HP 8920A, User Manual

Page: 465 / 658

465
Chapter 16, Radio Interface Screen
Using the Radio Interface (Manual Operation)
Inputting Data
Data can only be input through parallel data bus lines that are configured as
inputs. Data bus lines are configured as inputs by setting the output comparators
to their logic-high states (positive output level). This is best accomplished by
entering a hex value in the
I/O Config
field that will set the corresponding
bit(s) to a 1. For example, if you want to use lines 0-7 for input and lines 8-15 for
output, enter 00FF (which would be 0000000011111111 in binary) in the
I/O
Config
field.
To read data to the Test Set’s internal CPU, select the
Read Data
field on the
RADIO INTERFACE screen. A 16-bit binary value will be displayed under the
Parallel Data In
field near the top of the screen.
Important: When data is read, the
Parallel Data In
field will display the
value for all 16 parallel data bus lines, not just the lines designated as inputs.
See the
I/O Config
field description, on page 462
.
The need for configuring pins as inputs is a result of the “half-duplex” hardware
architecture of the bus. The 16 I/O pins on the parallel data bus are connected to
both the input data comparators and the output data comparators. Because the
output comparators are open-collector, a logic low makes it impossible to input a
logic high, since the output transistor is essentially clamped to ground.
NOTE: A logic “OR” function is implemented in the Radio Interface design between
I/O Config
field and the
Send Data
the. Entering a hex value in the
I/O Config
field writes that value to the output data lines, just as selecting
Send Data
writes the value entered in the
Output Data
field to the output data lines. The effect of this
operation is that it will not be possible to use the
Output Data
field to set a parallel data
bus line low if that line has been designated as an input by the value entered in the
I/O
Config
field.
The fastest rate at which input data can change is about 50 kHz. The practical
limits on data transfer, however, are based on the speed of the IBASIC or external
controller program since all data transfer is asynchronous.