Test Connections

The GT95014 is a breakout adapter board for all Geotest Digital I/O products with single-ended I/O signals. The board provides an interface between the 68-pin SCSI-3 connectors used on the Geotest DIO cards and 100-mil spaced pins – simplifying the connection to a UUT or fixture. In addition, the GT95014 provides the option of terminating all of the I/O signals to a matching the impedance which might be required by the UUT.


The basic connection between the GT95014 and the DIO card is accomplished using a SCSI-3 cable with 68-pin connectors on each end. The cable can be of any practical length as determined by the system’s needs. However, consideration should be given to how signal or system timing might be affected as cable lengths are increased. The best approach would be to use the shortest cable possible.

The GT95014 is connected to the UUT via a 68-pin, 100-mil pin grid. Cables connecting to the 100-mil pins may be individual wires, mass terminated connectors or combinations thereof. Care should be taken to provide adequate ground returns for the signals used, again considering how UUT loading, signal quality or system timing might be affected as cable lengths are increased.

The GT95014 supports two basic termination configurations, unterminated and parallel termination, using SIP resistor packs in the locations UR1, UR2, UR3 and UR4. Refer to Figures 1 and 2.

Parallel termination
Parallel termination places termination resistors in parallel from VCC and GND to the I/O signal using a 10-pin single inline resistor package (SIP). The values of the SIP resistors are selected to match the characteristic impedance of the logic device and the cable, with each SIP providing eight pairs of resistors connected to eight separate signals (see Figure 2, UR1 – UR4). For example, if LVTTL signals are being used to drive a 100-ohm cable, then the SIPs should provide the Thevenin-equivalent of 100-ohm termination resistors. Be careful to orient the SIPs so that pin 1 of the SIP is matched to pin 1 on the GT95014.

GT95014 Single-Ended DIO Adapter
Figure 1 — GT95014 Single-Ended DIO Adapter

The VCC termination voltage is provided via pins 33 and 67 on connector J1, and pins 65 and 67 on connector J2. This allows either the DIO instrument or the UUT to provide the termination voltage to the GT95014. However, caution should be exercised so that termination voltages are not applied to both the J1 and J2 pins simultaneously, especially if the voltages are of different values, as damage may result to the GT95104, the DIO instrument, the UUT or all of the above.

GT95014 Single-Ended DIO Adapter Schematic
Figure 2 — GT95014 Single-Ended DIO Adapter Schematic
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Geotest-Marvin Test Systems, Inc.
1770 Kettering
Irvine, CA 92614

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