Digital Equipment PDP-8 User manual

3:32;?flzz‘szzafisgazm"
mnannan
PDP-8 .
Maintenance
Manual
TCO1
Dectape
Control Unit

DEC-O8-I3AB-D
PDP-8
TCO1
*
DECTAPE CONTROL UNIT
DIGITAL
EQUIPMENT
CORPORATION
0
MAYNARD,
MASSACHUSETTS

1st Edition
September
1967
2nd
Printing
April
1968
3rd
Printing
April
1969
4th
Printing
July
1969
5th
Printing
November
1969
6th
Printing
February
1971
7th
Printing
December
1972
Copyright
©
1967,
1968, 1969, 1971,
1972
by
Digital
Equipment
Corporation
The
Following
are
trademarks of
Digital
Equipment
Corporation,
Maynard,
Massachusetts:
DEC
FLIP
CHIP
DIGITAL
PDP
FOCAL
COMPUTER
LAB

NNNMNNNNNNM'MMNNNMMNNNNNN
ODOJCON
——l—d
U1
00\|O~in:P-wr\>
01-th
000
_.d
CONTENTS
CHAPTER
I
INTRODUCTION
General
Description
Referenced
Documents
Physical
Description
Equipment
Characteristics
Electrical
Requirements
CHAPTER 2
OPERATION
AN D
PROGRAMMING
DECtape
Format
Mark—Track
Format
DECtape
Instructions
Status
Register
A
Functions
Status
Register
B
Functions
Control
Modes and Functions
Control Functions
Move
Search
Read
Data
Read
All
Write Data
Write
All
Write
Timing
and
Mark
Tracks
Enable
to the
Interrupt
Programmed
Operation
Simplified
Search
Procedure
Bootstrap
Loading
Upper
Bound
Protection for
Data Transfer
Write/Read
in
Opposite
Direction
Turn
Around
Specifications
Available Software
Subroutines
Library
Calling
System
00.
Ill

k)
wwmwwwwwwwmwwwmwwwwwwwwwwwwww
MNNNNNNNNNNNNNNNNNNNNNNNNN
‘OGDVOU‘IAOJOJN
.d
#(JJN—d
CON-d
.15.5
.l5.6
CONTENTS
(continued)
Performatting
Tape
Programs
Maintenance
Programs
Symbols
and
Abbreviations
CHAPTER 3
PRINCIPLES
OF
OPERATION
Functional
Description
Information Flow
Command
Flow
Registers
Detailed
Logic
Operations
Basic
Read/Write
Logic
Read and Write
Amplifiers
Device
Selector
Logic
STATUS
A
Decoding
Status
Register
B
and
Skip
Instructions
UnitSelectLogic
Motion
Control
Function
Selection
Interrupt
Enable
New
Unit/Motion
Select
Timing
Pulse
Generation
Counter
Register (C)
Window
Register
(W)
Counter
Synch
Level
(C-SYNC)
Start
Block Marks
(MK
BLK
MK)
Data
AAarks
State
Register
Memory
Field
Register
(MF)
Function
CDperations
Aone
Tape
Search
Read Data
Read
All
Function
VVHte
Data
FuncHon
Write All
Function
2-18
2-18
2-19
3-i
3—1
3-l
3-4
3-10
3—12
3-12
3-l2
3—13
3—14
3—15
3-15
3—15
3-15
3-l6
3-i7
3-17
3—23
3—23
3-24
3-25
3-27
3-27
3-27
3-28
3—28
3-29
3—30
3—31

#h-b-h-b-
wwwwwwwwwwwwwwwwwwww NNNNNNNNNNNNNNNNNNMN
01010101 030019
3.3313
.15.7
.16
.16.
.17
.17.
.18
.19
.20
.21
.22
.23
.24
.25
.25.
.25.
.25.
.25.
.25.
.26
.27
1
01-th
CONTENTS
(continued)
Write
Timing
and
Mark-Track
Read
and Write
Sequences
Read/Write
Control
Signals
Longitudinal
Parity
Buffer
Operation
LPB Control
Signals
Power Clear and Error
Stop
Logic
Increment
CA
Inhibit
(+1
CA
IN
H)
Address
Accepted
Transfer Direction
B
RUN
Level
Fixed Address
Interrupt
Request
ERROR FLAGS
(EF)
_
Mark-Track
Error
(MK
TR
K)
Select Error
(SE)
Parity
Error
(PAR)
Timing
Error
(TIM)
End
Error
(END)
DECtape
Flag (DTF)
Panel Indicator Drivers
CHAPTER 4
INSTALLATION
Installation Procedures
Site
Preparation
Environmental Conditions
Power
and Cable
Requirements
DECtape
Signal
Connectors
CHAPTER
5
MAINTENANCE
Maintenance
Equipment
Maintenance Control
Panel
DEC
Modules
Module
Locations
and
Complement
3-31
3-37
3-43
3-44
3-45
3-45
3-45
3-46
3—46
3—46
3-46
3-46
3-46
3-48
3-48
3-49
3-49
3-49
3-49
3-50
4-1
4-1
4-1
4-1
5-1
5-1
5-4
5-4

