ABSTRACT
The multifilar shielded lines electrical linking the various computer equipments as the lines between the computer and the disk controler may produce some failures in the transfer of the data traveled in the lines. The main faults observed in situ are of two types. The first one is an electrical failure between the data wire and the connector contacts. The second one concerns the electrical discontinuity of the various shielding components protecting the line and each connector against the electromagnetic interference. The first fault can be easily detected by a simple test which consists in verifying the dc. resistance of the lines located inside the shield. If the contact is good the dc. resistance is nearly equal to zero ; if the contact is bad this resistance is infinite. The second fault is rather the consequence of a random process which can occur when the lines are disturbed by an electromagnetic interference as the electrostatic discharge phenomena. When the discharges are flowed on the various shields protecting the lines and connectors a crosstalk voltage of very small amplitude appears at the ends of the cable. This voltage disturbs the data on the inner wires. If the electrical continuity of the shields is good the crosstalk voltage has no effect on the data. If the electrical continuity of the shield is no more ensured for various reasons the crosstalk voltage can take an amplitude enough to produce errors on the data transmission. The failure of the shield electrical continuity can be due for example to an interruption of the electrical contact between the shield of the lines and the metallic shell protecting the connectors. The failure can also increase the contact resistance between the shield and the shell. At the end of the manufacturing process of the multifilair shielded lines it is therefore necessary to verify this electrical continuity to insure the reliability of the cable against the damage caused by the electromagnetic interferences. We propose for this a test method consisting in injecting in the shield and shell assemblies a transient current simulating for example the effect of the electrostatic discharge. The principle of the test consists in analyzing the amplitude and the shape of the crosstalk voltage received at the ends of the lines and to make a comparison with a reference voltage obtained on a reference shielded line. If the peak voltage given by the test has a magnitude much higher than the amplitude 229of the reference signal we can decuce that the failure of the shield electrical continuity is due to an interruption of the electrical contact. If the shape of the voltage obtained is the same as the shape of the reference line we can deduce that the shield discontinuity is due to the increase of the contact resistance between the shield and the shell. This comparison can be used to reject the product or to give an information on the reliability of the cable. This test method is therefore very more accurate that a simple test of the electrical continuity as this is mentioned above. The test is controlled gy a micro computer and it can be made by a non specialist operator, furthermore some parameters can be stored during the test. These informations can be used to make a statistical study of the faults and also to improve the performances of a new product. We propose in this paper to give some informations on the economic impact of the failures produced in the shielded lines of a computer. In a second step a description of the benchtest developped by BULL SA and LILLE University is proposed.
