Cromemco Inc.

Curtis G Terwilliger

Roger D Melen

Szu C Sun

W James McKeefery

An arbitrary number of member computer stations are inter-connected by a 'party' communication network. Each member station is normally in a high impedance Monitor Mode, for monitoring in-coming messages. The incoming messages latch the monitoring stations into a Receive Only Mode for preventing 'interrupts' of on-going communications. Each station continuously monitors the input impedance from the network, to determine when the network is available for transmission. Member stations may enter the low impedance Transmit Mode only when the network is available, because all of the other stations are in the Monitor Mode. A high Z from the network indicates to each station that the network is available. A low Z indicates that another station is in the transmit Mode, and is in control of the network. The random entry of two (or more) parties at the same time is resolved by a Pre-Transmission Test Cycle. Full transmit status with actual data transmission, cannot be obtained unless the Test code transmission reveals a high network impedance indicating that the network is available. In the 'two-party' start situation, each party senses the other parties low Transmit Mode impedance. Both parties drop back to the Monitor Mode, and a data 'collision' is avoided.

Random entry intercomputer network with collision prevention

A phase detector generates an error signal by comparing an input data rate F:in with an output clock rate F:out. The error signal is amplified and applied to the base of an emitter follower circuit. Current from a discharging capacitor within a clock ramp generator flows through the emitter follower and the emitter impedance. The ramp current is controlled by the error signal. The ramp rate F:out is determined by the peak to peak ramp voltage and by the rate of discharge of the ramp capacitor. Variations in the error signal due to F:in drift, cause a compensating change in F:out for continuously maintaining sync between F:in and F:out within each passband. Multi-passband capability is provided by a multi-level current generator which also supplies current through the emitter impedance. The ramp current is reduced to a lower level by an amount equal to the current from the multi-level current source due to the emitter follower action. The ramp period is prolonged by the lower discharge ramp current placing the operation of the synchronizer in a lower passband. Each current level from the multi-level source establishes a separate passband of operation.

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Burlingame, CA, United States of America

Curtis G Terwilliger