Electronic Design

Create a small, low-current SCSI terminator

Many computers use the SCSI connection between the computer and its peripherals. Proper operation requires that the 18 signal lines (usually external) be properly “terminated.” Figure 1a represents the computer signal source. The termination must be able to source 10 milliamps on each of the 18 lines, through approximately 120 Ω (Fig. 1b). The termination also must be able to sink current, depending on how much the source voltage exceeds the termination voltage.

A passive terminator that requires 36 resistors is usually provided (Fig. 2a). This type of terminator draws about 140 milliamps residual from the “termpwr” bus. There are custom ICs that can provide active termination, but at a higher cost and limited availability for the individual builder.

The design shown uses an inexpensive low-voltage audio IC with internal biasing (Fig. 2b). It requires only 18 resistors and draws less than 10 milliamps residual from the “termpwr” bus. Crosstalk between data lines is about 1%, because the output impedance of the LM4861 measures about 1 Ω, with the loads split between pins 8 and 5 outputs. Tantalum capacitors (C1 and C2) are used to maintain this low crosstalk at higher frequencies. Capacitors C3 and C4 are required for stability. Diode D1 serves to center the quiescent output at 2.5 V. Because only one resistor is used for each line, less than 0.3-in.2 board space is occupied, and the circuitry may be assembled internally to a normal 50-pin SCSI connector.

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