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By STEVE CARROLL & BOB NICOL Build this low-cost AT keyboard translator This simple device converts the complex scan-codes from an AT keyboard to standard ASCII character and control codes. It was originally designed for use with the BASIC Stamp, Counterfeit and PIC series of microcontrollers but has lots of other uses as well. The rapid progress of computer technology has resulted in many old, fully-functional IBM AT keyboards being thrown away in favour of newer, fancier ones. If you’ve ever wanted to connect one of those discarded keyboards to a project which accepts standard ASCII codes, this AT Keyboard Translator could be just what you’re looking for. In operation, the device connects directly to any 101/104-key AT key- board with a 5-pin DIN connector and converts the key scan-codes to standard ASCII “character” and “control” codes. It then outputs these codes in standard inverted asynchronous format at 300, 1200, 2400 or 9600 baud. The baud rate chosen depends on your “receiver” and is selected using a single jumper designated HDR2 on the PC board. The output data is compatible with the RS-232 serial port of many devic- Information On PC Keyboard Standards Want to find out more about PC keyboard standards? You’ll find lots of information at these two websites: (1) http://www.hth.com/filelibrary/txtfiles/keyboard.txt (2) http://linton.csie.ntu.edu.tw/design-reference/pc/keyboard_FAQ.html 72  Silicon Chip es, allowing you to send text or control codes to your application. It’s just the shot for interfacing with microcontrollers such as the BASIC Stamp, Counterfeit and PIC series (in fact, the device was originally designed to do just that). There really is no easier way to connect over 60 switches to one pin of a Stamp1, Counterfeit Stamp1, Stamp2 or PIC chip (the non-ASCII keys are not used). Of course, it’s not just limited for use with microcontrollers. It can also interface with other serial devices such as a serial printer (via a suitable RS232 driver interface) or LCD drivers. One of the photographs with this article shows the AT Keyboard Translator driving a 4-line alphanumeric LCD via an “LCD Serial Backpack” (designed by Scott Edwards Electronics). Other possible applications include use in an ASCII User Terminal, an RF/Infrared Keyboard Link, a Video Text Generator and a Moving Message Display. The accompanying panel lists 12 possible applications but there are lots more. By the way, if your keyboard has a PS/2 connector (six pins) rather than the 5-pin DIN type, an adaptor can be purchased from most electronics sup- Fig.1: the circuit uses a PIC microcontroller to decode the complex scan-codes from an AT keyboard and convert them to ASCII character and control codes. pliers. Alternatively, an “off-board” PS/2 socket on a short length of cable can be used to replace the standard 5-pin DIN socket on the PC board. Power for the keyboard (+5V) is provided via the keyboard socket – it’s just a matter of plugging the keyboard in and applying power (12V DC) to the adjacent DC power socket. 10kHz to 30kHz. Fig.1 shows the circuit details of the AT Keyboard Translator. It’s deceptively simple, with all the “magic” taking place inside a pre-programmed PIC series microcontroller (IC2), either a PIC16F84-10 or PIC16F84-20. These devices have 1KB of “flash” EEPROM program-memory, 68 bytes of RAM and 64 bytes of EEPROM data storage. In this application, we require only a handful of external components to make a complete working circuit. How it works The AT keyboard has a fairly complex two-way communications protocol that is quite a handful to decipher. There’s no logical mathematical pattern to the “scan-codes” sent by the keyboard and certainly no similarity to ASCII. These AT scan-codes can involve up to 13 bytes of data being sent for a single keypress and release. Just to make things difficult, even the simple act of pressing “Caps Lock” does not automatically light the “Caps Lock” LED on the keyboard. Instead, the