Abstract:
The communication system has a first modem having a digital port for communicating a first data block in a first format between a first transmission channel and the digital port and a second modem having a digital port for communicating a second data block in a second format between a second transmission channel and the digital port. The first data block has a header block and a message block. The controller/processing device comprises a first interface means, a second interface means and a conversion means. The first interface means is adapted for receiving the first data block in the first format from the first digital port. The conversion means is coupled to the first interface means. The conversion means converts the first data block in the first format to the second data block in the second format. The second interface means is coupled to the conversion means. The second interface means is adapted for sending the second data block in the second format to the second digital port. The second interface means starts sending the second data block after said the interface means has received the header block and before it has received the message block.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to voice and data communications systems, and in particular, to a modem conversion system which allows secure communications between differing transmission channel types, with minimal call set-up delays. 
     BACKGROUND OF THE INVENTION 
     In contemporary voice and data communications systems, modulation schemes are generally selected to be most consistent with the characteristics of the particular transmission channel. As a result, a modulation type used for a wireless high frequency (HF) channel, for example, may not be the one best suited for a wire-line channel. In addition, the signaling, such as the information exchange that takes place during call set-up, is often matched to the characteristics of the channel type or application. For example, the start of signaling for HF transmissions usually contains Doppler tones that allow the determination of Doppler shift. These Doppler tones are used to correct for Doppler shifts in the transmitting frequency due to the velocity of the moving vehicle containing the communications equipment. On the other hand, the start of signaling for wire-line transmission typically contains tones that allow for the estimation and subsequent cancellation of echoes. 
     Many modern communications systems attempt to provide global connectivity, in which case the transmission channel may span several media. For instance, one part of the transmission may take place over a wireless HF channel and another part over a wire-line one. Since the modulation and signalling protocols on the wireless and wire-line sides differ, a modem converter is necessary to convert modulation and signaling to the one that is appropriate for each portion of the transmission channel. 
     In one type of wireless to wire-line communications link, one end of the link may be in a mobile vehicle several hundred miles from an HF base station that is connected to a public telephone line. The other end of the link is a public telephone subscriber at another location. Upon the start of a call from the mobile vehicle, modulated signaling and message data is transmitted to the HF base station. Two separate modems are located at the base station, a HF modem for interfacing with the HF part of the link, and a wire-line modem connected to the public telephone line. The HF modem demodulates the data received from the mobile vehicle and modulates data to be transmitted to the mobile vehicle in the reverse direction. The modem conversion occurs as the HF modem transfers the demodulated data to the wire-line modem and vice-versa in the reverse direction. Thus each modem acts as a bit source for the other. The wire line modem then modulates the data in the wire-line format. This modulated data is then conveyed via the public telephone company to the intended subscriber at the other end of the link. In the opposite communications direction, the sequence is reversed. 
     In the prior art system just described, the HF modem at the base station generally receives an entire block of signalling and message data in the HF format, prior to the wire-line modem transmitting any corresponding signalling or message data in the wire-line protocol towards the wire-line subscriber. Each modem is therefore unaware of the signaling that is occurring at the other modem, and only views the other as a source or sink of data. The modems do not notify each other of ongoing signaling stages, but instead wait until appropriate data blocks are collected which are then passed on. This approach can introduce significant delays in the call set-up portion of the communication. These delays are not only annoying, particularly for voice communication, but can also cause the data sources at the ends of the link to time out. 
     It is therefore an object of the present invention to provide a modulation and signalling conversion system to allow voice or data communication between differing transmission channel types, which system introduces only minimal delays in the signalling and message data transfers. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are achieved by providing a modulating and signalling conversion system. The communication system has a first modem for communicating a first data block in a first format between a first transmission channel and a first digital port and a second modem for communicating a second data block in a second format between a second transmission channel and a second digital port. The first data block has a header block and a message block. 
