Systems using line receivers for differential input signals require an adequate self-test capability. Using an LM339 (a quad differential comparator), you can provide this capability by constructing the configuration shown in the Figure 1. Resistors R1 and R5 pull up the input lines, and R2 pulls down the Input Signal Hi line whenever the differential driver and self-test drivers are OFF. In this state, R2 ensures the presence of a differential input voltage, which the receiver sees as a ZERO. The negative-input sides of the two self-test drivers are referenced to a threshold voltage determined by the ratio of R3 and R4; typically, this threshold is 1.5 V.
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| Figure 1. | Selecting the correct device lets you configure a differential line receiver with programmable self-test capability. |
Because the output stage of the LM339 is an open-collector transistor, whenever the comparator’s positive input exceeds its negative input, the comparator’s output turns OFF. Therefore, programming a ONE on the Test Rcvr Hi and Test Rcvr Lo comparator inputs allows the differential driver to control the receiver input lines during normal operation. Programming a ONE on Test Rcvr Hi and a ZERO on Test Rcvr Lo when the differential driver is OFF forces a differential ZERO state at the receiver inputs. The receiver output in this case should be a ZERO if the receiver is operational. Similarly, programming a ZERO on Test Rcvr Hi and a ONE on Test Rcvr Lo results in a ONE at the receiver output, again if the receiver is operational and the differential driver is OFF. These states are summarized in the truth table and can be easily programmed by means of latched bits from a computer or through manual switches.