keyboard sends a “Caps Lock” scancode to the host (normally a PC or, in this case, the Keyboard Translator itself), which then sends a “Light Caps Lock LED” message back to the keyboard. Finally, as if all that isn’t complicated enough, an AT keyboard can operate at anything from about The PC board should only take about 10 minutes to assemble. It really doesn’t get much simpler than this! May 2000  73 Parts List 1 PC board, 51mm x 61mm, 1 10MHz ceramic resonator (CR1) 1 18-pin DIL IC socket 1 2.1mm DC socket 1 5-pin DIN socket, PC-mount 1 2-way pin-header strip 2 4-way pin-header strips 1 5-way pin-header strip 1 pin header jumper 1 link wire Semiconductors 1 LM7805 3-terminal regulator (IC1) 1 PIC16F84-10/20 microcontroller with ATKB program (IC2) 1 1N4001 silicon diode (D1) 2 1N4148 signal diodes (D2, D3) Capacitors 1 100µF 25VW PC electrolytic (C1) 2 0.1µF ceramic (C2-C3) Resistors (0.25W, 5%) 3 10kΩ (R5-R7) 2 2.2kΩ (R1-R2) 2 1kΩ (R8, R9) 2 220Ω (R3-R4) Basically, the PIC microcontroller converts the complex IBM AT scancodes to standard ASCII codes. Much of the actual decoding function is achieved by the use of “lookup” tables. Each time a key is pressed, the keyboard sends the scan-code to pins 17 & 18 of IC2 via resistors R3 & R4. Pin 17 accepts the clock signal, while pin 18 accepts the data signal. The microcontroller separates the eight data bits and uses this value as a memory address offset to look up the appropriate ASCII value. Assum- ing that the code is a valid ASCII “character” or “control” code, the serial data appears on pin 13 and is fed to pin 4 of a 5-way pin header (HDR3) via resistor R8 (1kΩ). The CTS (clear to send) line (pin 8) is optional and in most cases only one pin on the receiver is needed to ensure clean communications. Note that communications between the keyboard and translator are almost exclusively one way. The only translator-to-keyboard commands involve turning the “Caps Lock” LED on or off as required. Optional CTS function Sometimes typing speeds can be too fast for the receiving device (Stamp, PIC, etc), so an optional CTS function has been programmed into the PIC microcontroller. In this circuit, the CTS output at pin 8 is normally tied to 0V by resistor R7. However, if necessary, it can be pulled high (+5V) by the receiver, taken low to receive the next byte, then immediately returned to the high state until the receiving device is ready again. A similar method of data flow control is used between the keyboard and the translator, utilising the keyboard’s inbuilt buffer to temporarily store key presses until the microcontroller is ready for them. Unfortunately, this buffer has a limited storage capacity so prolonged bursts of high-speed typing may cause some characters to be missed if the receiving device is too slow. The baud rate (ie, the rate at which data is transmitted from pin 13 of IC2) is set by placing a jumper across one of four pairs of header pins (HDR2). This can be set to either 300, 1200, 2400 or 9600 baud (bits per second) and must be set to match the receiving device. Specifications Supply voltage ����������������������7.5-15VDC Supply current ����������������������<1mA (idle). Note that a typical AT keyboard current of up to 300mA must be added to this. Keyboards supported �����������Most IBM-compatible 101/104/105-key AT keyboards with 5-pin DIN connector. A keyboard with a PS/2 connector can be used via a suitable adapter. Output data format ����������������Standard asynchronous (inverted) at 300, 1200, 2400 or 9600 baud (8N1). 74  Silicon Chip Fig.2: take care to ensure that all semi­ conductors and the electrolytic capacitor go in with the correct polarity. Depend­ ing on the keyboard, it may be necessary to fit a small heatsink to regulator IC1. For example, a “Stamp1” is limited to a maximum of 2400 baud, as is the LCD Serial Backpack, but many other devices will readily accept speeds up to the Keyboard Translator’s maximum 9600 baud rate. The logic levels on pins 6 & 7 of IC2 determine the baud rate. As shown in Fig.1, one side of the 4-way dual pin header is commoned and connected to the +5V rail. When the jumper is in the 300 baud position, pins 6 & 7 are both pulled low via R5 & R6. In the 1200 baud position, pin 6 is pulled high (+5V) and pin 7 is low, while for 2400 baud pin 6 is low and pin 7 is high. Finally, when the jumper is in the 9600 baud position, pins 6 & 7 are both pulled high via diodes D2 & D3. Clock signals for IC2 are derived from an internal oscillator between pins 15 & 16. Its frequency is set to 10MHz by ceramic resonator CR1. Power supply An AT keyboard requires 5V DC and typically draws a current of 100-300mA. This is provided by a 7805 regulator which also provides a regulated +5V rail for the rest of the circuit and to the 5-way “output” socket. The 78xx series of regulators can handle in excess of 1A and have internal current-limiting and thermal-protection circuitry, making them almost bullet-proof. Note that earlier keyboards may have higher power requirements than later models. For this reason, if you use an early keyboard, the regulator may get quite hot. If this happens, the answer is to fit a small heatsink. This view shows the AT Keyboard Translator driving a 4-line alphanumeric LCD (via the Scott Edwards LCD Serial Backpack). Note that the LCD Serial Backpack writes lines 1 & 3 first, then lines 2 & 4. The unit itself runs from 7.5-12V DC and this can come from a DC plugpack supply. The power can be applied via an on-board 2.1mm DC socket or to a nearby 2-pin header (HDR1). The centre pin on the DC input socket is positive, while the body contact has negative polarity. Building it The PC board is very easy to assemble and should cause no problems if the overlay illustration is carefully followed – see Fig.2. As with most boards, it’s a good idea to begin with the smallest parts and work up to the larger ones. Start by installing the wire link (this goes between the keyboard and DC power sockets), then install the resistors, diodes and capacitors. Take care also to ensure that the electrolytic capacitor and the diodes are mounted with the correct polarity. Note particularly that D1 is a 1N4004 power diode, while D2 & D3 are 1N4148 small signal diodes. The pin headers can now be installed, followed by the IC socket, the DC power socket and the keyboard socket. Make sure that the sockets are all seated correctly on the PC board before soldering their pins. The PIC16F84-10/20 is a static-sensitive device, so normal ESD (electrostatic discharge) precautions should be employed. This device should not be installed in its socket until all other assembly has been completed. Take care to ensure that it is installed the right way around. The 7805 3-terminal regulator (IC1) must be installed with its metal tab towards the centre of the board (see photo). Setup & testing Once the assembly has been com- pleted, carefully examine the rear of the PC board for solder bridges between pins, missed solder joints and vacant holes. You should also check that all the parts are in their correct positions and that all polarised parts are correctly oriented. If all appears OK, place the jumper across the appropriate pins to select the required baud rate and connect the +5V, DAT (Data), 0V and CTS pins to the receiver as required. Also ensure that the receiver is configured for 8,N,1 (ie, 8 data bits, no parity bit, 1 stop bit), inverted polarity. If using the AT Keyboard Translator with a BASIC Stamp1, the correct serial modes are N300, N1200 or N2400. The Stamp (or Where To Buy The Parts The Keyboard Translator is available pre-assembled or in kit form from two companies, as follows: (1) Control Electronics, 231D Timmsvale Rd, Timmsvale, NSW 2450. Phone (02) 6654 5458; email ctrl<at>mpx.com.au (2) Microzed Computers, PO Box 634, Armidale, NSW 2350. Phone (02) 6772 2777; fax (02) 6772 8987; email sales<at>microzed.com.au Fully assembled and tested PC board (no case)..............................$49.00 Short-form kit (PC board plus all on-board components)..................