     A controller/processing device comprises a first interface means, a second interface means and a conversion means. The first interface means is adapted for receiving the first data block in the first format from the first digital port. The conversion means is coupled to the first interface means. The conversion means converts the first data block in the first format to the second data block in the second format. The second interface means is coupled to the conversion means. The second interface means is adapted for sending the second data block in the second format to the second digital port. The second interface means starts sending the second data block after said the interface means has received the header block and before it has received the message block. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a full understanding of the present invention, the above objects and further features and advantages of the invention are described in detail in an exemplary embodiment below in conjunction with the drawings, of which: 
     FIG. 1 shows a wireless to wire-line communications link according to the present invention; 
     FIG. 2 depicts a block diagram of the radio to wire-line interface shown in FIG. 1; 
     FIG. 3 illustrates an HF signalling and messaging protocol; 
     FIG. 4 shows various signalling and messaging sequences communicated within the communications link of FIG. 1; 
     FIG. 5 shows an embodiment of the HF Modem of FIG. 2; 
     FIG. 6 shows an embodiment of the Wire-line Modem of FIG. 2; 
     FIG. 7 depicts a software flow chart for frame processing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     At the outset, it is noted that the present invention will be described in reference to a wireless HF to a wire-line communication link; however, it is understood that the invention is also applicable to other communication links that have differing transmission channel types in separate portions of the link. 
     With reference now to FIG. 1, there is shown a block diagram of a wireless to wire-line communications link  10  according to the present invention. The communications link  10  includes a novel radio to wire-line interface (RWI)  20  which will be shown to substantially reduce communication delays due to the differing signaling and message protocol requirements on both sides of the link. The shown communication link  10  is a link between a high frequency (HF) subscriber set  12  at a mobile site  25 , and a wire-line subscriber set  26  connected to a telephone line of a public service telephone network (PSTN) central office  24 . The HF subscriber set  12  may be a conventional digital telephone set suitable for mobile radio communications, such as a military standard KY-99. Advantageously, the subscriber set  12  may be controlled by a personal computer (not shown) to enable computer data files to be transmitted and received over the communication link. In addition, the HF subscriber set  12  of course includes an HF modem to modulate and demodulate either digitized voice information or computer data. Whether digitized voice or computer data is to be communicated, an HF radio transmit/receive unit  14  at the mobile site provides an HF input/output interface between the subscriber set  12  and an antenna  16 . 
     The modulated data to and from the mobile site  25  is communicated via a radio link of up to several hundred miles to a strategically located base site  27 . An antenna  18  coupled to another HF radio transmit/receive block  22 , together function as the radio interface. The RWI  20  located at the base site, converts the signaling, message and traffic data received by the transmit/receive block  22  in the HF format, to a wire-line format and vice versa. From the RWI  20 , the wire-line signaling and other communication data is transmitted to and from the PSTN central office  24  via a wire-line telephone company link. In this manner, the digitized information originating at the mobile site can finally be communicated to the intended wire-line subscriber set  26  via another telephone company link. The wire-line subscriber set  26  may be, for example, a two wire secure telephone unit such as the STU-III, a military standard telephone set. In any case, the subscriber set  26  includes a wire-line modem and preferably, a computer (both not shown) to transmit and receive computer data files in addition to digitized voice information. 
     Referring now to FIG. 2, a preferred configuration for the radio to wire-line interface (RWI)  20  is shown in block diagram format. A PC controller/processor  40  is employed to provide control signals to an HF modem  28  and to a wire-line modem  30 . Control and data signals are also communicated to and from the HF modem  28  and the HF radio transmit/receive block  22 , and to/from the wire-line modem  30  and wire-line input/output interface  32 . The data that flows to/from the block  22  through the RWI  20  to the central office  24 , and eventually to the subscriber sets  12  and  26  of FIG. 1, contains the signaling and communication data. It is noted that the shown RWI  20  is absent a communications security (COMSEC) means, and thus acts as a black gateway, thereby avoiding problems associated with red gateways. With a black gateway, the information bits of the transmissions do not have to be examined, so that additional delays are not introduced. 
     The HF modem  28  is advantageously a multi-mode Advanced Narrow Band Digital Voice Terminal (ANDVT), parallel-tone modem. One of the modes support voice data communications while several other modes support compute data file transfer at differing data rates. In the voice mode, a thirty-nine tone system is implemented, whereas a sixteen tone system is employed in the data file modes. 
     The wire-line modem  30  and input/output interface  32  may support a quaternary phase shift keying (QPSK) modulation scheme on the wire-line side, as is conventionally used by a STU III Modem. QPSK modulation schemes are well known, as are STU-III telephone sets, and therefore the details of QPSK modulation need not be elaborated upon. 
