Engineering Dictionary of Electronic Terms
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

SCSI Interface.

This is a basic description of a Single Ended SCSI interface [SE Interface]. The single-ended style interface was one of the first SCSI interfaces, as in SCSI-I. The SCSI-I interface has been obsolete for a number of years now, first replaced by many follow on SCSI Interfaces, then rendered completely obsolete by the introduction of Serial SCSI.

Interface Description; The schematic shows one interface line of an 8-bit data bus. The driver is a 7438, 2-Input NAND gate with Open-Collector Outputs [see below]. While the receiver is a 74LS14 Hex Inverter with Hysteresis. The bus is terminated at both ends of the line using a Thevenin Termination scheme of 220 and 330 ohm resistors [typical].

SCSI Single Ended Interface Schematic
SCSI Interface Schematic

Because the 7438 is an Open Collector NAND gate the output is pulled high by the 220 ohm termination resistors. The output of the 7438 gate is either open [pulled high] or low. A logic high is set by the termination resistors. The termination value turns out to be 1 / [1/330] + [1/220], or 132 ohms. The Thevenin voltage is 5 * [330 / [220 + 330]], 3 volts.

The line between the driver and receiver represents a cable between two devices. SCSI is an interface between a computer [driver] and a hard drive [receiver] for example. A 50-pin SCSI A Cable was used with SCSI I to interface between the two components. The nominal cable impedance is between 90 ohms and 140 ohms, matching the termination value of 132 ohms.

Because the circuit used first generation TTL devices this particular interface only operated up to 5MHz with a limited cable length of 6 meters.

The 7414 is a Hex Schmitt-Trigger Inverter. The Schmitt-Trigger function provides increased noise immunity by imparting Hysteresis into the switching or trigger point of the input. Hysteresis operates by having two different trip points or threshold levels, one level for a high [Vt+] and a different level for a low level [Vt-]. When the high set-point is reached the output will switch to a high output, but will not switch low if the input drops below the set-point. The input needs to be lowered to a different set-point before the output will switch to a low level. Having two different threshold levels eliminate false switching due to noise on the line.

Resistor Selection; The resistor terminations could be individual resistors or a resistor package containing all eight resistor pairs;
SIP Resistor Network Terminations
DIP Resistor Network Terminations

Interface IC Selection; A few 7438 ICs are still in production:
Quadruple 2-Input Positive-NAND Buffers with Open-Collector Output
7438; TTL Family [List of different types of IC Logic]
74LS38; LS TTL Family [Low-Power Schottky]
74ALS38; ALS TTL Family [Advanced Low-Power Schottky]
74F38; F Family [Fast]

74HC14; High Speed CMOS Family

Only the commercial [74xx] series is listed [IC Temp Ranges].
7438; Commercial Temperature Range.
5438; Military Temperature Range.

ICs listed without regard to Package Types

PC motherboard

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