$39.00 Programmed PIC microcontroller and 10MHz ceramic resonator.....$18.00 The BASIC Stamp, 4-line alphanumeric display and the Scott Edwards LCD Serial Backpack are available from Microzed Computers. Further information is available by phone or from www.microzed.com.au May 2000  75 Suggested Applications For The Keyboard Translator (1) STAMP “SERIN” COMPATIBLE: the Keyboard Translator’s output is directly compatible with the Stamp, Counterfeit and PICBASIC “SERIN” function. You can use it at 300, 1200 or 2400 baud for Stamp1 or Counterfeit and also 9600 baud for faster devices (PIC, Stamp2, etc), making keyboard input a simple matter for a wide variety of applications. A short program listing that enables the Stamp1 or Counterfeit to receive data from the Keyboard Translator is shown in the accompanying panel. (2) >60 SWITCHES, ONE STAMP PIN: an AT keyboard and Keyboard Translator combination is equivalent to more than 60 switches on one pin of a Stamp, Counterfeit or PIC, etc (two pins if CTS used). red link or RF data transmitter and receiver, the Keyboard Translator could be used to remotely send data to a PC running a QBASIC or terminal program. LCD display could allow a Stamp, Counterfeit, etc to accept ASCII data from a keyboard. You could then edit it and print it to a serial printer. (5) TV TEXT OVERLAY: a video text overlay generator could be designed to display text on a television screen using the Keyboard Translator and a suitable IC such as the STV 5730A. (9) RS-232 ASCII: with the addition of an RS-232 driver, standard ASCII character and control codes could be transmitted via cable to many devices and applications (eg, a serial printer). (6) MOVING-MESSAGE DISPLAY: the Keyboard Translator could be used, along with a Stamp or some other microcontroller and a suitable display, to design a moving-message display without tying up a PC. (10) HOME AUTOMATION: the Keyboard Translator could be used with an LCD display and a Stamp to control a simple home automation system. (3) ASCII USER TERMINAL: ap­ plication notes for a Stamp-based “User Terminal” extend only to a 3 x 4 or 4 x 4 numeric keypad. The Keyboard Translator, along with an LCD, can be used to build a far more versatile terminal, with all ASCII character and control codes available. (7) MOTOR CONTROL: an LCD display, a Stamp (or similar) and the Keyboard Translator could be used to program a wide variety of motor-control systems, especially if an extra memory chip (a serial EEPROM or similar) was used to store additional data for longer sequences, etc. Again, this could be done without a dedicated PC. (4) RF/INFRARED KEYBOARD LINK: in conjunction with an infra- (8) DO-IT-YOURSELF TYPEWRITER: the Keyboard Translator and an (11) EASY MORSE CODE: a Stamp and the Keyboard Translator could be used to send the Morse code tones for each keypress, without the user needing to learn Morse code. (12) TEXT WRITER/PLOTTER: using a microcontroller, a serial EEPROM or other memory IC and the Keyboard Translator, a writer/ plotter system could be devised to write text onto objects such as signs, etc. Fig.3: if you want to use the Keyboard Translator to send data to a terminal program, you will need to wire the connector as shown here. This tricks the port into thinking that it is connected to a serial device with full handshaking. The LCD Serial Backback is mounted on the back of the alphanumeric display via a 14-way pin header. It accepts serial data in ASCII format and decodes it to drive the display. 