Abstract: A modem with improved signal processing and handshaking capabilities as described. Two digital signal processors are used to perform independent, concurrent operations so that a faster execution rate is obtained and more precise calculations are made possible. The modem also uses an improved handshaking technique which allows the modem to maintain compatibility with existing 1200 and 2400 bps modems while allowing for negotiation for 4800 and 9600 bps communications. The modem also incorporates an improved baud clock recovery circuit which dynamically adjusts the actual sampling point in a manner dependent upon the difference between the actual sampling point and the optimal sampling point. This allows the actual sampling point to converge upon the desired sampling point at a high rate while minimizing jitter around the optimal sampling point.

Abstract: A speed correction circuit for use in a modem implementing a data communications protocol having tighter tolerances on bit timing for the transmitted data than for the input data. Selective insertion and deletion of stop bits is used to compensate for overspeed or underspeed conditions between the input and the transmitted data streams. The apparatus loads the incoming data into a FIFO register of predetermined length. Counters in a clock circuit keep track of character timing. Just before transmission of a stop bit, predetermined positions in the FIFO are tested to see if they are occupied in order to detect an overspeed or underspeed condition. If no overspeed or underspeed is detected the top bit is transmitted in its normal fashion. If underspeed is detected, a transmit controller sends a stop bit, and shifting out of the last bit in the FIFO is suppressed for one bit time. This inserts a stop bit in the data stream while allowing additional bits to come into the FIFO.

Abstract: A method and apparatus for allowing ISDN devices to communicate over an analog telephone line is shown. A first interface (12) accepts ISDN formatted data from an ISDN terminal equipment (TE) device (10), removes the D channel data from the ISDN data, and provides the D channel data to a synchronous modem (14). The modem (14) places onto an analog telephone line (15) a modulated carrier corresponding to the D channel data. The modulated carrier is passed through a telephone switch (16)onto another analog telephone line (17). A second synchronous modem (20) accepts the modulated carrier, demodulates the modulated carrier to recover the data, and provides the data to a second interface (22). The second interface (22) accepts the data from the modem (20), inserts the data into the D channel information in ISDN data stream, and provides the ISDN data stream to an ISDN network termination (NT) device or ISDN switch (24).

Abstract: An improved plate for mounting a plurality of receptacles on an electronics device, particularly a data communications peripheral, is disclosed. The plate is designed to be secured to an electronic subassembly which may be dropped into a portion of an enclosure and the plate placed proximate to a large opening in the enclosure. In order to compensate for mechanical tolerances on the fit of the plate within the enclosure, since it is attached to the subassembly, free edges of the plate which may act as an elongated antenna for emitting radio frequency interference are broken up into fingers of predetermined length. This attenuates the radiation from such edge. Additionally, the fingers are bent down to contact an electrically grounded portion of the enclosure to provide further RFI suppression.

Abstract: An improved V.22 bis 2400 bits per second (bps) handshake sequence detector. An incoming phase keyed (PSK) handshake sequence is autocorrelated using a frequency shift keyed (FSK) receiver (101). The autocorrelated signal is then filtered by a low pass filter (106). The autocorrelated, low pass filtered signal is then alternately fed, at a 1200 Hz rate, to two detectors (114,116). Each of the detectors (114,116) looks for one half of the handshake sequence. The output of each detector (114,116) is provided to an OR-gate (122). The 2400 bps handshake sequence is declared to be detected when either one or both of the detectors (114,116) detects its corresponding portion of the sequence.

Abstract: A cable management system for directing cables connected to the back of an equipment enclosure of the type containing a computer peripheral device or other electronics apparatus to enter and exit the equipment enclosure at a selected one of a plurality of predetermined orientations. A bottom cover of the equipment enclosure is provided with a recessed portion including a vertical wall upon which is mounted receptacles for receiving cable terminations. Upper stanchion portions at the rear of the recessed portion together with a hinged plate or with lower stanchion portions contacting the upper stanchion portions from the bottom form rear legs for the equipment enclosure. The rear legs and the vertical wall of the recessed portion confine cable entry and exit positions to particular angular segments corresponding to the sides and rear of the equipment enclosure.

Abstract: There is disclosed an improved adaptor for connecting a first connector to a data communications plug having at least one flange extending radially therefrom, comprising in combination a body, a first connector at the front end of the body, a well formed within the body and opening onto one of the body sides, a second connector within the well, a notched strain relief plate rotatably connected to one of the body sides of the body, and means for securing the strain relief plate in a closed position, which improved adaptor prevents, by the locking action of the strain relief plate and the flange, the withdrawal of the data communications plug when the plug is inserted into the well to a seated position.

Abstract: A method and apparatus for compressing data, particularly useful in a modem. The preferred method is implemented in a microprocessor within a modem, and dynamically adapts to changing data statistics. Parallel encoding and decoding tables are provided at the encoder and the decoder, and are updated for each character processed. Each table has a plurality of digital compression codes associated with characters of an alphabet. In response to an item of data presented for encoding, a compression code which corresponds to the character presented for encoding is selected using the encoding table. The selected compression code is provided as an output. Periodically, the association between the codes and the characters of the alphabet in the table is adjusted as a function of the frequency of occurrence of characters of the alphabet, over a plurality of characters.

Abstract: A separately formed slanted front panel bezel manufactured using injection molding techniques. The interior slanted surface has a plurality of positioning tabs extending perpendicular thereto. An arch shaped screw receptacle extends above the surface, the geometry which allows the mold for same to be withdrawn in a conventional fashion. When the bezel is used in final assembly, a self-tapping screw bites a semi-circular section of the arch shaped protrusion.

