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HP-IB INTERFACE/26067B
I/O
2-6.
CABLE
LOADING
DRIVER
When calculating
loading
for a system, the
number
of loads (load units) in the system
controller
plus, all other bus devices must be
added together to get the total number of I/O
driver loads. The cable length is the total of all
lengths (number of metres) of cable.
NOTE
When
adding
the
number
of
metres
of cable
in
a system,
remember to include the internal
cable in the bus devices if there is
any.
The maximum number of loads is 15 for any bus,
thus the maximum length of cable is 15 metres.
Bus devices are designed with a certain number
of loads built into the circuit which are not
removable. This type of load is referred to as an
"internal"
load
where
as
the
installable
(removable) resistor pack loads are referred to as
"installable" loads (or load packs).
The HP-IB
Interface I/O driver loading can be changed by
inserting
installable
load packs (that
contain
either 1 load, 2 loads, or 4 loads) into the I/O
driver load sockets (XR96, XRI06, and XR116)
on the interface PCA (see figure 2-2). One, two,
or three load packs (that contain
1
load, 2 loads,
and
4 loads), which
are
provided
with
the
interface
PCA, may be installed into the I/O
driver circuitry load sockets. The total loading is
the sum of the installed loads plus one (there is
one internal load designed into the circuit which
must also be added). This provides a variable
WARNING
Any operation which requires the
removal
of
the
printer's
protective
covers
must
be
performed
by
a
qualified
Hewlett - Packard
service
representative.
Unqualified
personel inside the printer
may
contact
voltage
points
causing
serious personal injury or death.
An
HP-IB
cable
connected
to
the
HP-IB
interface presents a load to the HP-IB interface
driver
circuits.
Different
bus cable lengths
present different
loads to the bus devices.
To
eliminate possible data rate reduction or data loss
which might occur as a result of mismatched
loading between the HP-IB devices and the cable
loading, the HP-IB Interface
(5062-0477)
I/O
driver is designed with variable loading.
This
variable
loading enables compensation
of the
HP-IB interface
for changes in cable loading
(length).
The interface loading is designed such that one
load unit (1 load) compensates for one metre of
cable. For a good loading match the HP-IB bus
requires that
the number
of metres of cable
should be less than or equal to the total number
of system device loads.
The equation
below
describes these loading requirements.
TOTAL CABLE LENGTH
(in metres)
~
#
of System
(GIC) Loads
+
#
of Printer
Loads
+
#
of Loads Of All
Other Bus Devices
2-4
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