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Table 2-3 NOTES
* The input signal is active low and requires a positive-voltage external circuit, which
must pull the signal line down below 0.4V, but not less than 0.0V, i.e. not negative. The
logic-low current is 0.4mA (max). For the inactive state (logic high), the external circuit
must pull the signal line above +2.5V, but not above +5V. This can be converted to a
isolated active low input by removing jumper JP3 on the I/O PCB.
** Outputs can be active low optically isolated open - collector drivers that pull each
signal line to IGND (isolated common) when asserted. All outputs require a positive
+5V to +24V external source (referenced to IGND) and pull-up resistor to operate as
fully isolated signals. IGND can be isolated from system GND by removing jumper JP2
on the I/O PCB. With jumper JP2 in place optical isolation is defeated allowing the
outputs to be pulled up to the system +5V with external resistors. The outputs can sink up
to 80mA of current. Greater currents can damage the instrument.
*** Pin 34 is connected to the + input of the optical isolator through a 620Ω current
limiting resistor. Pin 35 is connected to the - side of the optical isolator. Recommended
force current through this circuit should be between 1 and 5mA nominal (10mA max).
2.7.2 IEEE-488 Interface
The IEEE-488 interface is available as an option to the 1865. When this option is present
connection is made through a connector (24 pin) on the rear panel. This interface can be
used to connect to a system containing a number of instruments and a controller in which
each meets IEEE Standard 488.2-1987 (Standard Digital Interface for Programmable
Instrumentation) Refer to Table 2.4 below for a full tabulation of connections and Table
2.5 for the command set.
The following functions have been implemented.
explanation of the function subsets, represented by the identifications below.
SH1
Source Handshake
DC1
Device Clear
RL1
Remote Local
E2
Electrical Interface
Page 60 of 87
+
pin
34
620
ohms
pin
35
-
Figure 2-33: Isolated Connection
PP1
Parallel Poll
DT1
Device Trigger
L3
Listener
T5
Talker
start
Refer to the standard for an
AH1
Acceptor Handshake
SR1
Service Request
C0
Controller
Operation
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