Loop back test apparatus and technique

A combination comprising:

• a first section and a second section, each section having an input and an output;
• the first section including circuitry for receiving digital signals at its input and producing analog signals at its output corresponding to the digital signals at its input;
• the second section including circuitry for receiving analog signals at its input and producing digital signals at its output corresponding to the analog signals at its input;
• a lock-out circuit coupled between said first and second sections for disabling the second section while signals are being propagated in the first section and for disabling the first section while signals are being propagated in the second section;
• a selectively enabled delay network;
• and
• means coupling said selectively enabled delay network between the input and the output of the second section for delaying the passage of signals along its transmission path during the time the delay network is enabled.

A method for testing a circuit having first and second sections;

• each section having an input and an output;
• the first section for receiving digital signals at its input and for producing analog signals at its output corresponding to the digital signals at its input;
• the second section for receiving analog signals at its input and for producing digital signals at its output corresponding to the analog signals at its input;
• and further including a lock-out circuit coupled between said first and second sections for disabling the second section while signals are being propagated in the first section and for disabling the first section while signals are being propagated in the second section, comprising the steps of:
• supplying known test signals to the input of the first section;
• connecting a delay network between the output of the first section and the input of the second section for delaying the application of the output signals from the first section to the input of the second section until propagation of the test signals through the first section has been completed;
• and
• comparing the supplied signals with the signals produced at the output of the second section.

In combination with a circuit having first and second sections, each section having an input and an output;

• the first section for receiving digital signals at its input and producing an analog signal at its output corresponding to the digital signals at its input;
• the second section for receiving analog signals at its input and producing digital signals at its output corresponding to the analog signals at its input;
• and further including inhibiting circuitry coupled between said first and second sections for disabling the second section while signals are being propagated in the first section and for disabling the first section while signals are being propagated in the second section, the improvement comprising:
• a delay network;
• means connecting the delay network between the output of the first section and the input of the second section, said delay network having a time delay which is equal to or greater than the time it takes a signal to propagate through the first section.

A combination comprising:

• first and second transmission paths, each transmission path having an input and an output;
• the first transmission path including circuitry for receiving digital signals at its input and producing analog signals at its output corresponding to the digital signals at its input;
• the second transmission path including circuitry for receiving analog signals at its input and producing digital signals at its output corresponding to the analog signals at its input;
• an inhibiting circuit coupled between said first and second transmission paths for disabling the second transmission path when signals are being propagated along the first transmission path and for disabling the first transmission path when signals are being propagated along the second transmission path;
• means for supplying an input test pattern to the input of said first transmission path;
• a delay network;
• and
• means coupling said delay network between the output of the first transmission path and a point along the second transmission path for producing signals at the output of the second transmission path corresponding to said input test pattern.