Radio Shack TRS-80 Service Manual page 112

PART 4
MAINTENANCE
PHYSICAL DESCRIPTION OF THE PC BOARDS
The logic PC board is approximately 6 inches (152 mm)
long by 5.5 inches (139 mm) wide. Figure 10 illustrates the
placement of test points and connectors.
CIRCUIT BOARD TEST POINTS
The following test point description assumes that the Logic
and Servo P.C. boards are installed in the Drive and that the
Drive is in an operational mode with a diskette installed:
Logic Ground (TP6)
Digital Logic ground is referenced at TP6.
Diffeftntiated Rtad Signal (TP3, TP4)
These test points are provided to observe the differ·
ential output of the second stage amplifier and differ·
entiated read signal.
Read Data Singhl Shot (TP5)
The output of the single shot used in the read section is
nominally 1.0 /.lsec for each flux transition dt!tected.
Index Pulse (TP7)
With a standard soft seclort!d diskt!t1e installed, the
signal is a high going pulse, nominallv 4.0 msec in dura-
ation every 200 msec.
Amplifitd Read Signal (TP1, TP2)
These test points are provided to observe the differ·
ential output of the first stage of read signal amplifi·
cation.
Motor On (TP13)
This signal is low true for the "motor on" condition.
Track
ge
(TP8)
This signal is low true when the carriage is positioned
al track (;10 and the step motor phase is correct.
Analog Ground (TP10)
The analog ground reference point is provided for
measuring read/write waveforms.
Step Pulse (TP12)
When stepping in or out, the signal is a high going pulse
for t!ach step of the carriage.
Write Protect Switch (Tpg)
When a write protl:lcted diskette is installed in the Drive,
the signal is high.
OPTION SELECT
Input line Terminations
The Disk Drive has been provided with the capability of
lerminating the input lines listed below:
*
MotorOn
*
Dieection Select
'II:
Step
'It
Write Data
'II:
Write Gate
These lines arc terminllted through a 150 ohm resistor pack
instlliled in II DIP socket locllted llt IC location 2F.
In a multiple drive system (Catalog Numbers 26-1163/1164/
1161) only the last drive on the interface is to be termi-
nated. All other drives on the interface must have the resis-
tor pack removed. Catalog Number 26-1164 is shipped
with the termination resistor installed. Catalog Numbers
26·1161/1162/1163 are shipped without a resistor pack.
105