     The signaling information required on the wire-line side of the link, such as the path in FIG. 1 from the RWI  20  to the wire-line subscriber set  26 , may include pusedo 1800 Hz (P1800) tones which are necessary for echo cancellation. As is well known, echo cancelers may be used at both ends of a wire-line communication link to eliminate only reflected signals and not the electrical signals representing the party&#39;s speech. An echo canceler operates by storing transmitted speech for a period of time equal to the round-trip delay of the circuit. Then, the stored signal is properly attenuated and subtracted from the incoming return signal. Thus echo cancellation requires knowledge of circuit length, the echo return loss and continuous storage of the transmitted signal. By transmitting the P1800 tones on one side of the wire-line link and then gathering the energy reflected from the other side, the requisite parameters become known and echo-cancellation becomes possible. 
     The signaling on the wireless side, from the RWI  20  to the HF subscriber set  12 , does not include P1800 echo cancellation tones but rather includes Doppler tones to compensate for transmitted frequency errors due to the velocity of the mobile vehicle at the mobile site  25 . The Doppler tones are also used to correct for differences in the actual transmitting frequency and the tuned receiver frequency due to tolerance variations and the like. 
     Shown in FIG. 3 is the signalling and message protocol for data communication on the HF side of the link, i.e. between the RWI  20  and the HF subscriber set  12 . Each data transmission during the call set up portion of the communication will begin with an HF preamble on the order of 700 ms, followed by a Message Block. The HF Preamble data sequence consists of a data stream of Doppler tones  62  followed by a stream of synchronization tones  64 , reference tones  66  and a framing sequence  68 . This HF Preamble is one that would be conventionally used for tactical wireless communications between a mobile site and a strategic base site, and therefore the details of the data signals there within will not be elaborated upon. Following the HF Preamble, the first portion of the message block is a control word (CW) sequence which may be a four bit codeword. The Control Word will identify the type of data to be transmitted, thus one codeword may indicate a point to point voice communication; another may correspond to a data transmission at 300 bits/sec., while yet another codeword may indicate data transmission at 2400 bits/sec., and so on. Following the CW sequence, an eight bit message identifier (MID) word is transmitted, which will identify the type of HF link message to be transmitted. A distinct 8 bit MID codeword can be used to identify one of the following HF link messages: Dialing; Dialing Status; Call status; Capabilities/Status Vector (CAP/SV); Capabilities/Status Vector/Terminal Cipher (CAP/SV/TC); Terminal Cipher/Random Component Cipher (TC/RCC); Random Component Cipher (RCC); Crypto Synchronization (CS); and traffic data. Following the MID codeword, the corresponding HF link message is transmitted which may be 116 information bits followed by a 124 parity bit block. The HF link messages of Dialing, Dialing Status and Call Status may be single Bose-Chaudhuri-Hocquenghem (BCH) binary block codes. The traffic data HF link messages consist of encrypted voice traffic data where the encryption codes are deciphered by the microprocessors within the receiving subscriber set  12  or  26  at both ends of the communications link. The other HF Link messages are used for secure call set up between the HF subscriber set  12  embodied as a KY-99 and the wire-line subscriber set  26  embodied as a STU-III. 
     Referring now to FIG. 4, the various signalling and messaging sequences to be transmitted within the communications link  10  of FIG.  1 . are shown. The shown timing diagrams illustrate call set up data communication for the case of a call originating at the mobile site  25 . In the illustrative example, a KY-99 telephone set is used for the HF subscriber set  12  while a STU-III is employed for the wire-line subscriber set  26 . At the time t o , the KY-99 transmits a data block  41  towards the base site  27  using phase shift keying (PSK) modulation which is the standard modulation of KY-99 phone sets. It is noted that the HF modulation scheme employed is not critical to the invention; as such, other modulation schemes suitable for an HF radio link can conceivably be utilized. The data block  41  consists of an HF preamble  42  data sequence followed by an HF Message Block  43  in the time interval t 1 -t 2 , for which the HF link message is a Capabilities/Status Vector (CAP/SV). The CAP/SV message functions to announce the capabilities of the HF subscriber set  12 . In addition, the CAP/SV block identifies that the call originating from the KY-99 has a predetermined final destination i.e., the wire-line subscriber set  26 . Therefore, there is no need for a Dialing sequence to be entered when the CAP/SV option is taken by a KY-99 user. A push-button on the KY-99 set or a programming instruction within a controlling computer (if used) at the mobile site  25 , will access the CAP/SV option. For convenience, the CW and MID codeword within the HF message block  43  are not shown, these codewords are understood to immediately follow all HF preambles shown in FIG.  4 . It is noted that time interval between t 1  and t 2  is approximately 250 ms in this example. 
