an ask modem in accordance with the present invention may be implemented as part of a custom asic designed to support multiple ir modulation formats and protocols . the asic may include a standard 16550 uart ( obtainable from vlsi technology , inc ., burlington , mass .) which is used to produce and receive serial data in an asynchronous format . the present ask modulator and demodulator are designed to be compatible with other ask systems using a 500 khz carrier . referring first to fig4 a modulator circuit for producing an ask transmit signal in accordance with the invention is shown with a carrier frequency generator and adder circuit wherein a seven bit counter 10 may be used to divide the input of about 36 . 864 mhz master clock source counts by 74 , with 36 of those counts producing a high output signal , in order to generate a square wave output of about 498 . 16 khz from latch 13 . a latch 14 is set on the clock edge after the seven bit counter 10 reaches the count of 72 . the output of the latch 14 is fed back on line 15 to reset the counter 10 so that the counter cycle always consists of 74 clock counts . the value of the counter 10 output is sampled by latch 13 , the output of which goes high if the count value is in the range from 28 to 63 . the output of latch 13 is a square wave of 498 . 16 khz and is used to modulate the signal from the transmit line 12 of a standard uart by logically gating it through and gate 16 . accordingly , a square wave is generated on line 11 whenever the uart output transmit signal on line 12 is low . the forms of the transmit signal and square wave are shown in fig5 . thus , an approximately 500 khz carrier signal is produced during the start bit of each character in the input signal and during each zero bit period . the 500 khz output signal is then fed on line 11 to a suitable output device , such as an ir source 17 , in order to produce the desired ask modulated ir light signal . the clock to the seven bit counter 10 is disabled whenever the uart transmit line 12 is high in order to conserve power . the system clock is divided down to generate a clock with the desired frequency and an almost 50 percent duty cycle . in some applications it may be desirable to make both the frequency and duty cycle programmable . the output of the carrier frequency generator is anded with inverted raw transmit data to create the required ask modulated transmit signal . on the receiver side , as seen in fig1 the ask modem of the invention , has a demodulator comprising an ir transceiver 18 , acting as a receiver , and an analog - emulating digital bandpass filter . the ir receiver 18 amplifies the incoming signal with an analog circuit and takes the amplified incoming signal and runs it through a schmitt - triggered input buffer to recondition the signal as well as to digitize it . the analog circuit just amplifies the input signals and is ac coupled and dc offset so that it has a minimum at zero volts and a maximum at the supply voltage of the digital logic . the reconditioned signal is fed to the digital bandpass filter after synchronization with the system clock . the bandpass filter is variously shown in fig1 - 3 and consists of a carrier frequency qualifier ( fig1 ), carrier detect logic ( fig2 ), and a delay block ( fig3 ) that emulates the discharge characteristics of an analog band pass filter implementation . briefly the filter operates as follows . the carrier frequency qualifier of fig1 has a window that accepts a range of pulse frequencies . the time between the receipt of the leading edge of one pulse to the leading edge of the next pulse is measured with a very high frequency clock . if this time is within the acceptable tolerance or within the desired band frequency , the carrier frequency qualifier activates a carrier -- good signal . since this is done in the digital logic , the width of the band and the cut off frequency may readily be made programmable . the carrier detect logic then counts the number of consecutive incoming pulses while carrier -- good is active . if it counts a predefined number of pulses , it activates an ask carrier detect circuit , shown in fig2 which outputs a carrier -- detect signal . this predefined number may also be programmed so that the accuracy of the carrier detect logic may be adjusted depending on the application . at the same time , the delay block logic of fig3 prolongs the carrier -- good signal for a predefined amount of time , which also can be designed to be programmable . the output of this delay logic is the output of the bandpass filter . more particularly , referring to fig1 the digital bandpass filter for emulating an ask analog bandpass filter , and centered around a carrier frequency of 500 khz , consists of a seven bit counter 19 which is clocked by a master 36 . 864 mhz clock . counter 19 is reset on each edge of an incoming ir pulse , from ir transceiver 18 , which is detected by latches 20 and 21 . the current count value is compared each time the counter 19 is reset and a latch 22 is set if the count value is in the range of 68 to 79 . the output of latch 22 is gated by an and gate with the inverted output of latch 26 which becomes set when counter 19 reaches the count of 80 . the carrier -- good signal is therefore shut off if no subsequent ir pulse is received within 80 clock pulse cycles of the previous ir pulse . by setting latch 22 in the 68 to 79 count range , latch 22 is therefore set when the period between two incoming ir pulses corresponds to a frequency of anywhere from 467 khz to 542 khz and its anded output produces the carrier -- good signal . since such signal sensing is an overly strict bandpass indicator , an additional stage ( fig3 ) may be added to allow the carrier -- good output of the filter to stay set for up to three carrier pulse periods after latch 22 becomes set , regardless of the incoming ir signal . this additional stage functions to simulate the time constant of discharging which occurs in an analog bandpass circuit . specifically , as seen in fig3 a four bit counter 23 is reset , in response to the carrier -- good signal output which results whenever a valid ir signal is detected by the band pass filter of fig1 . the valid carrier -- good ir signal is then prolonged by a latch 24 which is only reset when the output of the counter 23 reaches a count of 15 . latch 24 accordingly controls the output of the digital band pass filter , which output is then fed to a uart for implementing . thus this additional stage simulates the time constant of discharging of an analog band pass filter . an additional circuit shown in fig2 is included which sets a flag , the carrier -- detect signal , when a minimum of eight consecutive ir pulses satisfy the frequency range requirement . this circuit uses a four bit counter 25 to count the number of valid ir pulses of the carrier -- good signal . the carrier -- detect signal flag is used to provide an indication to the controlling software that a valid ask modulated signal is being received . by using the digital bandpass filter in the ask modem , a less expensive and yet effective demodulator is provided which may be used for handling and facilitating infrared ( ir ) communications . the ask modem described herein is particularly suitable for use in the interoperable multiprotocol directed infrared wireless communication controller disclosed in our u . s . patent application ser . no . 08 / 323 , 282 filed concurrently herewith and incorporated by reference . this modem also can be used in any communication controller that uses ask modulation . the programmable digital bandpass filter of the invention can be used in any application that requires a digital bandpass filter . this modem has been found to be compatible , for example , with the sharp wizard personal organizer unit which employs a 500 khz analog ir transceiver . additionally , the ask modulation has been tested between two custom asic units up to data rates of 38 . 4 kbps . while the invention has been particularly shown and described with respect to a preferred embodiment thereof , it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention .