Abstract: An autocorrelator for FSK signals. An FSK signal, incoming on a telephone line is filtered by a bandpass filter (13) and sampled by an A/D converter (14). A sample clock (15) provides a fixed sampling frequency, FS. A first interpolating filter (20B) provides selectable delays which are non-integer multiples of 1/FS, and an integer delay (25) provides selectable delays which are integer multiples of 1/FS. The delayed signals are multiplied by a multiplier (26) and then filtered by a low pass filter (27) to provide a filtered autocorrelated signal. A second interpolating filter (20A) provides for reduced jitter in the baud timing by increasing the number of samples provided to the decision logic (23). The decision logic (23) provides the decoded data output.

Abstract: An improved circuit for providing automatic gain control (AGC) for incoming phase shift keyed (PSK) and quadrature amplitude modulated (QAM) signals. An absolute value circuit (193) and a comparator (195) provide a first error signal (197). An integrater (200,202) smoothes the first error signal (197) to provide a second error signal (201) to a variable-threshold threshold detector (204). The threshold (209) is initially set at a low value to allow the AGC circuit to quickly respond. A larger value is then used to reduce susceptibility to noise and provide for proper QAM operation. An error circuit (208,211,213) provides a non-linear response so that the gain variations will be small when the input signal (190) is large. This non-linear response further reduces the effects of noise on the AGC circuit.

Abstract: An apparatus and method is provided for detecting and amplifying the level of a received signal in the analog front end of a modem so as to maintain the level of the signal below a predetermined signal clipping level. An incoming signal is amplified by a predetermined amount of gain to provide a received signal. The level of the received signal is compared to a first reference level and a second reference level by a signal level detector. The comparative information regarding the level of the received signal is provided to a signal adjustment controller which provides control signals to the amplifier to maintain the received signal level between the first and second reference levels.

Abstract: A data communications receiver for use in a modem. A fixed sample clock and a dominated tap tracking algorithm lock the local baud timing in the receiver to the baud timing in a remote transmitter. An interpolating filter provides a plurality of discrete delays. A filter control circuit inspects the tap coefficients of an adaptive equalizer to determine the location of and any movement of the dominant tap. The filter control circuit selects the rate of cycling through the discrete delays to compensate for any frequency difference between the local baud timing and the remote baud timing, and to prevent movement of the dominant tap. A baud detector circuit monitors the sample clock and the operating state of the filter control circuit to identify the end of a baud and detects and corrects for any frequency difference between the remote baud timing and the local baud timing by providing one additional sample or one less sample to the adaptive equalizer.

Abstract: A method for communicating with a plurality of hosts connected to an X.25 network and for selectably using a character as a control character or a data character. Each character received from the X.25 network is inspected to determine if it is one of a set of reserved characters. If so, the character is encoded before being sent to its final destination. Data intended for the X.25 network is inspected for the presence of a predetermined control character. If the control character is present, the next character is inspected to determine whether it is a command character or an encoded data character. If this next character is an encoded data character, it is decoded and provided to the X.25 network. If this next character is a command character, then the command is executed. The method allows the use of any character as either a command character or data character. Data transfer between the DTE and DCE is assigned a PAD and a virtual channel number which corresponds to a particular host on the X.25 network.

Abstract: An improved memory control and refresh apparatus as shown. For memory read operations, the read only memory and the random access memory are controlled by the NCSROM conductor (41) and the NCSRAM conductor (40), respectively, so that the read only memory and the random access memory do not compete for control of the data bus (20). However, the NCSRAM conductor (40) is not used to control the random access memory during write operations. Therefore, when transferring blocks of data from the read only memory (60) to the random access memory (61,62) the NCSROM conductor (41) is active during both reading of data from the read only memory (60) and writing of the data to the random access memory (61).

Abstract: A phase locked loop circuit which eliminates the phase difference between an incoming reference signal and a sampling signal by sampling the incoming reference signal to produce a sampled signal. The sign of the sampled signal at two sample points is compared to determine in which quadrant a predetermined one of these sample points is located. The phase adjustment to the sampling signal is dependent upon the quadrant in which this sample point is located and the magnitude of this sample point. A large phase difference produces a large phase adjustment so that this sample point is quickly locked onto the zero-crossing points of the incoming reference signal. A small phase difference produces a small phase adjustment and prevents jitter. The lock onto the zero-crossing point of the incoming reference signal minimizes the data error rate of the modem.