     Once the RWI  20  has received the HF preamble  42  of the data block  43 , it begins transmitting a signalling/message block  44  towards the wire-line subscriber set  26  via the central office  24 . The first portion of signalling/message block  44  is a pusedo 1800 (P1800) signalling preamble  52 . The P1800 preamble begins at time t 1  which is also the approximate time that the RWI  20  begins receiving the HF Message Block (CAP/SV block)  43 . Thus the RWI  20  does not wait until the CAP/SV block  43  is received in its entirety prior to transmitting the P1800 preamble  52 . This is a key aspect of the present invention, which is distinguishable from the store and forward systems utilized in the prior art that were designed to store the entire portion of the messages following the signalling preamble, prior to the commencement of signalling on the other side of the communications link. Thus, if a store and forward system were to be utilized in FIG. 4, the P1800 sequence  52  would not commence until at least the time t 2  corresponding to the end of the CAP/SV block  43 . Accordingly, the system of the present invention affords a substantial reduction in call set up time as compared to the store and forward modem conversion systems used previously. 
     Following the P1800 block  52 , another P1800 block  54  is transmitted after a predetermined GAP frame  53 . The GAP frame  53  is a necessary silent time frame to enable echoes from the P1800 sequence  52  to be received, thereby enabling subsequent echo cancellation to be performed. In the STU-III wire-line protocol, the two P1800 sequences  52  and  54  separated by the GAP  53  identifies the commencement of a call. Then, to provide for a secure communication link, a scrambled message block SCRA is transmitted which will excite the current communication link and allows the subsequent traffic data transmission to be demodulated properly. After the SCRA block, a start bit block S, CAP/SV block  47  and an end bit block E are sent. The CAP/SV block  47  contains the information within the CAP/SV block  43  transmitted over the HF radio link. The S and E bits are necessary in the STU-III wire-line format. 
     Once the STU-III receives the signalling/message block  44 , the STU III begins transmitting a signalling/message block  46  at the time t 4 . The time interval between t 3  and t 4  will of course depend upon the switching capabilities of the PSTN Central Office. At the start of the block  46 , a P1800 signalling sequence is followed by a scrambling sequence SCRC which is essentially a favorable response to the SCRA scrambling sequence received by the STU III. Thereafter, a start block S is followed by a CAP/SV block which functions to announce the capabilities of the STU III. A FILLER block then follows, which allows information associated with cryptographic key transfers to be processed. Another S block, a Terminal Cipher (TC) block and an end block E then complete the signalling/message block  46 . 
     Once the RWI  20  receives the beginning portion of the STU III transmitted block  46 , it begins a corresponding transmission of a signalling/message block  48  on the HF side towards the KY-99. The block  48  begins with a HF preamble  51  at time t 6  which is in the example, a short time duration on the order of 100 ms following the end of the SCRC block at time t 5 . Hence, the RWI  20  begins the corresponding transmission on the HF side, prior to receiving the entire signalling/message sequence  46  from the STU III. Accordingly, a significant reduction in call set-up time over store and forward systems is obtained with the current invention. 
     In the HF Preamble  51  of the signalling/message block  48 , the bits within the Framing Sequence  49  are inverted as compared to those within the Framing Sequence of the HF Preamble  42  originally transmitted. This inversion of the Framing Sequence bits identifies that the return communication is one that has originated at a STU III. Moreover, if the original call had been placed from the STU III rather than from the KY-99, the framing sequence is the HF Preamble transmitted by the RWI  20  towards the KY-99, would also be inverted as compared to that transmitted by the KY-99 in order to identify that the call was originating from a STU III. 
     The signalling/message block  50  transmitted by the KY-99 is then followed by a signalling/message block  52  transmitted by the RWI  20  towards the STU III again. It is seen that the RWI  20  sends the P1800 tones within block  52  while the TC, FILLER and RCC messages of block  50  are still being transmitted towards the RWI  20 , once again minimizing call set-up delays. 
