullamarine, Victoria 3043. Note: copyright of the PCB patterns and the software in the EPROM is retained by Don McKenzie. OCT0BER1989 45