Abstract: An improved analog front end circuit for a high performance modem comprising an oversampling sigma delta modulator analog-to-digital converter which employs a novel four phase clocked MOSFET switched capacitor integrator. The integrator is switched in a manner as to eliminate signal dependent charges in the MOSFET switches. The sigma delta modulator shifts quantization noise of the analog-to-digital conversion process out of baseband of the analog signal. A novel integrated decimating FIR low pass filter filters the quantization noise from the digital output signals, and reduces the number of digital signals to obtain a sufficient number of signal samples in order to provide operation at high speeds, for example 9600 bps. The improved four phase switched capacitor integrator is also suitable for use in sigma delta modulator circuits, analog-to-digital converter circuits, integrating cirucits, and the like.

Abstract: A method for feature negotiations between two synchronous modems. To engage in negotiations a modem must be configured to support synchronous operation, a data transfer rate of at least 4800 bits per second, and at least one non-default feature. After the orginating modem and the answering modem have completed standard handshaking sequences the originating modem initiates the negotiations by sending a list of supported features. The answering modem responds by confirming that all the features are supported, by returning a subset of the features list, or by sending a different features list. If the answering modem did not confirm the list then the originating modem responds to the subset or to the different list by confirming the subset or different list, by returning a subset of the different list, or by sending another different list.

Abstract: The present invention provides an interconnecting bracket for releasably connecting two circuit boards in an end-abutting relationship. The bracket is fastened to one circuit board in a relatively permanent manner, and is fastened to the second circuit board by the combination of a barbed locking finger which downwardly engages a hole defined by the second circuit board, a noduled foot which upwardly bears upon the lower primary planar surface of the second circuit board, and a positioning flange which nestles within an open-ended slot defined by the second circuit board. The resulting configuration allows a typical computer user to releasably fasten the two boards without tools and special manual skills.

Abstract: A multi-port message buffer which is responsive to an escape sequence. A control unit controls whether the message buffer is in a transparent mode or a command mode with respect to a particular port. The control unit monitors a data port for the escape sequence and, if the escape sequence is detected, the control unit causes the message buffer to enter into the command mode with respect to that port. The escape sequence is a break sequence of a predetermined period, followed within a predetermined time by a predetermined character at a predetermined data transmission speed. The escape sequence can therefore be detected even if the currently programmed data transmission speed for that particular port is unknown.

Abstract: An improved memory control and refresh apparatus as shown. For memory read operations, the read only memory and the random access memory are controlled by a signal on conductor (41) and a signal on conductor (40), respectively, so that the read only memory and the random access memory do not compete for control of the data bus (20). However, the signal on conductor (40) is not used to control the random access memory during write operations. Therefore, when transferring blocks of data from the read only memory (60) to the random access memory (61,62) the signal on conductor (41) is active during both reading of data from the read only memory (60) and writing of the data to the random access memory (61).

Abstract: A gas discharge device is provided for protecting a telephone line from certain voltage surges. The gas discharge device provides low voltage surge protection from tip to ring but still retains higher voltage surge protection from tip or ring to ground. The device is a three terminal device constructed with a rod extending from the tip or ring toward the other tip or ring to control the electrical distance between the tip or ring. The ground element may be positioned at the other end of the device or it may be positioned intermediate the tip and ring so that the rod extends through an opening in the ground conductor. In either position, the distance between tip or ring and ground is greater than the distance between tip and ring, and thus the breakdown voltage is higher.

Abstract: An improved circuit for generating phase and amplitude modulated signals in a modem. A first circuit (161) generates a first signal (163) using a 1200 Hz carrier (160) and an input data stream (162). This signal (163) is generated at the rate of 7200 samples per second. The first signal (163) is then sampled by a sampler (164) at a 3600 Hz rate (165). The sampled signal (166) contains both a 1200 Hz signal and a 2400 Hz signal. A bandpass filter (43) selects a 1200 Hz or a 2400 Hz center frequency. The resulting output (44) is a selectable 1200 Hz or 2400 Hz signal which is generated using only the data sampling points necessary to generate the first, 1200 Hz signal (163).

Abstract: A feature negotiation protocol for synchronous operation and a dynamically adjustable retraining sequence. An originating modem, after completing conventional or standard handshaking sequences, sends a list of desired features to an answering modem. The answering modem either concurs with the desired features, if they are all supported, or sends, back to the originating modem, a list of those features which are supported by the answering modem. After the end of a negotiation time-out period, both modems begin operation using the features which are common to both modems or by using default (standard) features for operation. If the adjustable duration retraining sequence is selected then each modem will measure the elapsed time since the end of its last transmission and, at the beginning of its next transmission, send a retraining sequence which is dependent upon this elapsed time.