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It is recommended that the interface cable be flat ribbon cable, with a characteristic impedance of
100 ohms.
Interface connector pin assignments are given in Table 3-1.
3.3
INPUT CONTROL LINES
A. DS1-DS4, Select Lines
Functional Description
The select lines (see Figure 3-3) provide a means of selecting and deselecting a drive. These four
lines, DS1 through DS4, select one of the four drives attached to the controller. When the signal logic
level is true (low), the drive electronics are activated, and the drive is conditioned to respond to Step or
Read/Write commands. When the logic level is false (high), the input control lines and output status
lines are disabled.
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Figure 3-3
Select Lines Schematic Diagram
A select line must remain stable in the true (low) state until after a Step or Read/Write command has
been executed.
The drive select address (1-4) is determined by shorting plugs on the circuit board. Select lines 1
through 4 provide a means of daisy chaining a maximum of four drives to a controller. Only one line
can be true (low) at a time. An undefined operation might result if two or more units are assigned the
same address or if two or more select lines are in the true (low) state simultaneously.
Circuit Description
An 150-ohm resistor holds the output of the appropriate Select line high (false) until the line is driven
low (true). Then, the Select signal is gated through U1 and US to derive the signal DRSEL
+.
DRSEL
+
is used to gate all I/O Port, ensuring that only one drive's data is present on the interface at a time.
Writing and stepping are also gated by the signal DRSEL
+.
This signal is inverted by U13, then
buffered by U15 to drive the Activity L. E. D., which is located on the front panel. Option Z is factory
installed. It should be installed for the Activity L. E. D. to operate with Drive Select.
B. MOTOR ON, Drive Motor Enable
3-2
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