     Having thus discussed one example of signalling, message and traffic data transmission sequences within the communications link  10 , the preferred configurations for the HF and wire-line modems  28  and  30  of the RWI  20  are now presented. Shown in FIG. 5 is an embodiment of the HF modem  28 , in which a Host Bus  70  is the conduit for control signals received from the PC Controller/Processor  40  and for the control signals passing to and from the HF Radio Transmit/Receive (T/R) unit  22 . Modulated signalling and message data received by the T/R unit  22  on the HF side is demodulated by a conventional demodulator  73  and inputted to a processor  72 , such as a Texas Instruments TMS320C25 processor. In the reverse direction, the processor  72  outputs data to a conventional HF modulator  75  where it is modulated in the proper HF format and outputed to the T/R unit  22 . The modulator  75  modulates voice data in a thirty-nine tone, parallel tone format whereas other types of data are modulated in a sixteen tone, parallel tone format. A control signal supplied from a Status/Control circuit  82  to the modulator  75  will control the modulator operating mode, i.e. sixteen tone or thirty-nine tone. 
     A Serial In Interface  74  and Serial Out Interface  76  are employed as data interfaces to the wire-line modem  30  of FIG. 6 to allow data transfer between the two modems. The output line  83  will output data to the input line  87  of the wire line modem  30 ; the input line  81  receives data from the output line  85 . The modem  30  of FIG. 6 is advantageously of the same basic configuration as the modem  28 , with the exception of the wire-line modulator  93  and demodulator  91  replacing the respective HF modulator  75  and demodulator  73 . Otherwise, the same components can be utilized for both modems  28  and  30 , including a Dual Access Memory  786 , a Dynamic Memory Access Interface  806 , an Environmental Control Block  886 , a Data Interchange Circuit  866 , a Host Bus Interface  846  and an Optional Analog Interface  906 . 
     The software routines which run on the PC Controller/Processor  40  of FIG.  2  and within the processors  72  of the HF and wire-line modems to provide the signalling and messaging communications as shown in FIG. 4, shall now be described. It is noted at the outset that each of the processors  72  are located on a separate PC ISA card, such as a Chimera card, which is a commercially available card, manufactured by Atlanta Signal Processor Corp. based in Atlanta, Ga. The PC ISA card interfaces to the system via the ISA bus. 
     The PC Controller/Processor  40  of FIG.  2 . loads executable code from its disk into the processors  72  of the HF and wire-line modems, facilitates communications between the processors  72 , and displays status and debug information from the processors  72 . 
     The RWI  20  executable code is invoked from MS-DOS with the command: RWI hfmod.out wlmod.out. The two arguments to the command, hfmod.out and wlmod.out, are the names of files containing the executable code in Common Object File Format which will run in the processors  72  of the HF wire-line modems. 
     When the command is invoked, the PC Controller/Processor  40  of FIG. 2 generates environmental variables, HFMOD and WLMOD, which describe information required so that the PC Controller Processor  40  can communicate with the processors  72 . This information includes the PC base address, Input/Output (I/O) port number, PC interrupt level, etc. 
     When a valid environmental variable is found, the PC Controller/Processor  40  loads the corresponding Common Object File Format file into the corresponding processor  72 . The PC Controller/Processor  40  releases the hold lines and resets the processors  72  all the Common Object File Format file has been loaded, resulting in each processor  72  receiving a clean boot. By utilizing this scheme, the system can grow without adding any new software to the PC Controller/Processor  40 . 
     The PC Controller/Processor  40  enables all interrupts and Direct Memory Access channels specified by the environment variables, HFMOD and WLMOD. The Direct Memory Access channels allow the processors  72  to send status and debug information to the PC Controller/Processor  40 . 
     The PC Controller/Processor  40  will loop reading status and debug information from PC memory buffers where the Direct Memory Access controllers are programmed to write and intelligently display this information. A keyboard can be used to specify which status and debug data to display as well as to terminate the execution. Direct Memory Access communication from the PC Controller/Processor  40  to the processors  72  is possible, but has not been implemented. 
