1 bit Reg
Second
Byte
Output
Register
First
Byte
Output
Register
First
Byte
Holding
Register
Internal
Data Bus
EOI
oEND
DOUT8
thru
DOUT15
DOUT0
thru
DOUT7
First Byte
Clock
Second Byte
Clock
Simplified Output Port Block Diagram
3.1.1 Output Port Data Lines
The first and second byte output port drivers are externally controlled. The Output
Enable control line (/OE) must be forced low to allow the digital output data to be
presented on the output connector. If /OE is disconnected or forced high, the output
drivers are disabed and the output connector is not driven. This allows the input and
output ports on the Digital488HS/32 to be connected to form a bidirectional bus. The
oEND output, however, is always enabled. /OE does not affect the oEND control
line.
The Output Enable control line (/OE) places the output port buffers, both first and
second byte, into tri-state operation if left floating or forced active high. To enable the
output port, this line must be driven active low. The state of the /OE line does not
effect the oEND control line. This allows the input and output ports to be used as a
bi-directional interface, connecting the input and output port data lines together.
The IEEE 488 (EOI) End or Identify bus control line is often used to designate the
last byte of a multiple byte transfer. The Digital488HS/32 latches the EOI signal into
the ninth bit (oEND) of the second byte output port. This oEND signal, derived
directly from the EOI line, can be used to signal the end of the transfer to the external
device.
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