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Tester for an RS-422 Interface Cable

IP.com Disclosure Number: IPCOM000039982D
Original Publication Date: 1987-Sep-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 78K

Publishing Venue

IBM

Related People

Frey, DE: AUTHOR [+2]

Abstract

This tester verifies proper wiring of a four-conductor cable generally, and an RS-422 interface cable, in particular. If the cable test fails, the tester indicates the probable cause of failure. This test apparatus and procedure allows the user to verify the integrity of the interface cable by attaching the tester and running diagnostic software. A schematic of the tester circuit is shown in Fig. 1. The concept by which the tester operates is that the receive lines are connected to the send lines, and a computer attempts to read back the same data that it writes. The tester circuit allows the data to flow in only one direction so that the interchange of send and receive lines may be detected.

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Tester for an RS-422 Interface Cable

This tester verifies proper wiring of a four-conductor cable generally, and an RS-422 interface cable, in particular. If the cable test fails, the tester indicates the probable cause of failure. This test apparatus and procedure allows the user to verify the integrity of the interface cable by attaching the tester and running diagnostic software. A schematic of the tester circuit is shown in Fig. 1. The concept by which the tester operates is that the receive lines are connected to the send lines, and a computer attempts to read back the same data that it writes. The tester circuit allows the data to flow in only one direction so that the interchange of send and receive lines may be detected. The tester provides an indication of polarity so that the interchange of + and - lines is detected as are open lines which might not always cause immediately observable functional problems.

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Assuming that the +Send line is high and the -Send line is low, the emitter-base junction of transistor Qa is forward biased, and a high level is applied to +Receive through Qa and resistor R1a. At the same time, -Receive is held low through diode CR3a and resistor R1b. Thus, the logic levels present on the Send lines are applied directly to the Receive lines. If the Receive lines are unbroken, base current flows in Qa, which will turn Qa on, and apply voltage to resistor R2 and the light- emitting diode (LED) identified as CR1a. Current through this LED flows through diode CR2a back to -Send. When light is observed from CR1a, it indicates that both Send lines are intact, at least the +Receive line is intact (there could be a ground which is common to the two systems which would compensate for a break in the -Receive line), the polarity of the applied signal is +Send high (at the tester), and the Send and Receive lines have not been interchanged between the computer and tester. (This is known because the power to light the LEDs is drawn from the Send lines; the Receive lines cannot provide this power.) LED CR1b, diode CR2b, diode CR3b, and transistor Qb are back- biased under these conditions, and do not take part in the test. The tester circuit is symmetrical; therefore, the operation is identical when the -Send line is high with respect to the +Send line. In this case, CR1b will light to indicate a successful test. The -Receive line must be unbroken to pass this test. Capacitor C functions under conditions of rapidly changing data to prevent the LEDs from lighting by currents flowing through the junction capacitances of various semiconductors, even when they are supposed to be off. This capacitor absorbs these currents, and ensures that the LEDs will light only for a successful test. Diagnostic software is used in conjunction with this tester for the purpose...