     As the PC Controller/Processor  40  releases the hold and reset lines of the processors  72 , their software begins to run. The following steps are then taken by the software to initialize the system: 
     1. Initialize all internal resources. 
     2. Initialize all hardware resources. 
     3. Initialize all application software data and structures. 
     4. Enable interrupts. 
     Referring to FIG. 7 the operational flow of the software for frame processing that runs on processors  72  will now be explained. 
     In the first step  100 , the system loops until N serial interrupts have occurred. This condition signals the start of a frame processing. 
     In the next step the received data processing block  102  (rcvIpc) is responsible for taking all data that has been received from the PC Controller/Processor  40  since the last time a frame was processed and distributing the data. Received data processing block  102  (rcvIpc) examines the upper eight bits which specify if the data is a message from another processor  72  or some kind of traffic data. If the data is a message, it is placed into a message buffer. If it is data, then the upper 8 bits are again examined in order to determine which state machine&#39;s holding buffer this data should be placed into. 
     During the next step, the message processing block  104  (msgproc) processes all messages that were enqueued by received data processing block  102  (rcvIpc). Each message specifies a particular state machine in the upper eight bits and a unique 1 out of 256 bits in the lower 8 bits. The messages will cause particular event bits to be set in an event bit field that is “visible” only to the message processing block  104  (msgProc) and the state processing block  110  (statProc). Each state machine has its own private event bit field which is used in conjunction with application specific state tables to control the current state of the particular machine. 
     In the next step  106 , the system tests if the RWI  20  is idle or the particular processor  72  is in modem receive mode. When this is true, the next step get A/D data  108  (getanadt) is preformed, wherein all A/D data that has been acquired, since the last time a frame was processed, is removed from the SINT receive buffer and is placed in an application specific input buffer for processing. Otherwise, processing step getanadt  108  is skipped. 
     The central part of processing a frame is the state processing block  110  (statProc), which is responsible for executing up to eight independent threads of processing as though each thread was a state machine. Each thread is executed once per frame. A State Variable Block, associated with each state machine, saves the state machine&#39;s state and determines when to change states. The State Variable Block contains: 
     1. A state table entry, also known as a procedure list entry. This contains the current state of the machine, which is a pointer to a ROM based list of event numbers and an associated function list. 
     2. The current sub state of the machine, which is a pointer to a ROM based list of functions run unconditionally without the association of an event bit. 
     3. The event bit field, which is set by msgProc and cleared by statProc when a set event is found which has been specified in the current state table entry. 
     4. Two frame down counters. The counters are used to generate delays. They decrement once a frame and specific bits within the current machine&#39;s event bit field are set when a non zero timer decrements to zero. 
     When state processing starts, a table of pointers to all active State Variable Blocks will be examined. The sub state pointer which is copied to a global location, accesses a list of routines to be run unconditionally. The global location permits any of the application routines access parameters and/or alter which list of routines will be run. After running the unconditional list, the machine&#39;s state pointer is used to examine a ROM based list of state table entries. Each entry in the table contains an event bit number and a list of routines to run if the specified event bit is set in the event bit field within the State Variable Block. When a set event is located, the associated list of application functions are run. After the last function has been run, control is returned to statproc. A pointer to the list of routines running is globally available in order that any of the application functions can access parameters and/or alter which list of routines will be run. Any of the application routines can call the system level function sndMsg. The sndMsg function will enque a message to be sent to any machine, even to itself. 
     The next step send data processing  112  (sndIpc) takes all messages and/or data, enqued by the application routines run by statproc, and distributes the data utilizing the routing information in the upper eight bits of each data value. The routing information provides an index into a table of drivers. The drivers contain the low level information with respect to how to get data to another machine, inter or intra processor. The system is easily expanded by adding additional processors. If data is enqued to be sent to the PC Controller/Processor  40  the timer interrupt (TINT) is enabled. By writing drivers to access the additional processors, they become part of the heterogenous system. 
     The next step  114 , the system tests if the modem application is in the transmit mode. If it is in the transmit mode the step put D/A data  116  (putanadt) is run. Processor step putanadt  116  takes the D/A data created during the application during the last frame and transfers it to a circular buffer. The circular buffer is read by the serial transmit interrupt handler. While this routine is application specific, the serial transmit interrupt handler and the circular transmit buffer are common to all applications. 
     The system loops back to step  100 . 
     It should be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as defined in the appended claims.