76  Silicon Chip similar) can also be powered from the +5V and 0V pins on the 5-pin output header. In our example, we are using the AT Keyboard Translator to drive the Stamp/Counterfeit Program Listing ASCII Control Codes Following is a short BASIC Stamp1 (or Counterfeit) program to receive serial data from the AT Keyboard Translator and display it on a PC monitor. After typing in the listing exactly as it appears below and connecting the keyboard to the Keyboard Translator, connect the 0V pin on the Stamp to the 0V pin on the Keyboard Translator, then connect Pin 0 on the Stamp to the Data pin on the Keyboard Translator. Next connect Pin 1 on the Stamp to the CTS pin on the Keyboard Translator and ensure that the baud-rate jumper is set to the required bit-rate. Finally, connect the Stamp’s download cable and press <Alt><R> as usual to download the program and execute the DEBUG instruction. At this stage, pressing any ASCII keys on the keyboard should result in the corresponding characters appearing on the PC monitor. NUL Ctrl <at> Null SO H Ctrl A Start of heading S TX Ctrl B Start of text ETX Ctrl C End of text EO T Ctrl D End of transmission EN Q Ctrl E Enquiry LF Ctrl J Line feed ‘KB_READ.BAS ‘BS1 PROGRAM TO RECEIVE SERIAL ASCII DATA FROM THE “AT KEYBOARD ‘TRANSLATOR” AND DISPLAY IT ON THE PC MONITOR VIA “DEBUG”. ‘BAUD-RATES OF 300, 1200 AND 2400 ARE SELECTABLE. (PLACE ‘ AT BEGINNING ‘OF UNUSED LINES) ‘DATA FORMAT IS INVERTED, 8,N,1, (N300, N1200, N2400). VT Ctrl K Verti cal tab ‘NB: FALSE CHARACTERS WILL BE DISPLAYED BY THE STAMP1 “DEBUG” ‘FIRMWARE IF CONTROL-CODES ARE SENT. SYMBOL COMS=0 SYMBOL CTS=1 PAUSE 1000 LOOP: LOW CTS ‘SERIN COMS,N300,B2 ‘SERIN COMS,N1200,B2 SERIN COMS,N2400,B2 HIGH CTS DEBUG #<at>B2 GOTO LOOP ‘SERIAL COMS ON PIN0 ‘CTS ON PIN1 ‘WAIT FOR KEYBOARD AND TRANSLATOR ‘TO INITIALISE ‘ENABLE KEYBOARD TRANSLATOR TX ‘N300 ‘N1200 ‘N2400 ‘DISABLE KEYBOARD TRANSLATOR TX ‘DISPLAY ASCII CHARACTER ON PC MONITOR ‘GET NEXT CHARACTER LCD Serial Backback and this requires connections from the +5V, 0V and data (DAT) pins. These points respectively go to +5V, GND and SER (serial) on the LCD Serial Backpack. Note that this device does not write to lines 1-4 of the LCD in numerical order. Instead it writes to line 1 first, then line 3, then line 2 and finally line 4 (ie, the order is 1, 3, 2, 4). The keys As mentioned earlier, the keys that have no ASCII equivalent are unused. All ASCII character-codes, both shifted and unshifted, are generated in the usual way by pressing the appropriate key/s. The “Caps Lock” function operates as usual and lights the “Caps Lock” LED when it is enabled. The alternative characters on the numeric keypad are all non-ASCII, so these have been disabled and the keypad operates in numeric mode only. Similarly, all the arrow keys are disabled with the exception of the “Backspace” key, which sends 08 hex, the ASCII code for “BS”. Pressing either “Enter” key will generate an ASCII “Carriage-Return” (CR). The “control” codes are accessed by holding down the “Ctrl” key or “Ctrl” + “Shift” and the relevant key. The <Esc>, <Backspace>, <Del>, <Enter> and <Tab> keys also generate the corresponding ASCII control codes. You can refer to the accompanying “ASCII Control Codes” table for a full ACK Ctrl F B EL Ctrl G Bell Acknowledge BS Ctrl H Backspace HT Ctrl I Horizontal tab FF Ctrl L Form feed CR Ctrl M Carri age return SO Ctrl N Sh wt out SI Ctrl O Sh wt in D LE Ctrl P Data li nk escape D C1 Ctrl Q Devi ce control 1 D C2 Ctrl R Devi ce control 2 D C3 Ctrl S Devi ce control 3 D C4 Ctrl T Devi ce control 4 N AK Ctrl U Negati ve acknowledge SYN Ctrl V Synchronous idl e ETB Ctrl W End of transmission block CAN Ctrl X Cancel EM Ctrl Y End of medium SU B Ctrl Z Substi tute ES C Ctrl [ Escape FS Ctrl \ Fil e separator GS Ctrl ] Group separator RS Ctrl ^ Record separator US Ctrl _ Uni t separator listing of the control codes as defined in ANSI X3.4. Keyboard compatibility Finally, although the device works with the vast majority of keyboards, you will inevitably come across the odd keyboard that won’t work with the translator. Typically, if you press the caps lock key on these keyboards, the keyboard LED indicator comes on and the device appears to lock up. The current answer is to use a different keyboard, although further refinements to the ATKB PIC program to include a simple error handling routine (and still fit the program in the available space) may eliminate this problem further down the track. SC May 2000  77