Abstract: An improved standoff for use in assembly of an electronics device includes a generally cylindrical body with a bore hole through the longitudinal axis. A plurality of reinforcing fin-shaped members disposed radially about the outer surface provide additional rigidity to the structure as well as providing surfaces at each end of the standoff upon which circuit boards may rest. A plurality of clips may be moved under compression and passed through holes on a circuit board. Once they have passed through, they will elastically return to their normal position thus loosely, but securely, holding the circuit boards together. This allows the standoffs to hold the circuit boards together, without the use of additional nuts and bolts and conventional spacers, and also allows the passing of final assembly screws through the longitudinally bore of the spacer.

Abstract: An improved message buffer for transferring data between two or more digital data devices is disclosed. A dual universal asynchronous receiver-transmitter (13) provides bidirectional data flow to two bidirectional ports (B and C) through two interface circuits (14 and 15). A peripheral interface adapter (16) and a third interface circuit (17) provide for sending data to an output port (A) in a serial or a parallel data format, and provides access to a real time clock (20), an electrically erasable programmable read only memory (41), a bank of switches (36), and a bank of display indicators (37). A read only memory (11) provides operating instructions to a control unit (10). The control unit (10) reads data and handshake signals from the two bidirectional ports (B and C), stores the data in a random access memory (12), reads the data from the memory (12) and sends it to the output port (A).

Abstract: A modem is adapted to accommodate a call waiting feature so that a user can select to continue an ongoing call or accept an incoming call. The modem detects a call waiting signal and generates a call waiting message. Where a call waiting signal is preceded by a loss of carrier, the modem detects the loss of carrier and then detects the presence of energy in a call progress band indicating that a new call is waiting. The modem can be selectably programmed to process the call waiting signal or to provide a call waiting message to a communications software package that can process the call waiting signal. The modem, in conjunction with the communications of software package, responds to the call waiting signal by asking whether the user wants to answer the incoming call or continue with the ongoing communication. The modem generates a programmable default response if the user does not respond within a prescribed time.

Abstract: An improved baud clock recovery, synchronization and data sampling circuit for a modem. A CODEC (41) samples the incoming signal at a rate determined by the sample clock output of a presettable counter (236). The sampled signal is then squared (231) and bandpass filtered (232) to provide a recovered baud clock. A detector (233) signals the positive going zero-crossing points of the recovered baud clock. A lead/lag calculator (234) determines which of the signal samples is nearest the zero-crossing point. The calculator (234) then determines whether this and every subsequent 12th sampling point leads or lags the zero-crossing point by inspecting the sign of the recovered baud clock and adjusts the preset inputs of the counter (236) to cause the sample points to occur at the zero-crossing point.

Abstract: An enclosure for housing electrical equipment and components includes a base and a cover that are releasably connected with internal fasteners. The base has an upturned peripheral portion and the cover has a downturned peripheral portion. The fastener assembly includes a tooth and a detent that are located internally on the peripheral portions for sliding engagement with one another to releasably connect the base and cover.

Abstract: A modem and an improved phase-locked loop used in the modem. A presettable counter (251) generates the desired signal (TXCLKOUT). The reference input clock signal (TXCLKIN) and the generated clock signal (TXCLKOUT) are compared by an exclusive-OR gate (254). The output of the gate (254) is sampled by two flip-flops (256,266) before and after a rising edge of the TXCLKOUT signal. If the TXCLKIN and TXCLKOUT signals differ by more than a desired phase window a logic circuit (264) adjusts the preset inputs of the counter (251) so that the TXCLKIN and TXCLKOUT signals are in phase within the desired phase window.

Abstract: A low profile modular telephone receptacle (15) is shown. A unitary housing (16) having a pair of transverse support members (35, 36) is provided. A top plate (17) having the necessary plurality of conductors (20) embedded therein is also provided. During assembly, the ends (22, 25) of the conductor are bent to predetermined angles (62, 65) and the top plate is joined to the bottom plate by ultrasonic welding. The resultant receptacle is one in which a minimum height dimension (h) above the necessary height dimension of the opening (18) to accept a standard modular plug is provided. In the preferred method of construction, the top plates (17) are injection molded onto continuous strands of filaments to form the conductors (20). A plurality of slots (55, 56) are provided at one end of the top plate to guide the conductors corresponding openings (46) in the bottom plate when the ends of the conductors forming a terminal set for connecting the receptacle to a utilization circuit are fabricated.

Abstract: The preferred embodiment (11) of the modem comprises a control unit (13), a memory (27), a switch (14), and a modem engine (17). The modem engine (17) establishes communications with a modem (30) using conventional handshake methods. The control unit (13) then initiates a special handshake sequence composed of nonprintable, opposing characters to the modem (30) via the switch (14) and the modem engine (17). If the modem (30) completes the special handshake sequence then the control unit (13) and the modem (30) exchange the desired information. If the modem (30) does not complete the special handshake sequence then the preferred embodiment (11) functions as a conventional modem. The use of nonprinting, opposing characters for the special handshake sequence prevents the special handshake sequence from adversely affecting the external devices (10) (34).

Abstract: A fast phase reference recovery apparatus for use in a data communication system employing a signal constellation which is variant for certain rotations which will produce local minima in the phase error signal output of a phase lock loop. The incoming signal is rotated by one or more phase shifting networks, the outputs of which are provided to constellation decision circuits for the constellation in use. One of the constellation decision circuits will be locked on to the correct local minimum and thus on to the proper phase constellation, and the others will be locked onto incorrect rotations of the constellation. The magnitudes of the errors produced by the constellation decision circuits are compared and selection of the data signals from the proper circuit is made based on the relative magnitudes of these error signals. In preferred forms, the comparison of the magnitudes of the error signals is averaged over a predetermined number of baud times to improve reliability.