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This section describes the tools and programmed diagnostic
aids used to maintain 3277 Display Stations. A compre-
hensive description of the logic probe is included because
proper use of this tool is essential to successful display
station maintenance.
2.1 MAINTENANCE TOO
LS
2.1.1 Customer Engineer Tool Kit
The Customer Engineer tool kit contains all basic tools
necessary
to maintain IBM 3277 Display Stations. The
standard IBM volt-ohmmeter (VOM) is adequate for all
voltage measurements. The meter's input impedance of
20,000 ohms per volt causes an erroneous reading when
checking the 400V dc power supply. This effect is noted
when a check of that power supply is called out. When
using the VOM, all dc voltage measurements should be
referenced to dc return rather than to frame ground. DC
return and frame ground are at different levels when the
display station is not connected to a control unit.
2.1.2
Oscilloscope
I n some cases of trouble analysis, it may be necessary to use
an oscilloscope. The Tektronix
*
model 453 oscilloscope, or
equivalent, should
be
used when an oscilloscope is required.
However, the
logic probe
is recommended for use,
whenever possible, rather than an oscilloscope.
2.1.3
Logic Probes
Either of two styles of logic probes can be used to probe
signal levels while using the Troubleshooting Diagrams or
FE ALDs. The older-style probe (PN453652) is described
completely in the following paragraphs and shown in
Diagram 2·1 A. The newer General Logic Probe (G LP) is
shown in Diagram 2-18. An available GLP Kit (PN 453212)
includes the G LP, standard accessories, and the General
Logic Probe Manual, SY27-0113. The manual describes the
features of the GLP, the functions and limitations of each
feature, and a checkout procedure. (That information is not
duplicated here, so the user is referred to SY27-0113.)
A probe should be obtained from Mechanicsburg by all
Customer Engineers who service 3270 units. Normal tool-
ordering procedures should be used to obtain the probe.
*Trademark of Tektronix, Inc.
Section 2. Tools and Diagnostic Programs
2. 1.3. 1
Description
The logic probe (Diagram 2-1 A) is a self-contained device
consisting of an anodized aluminum tube with a red and
green indicator incorporated near the probe end. A cable
containing two wires and SL T-type push-on terminals leads
from the sealed top end of the probe. A plastic head with a
threaded stud is at the probe end of the device. Red and
green indicators tell the state of the net being probed. The
three signal states that can be distinguished are: (1) a solid
plus level, (2) a solid minus level, and (3) a pulse or the
presence of pulses. The two wires with the SL T-type
push-on connectors are attached to pins on the back of the
logic board being probed. They carry the operating voltage
(+5V dc and ground) for the probe. An oscilloscope probe
tip screws on the threaded stud at the tip of the probe. This
carries the input signals to the indicators.
2.1.3.2 Probe Usage
To use the logic probe, a probe tip must be attached to the
threaded
stud.
An
SL T probe tip
(PN 453163, or
equivalent) should be used. The two wires at the top of the
probe are attached to the back of the logic board being
probed. The leads are clearly labeled: GND and +6V.
Connect the GND lead to any D08 pin, and connect the
+6V lead to any D03 pin (+5V dc). The leads are connected
to the board by pushing the connector at the end of each
lead onto the designated pin. When the last wire is
connected, the red indicator should light.
Connect the oscilloscope probe tip to the pin designated
in the Troubleshooting Diagrams or as determined in logic.
The following conditions can be observed by the logic
probe:
1. Red indication
a. Probe not attached to a pin (floating condition), or
b. Plus signal on the net being probed.
2. Green indication - Ground (negative) signal on the net
being probed.
3. Red and green indication - A pulsing signal (alternately
plus and ground) is present on the net being probed.
This condition can appear either as the red and the green
indicator pulsing on and off, or as both indicators
seemingly on at the same time (depending on the
frequency of the pulsing signal). Single pulses can also be
seen.
2.1.3.3 Probe Checkout
Note: This checkout procedure applies to the Older-style
logic probe (PN 453652). G LP checkout is described in
General Logic Probe Manual, SY27-0113, which accom-
panies the GLP.
Tools and Diagnostic Programs
2-1
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