System monitor for credit system

Claim 1

A credit checking system comprising: a memory at a central location containing representations of the credit status of account numbers, a plurality of counter units at remote locations for encoding account numbers, means in each counter unit for generating request signals in response to transmit signals originating at said central location, comparison means at said central location for comparing encoded account numbers with the representations of the credit status of the account numbers stored in the said central memory, means responsive to said comparison means for generating valid or invalid response signals, a communication link for transmitting encoded account numbers, and valid or invalid response signals between said counter units and said central location, a system monitor for testing the communication link between counter units and said central location, said system monitor including means for transmitting valid signals to counter units not operatively connected over said communication link to said central location, said communications link including: distributor units, a plurality of counter units being connected to each distributor unit, each distributor unit being connected over a voice grade telepHone line to said central location, and wherein said system monitor includes the following in each distributor unit: means for periodically sending request signals over said telephone line and for receiving transmit signals from said central location, and means for sending valid signals to said counter units when a transmit signal is not received within a predetermined time period after a request signal.