.3.2
.3.3
.4.1
.4.2
.4.3
UIUIU‘IU‘IU‘IU‘IU‘IU‘IU‘I .6.l
CONTENTS
(continued)
Circuit
Descriptions
Module
Replacement
Procedure
Power
Supply
779
Mechanical
Characteristics
Power
Supply
Checks
Marginal
Checks
Power Control
Panel
(Type
834)
Preventive Maintenance
Mechanical
Checks
CHAPTER 6
ENGINEERING DRAWINGS
Symbols
and
Designations
Drawing
List
ILLUSTRATIONS
TCOl
System
Configuration
DECtape
Track
Allocation
DECtape
Control and
Data Word
Assignments
DECtape
Recording
Format
DECtape
Mark Track Format
Status
Register
A,
Format
Status
Register
B,
Format
Turn Around
Sequence
Diagram
Type
TCOl
DECtape
Control
Functional Block
Diagram
Status
Register
A,
Instruetion Flow
Diagram
(Part I)
Status
Register
.A,
Instruction
Flow
Diagram
(Part
II)
Status
Register
A,
Instruction Flow
Diagram (Part
III)
Status
Register
A,
Instruction
Flow
Diagram
(Part IV)
Status
Register
A,
Instruction
Flow
Diagram
(Part
V)
Status
Register
B,
Instructions
Flow
Diagram
Read/Write Logic
and
Waveforms
Slicer Network
Waveforms
Timing
and State
Sequence
Diagram
(Sheet
l)
Timing
and
State
Sequence
Diagram
(Sheet 2)
vi
5—5
5-18
5—1
8
5-19
5-19
5—20
5—2l
5-22
5-22
6-I
6-1
3-10
3-10
3-14
3-19
3—21

5-10
5—H
5—12
5—13
5—14
5-15
5-16
5—1
7
5-18
5—19
5-20
5—21
5—22
5—23
CONTENTS
(continued)
Mark-Track
Decoding (C-SYNCH)
Mark-Track
Decoding
(MK
BLK
MK)
Mark-Track
Decoding (MK
BLK
START—2l0)
Mark-Track
Decoding
(MK
BLK
START—010)
Mark-Track
Decoding
(MK
DATA
070)
Mark-Track
Decoding
(MK
BLK
END)
Write
All
Function,
Timing
Sequence
Diagram
(Sheet 1)
Write All
Function,
Timing
Sequence
Diagram (Sheet
2)
Error
Check
Flow
Diagram
I
TCOI
Unit
Single
Cabinet Installation Dimensions
TCOI
Control,
Cable
Diagram
Maintenance Control Panels
(Switch
and
Indicators)
Master Slice
Control,
G008
Sense
Amplifier,
G009.
Manchester
Reader/Writer,
G882
Diode
Network,
R002
Inverter,
$107
Diode
Gate,
51
II
Diode
Gate,
R113
Diode
Gate,
R123
Diode
Gate,
Rl4l
Binary-to-Octal
Decoder,
5151
Flip-Flop,
R201
Dual
Flip-Flop,
$202
Triple Flip-Flop,
5203
Dual
Flip-Flop,
$205
Delay
(One
Shot),
R302
Integrating
(One
Shot),
R303
Variable
Clock,
R401
Pulse
Amplifier,
$602
Pulse
Amplifier,
$603
Clamped
Load,
W005
30 MA Indicator
Driver,
W050
Device
Selector,
W103
3-23
3-24
3-24
3-24
3-25
3-25
3—33
3-35
3-47
4-2
4-3
5-10
5-10
5-11
5-11
5—12
5-12
5-13
5-13
5-14
5-14
5-15
5-15
5—16
5-16
5-17
5-17

5-24
5-25
5—26
CONTENTS
(continued)
Comparator,
W520
Power
Supply Type
779,
Schematic
Diagram
Power
Control Panel
Type
834
TABLES
DEC
Documents
Summary
of
Equipment
Characteristics For
the
TCOI
DECtape
Control
Mark-Track
Coding
TCOl
DECtape
Instruction List
Status
A—bit
Assignments
Status
B,
Bit
Assignment
Control
Function Procedures and Errors
Summary
of
Timing
Data For
TCOI
Operation
Symbols
and
Abbreviations
Counter
Register
Sequence
Sequence
of Block
Marks and
Control States
Sequence
of Events
During
Write
Operation“)
Sequence
of
Events
During
Write
Operation“)
Maintenance
Equipment
Switch
and Indicators
(Maintenance
Control
Panel)
TCOI
Module
Complement
Engineering
Drawing
List
ENGINEERING DRAWINGS
BS-D-TCOi-O-Z
MF, USR, MR,
FR
BS—D-TCOi—O-B
WINDOW,
MK
TRK, STATE,
LPB
BS-D-TCOl-O-4
Error
Flags
BS-D-TCOl-0-5
Control
BS-D-TCOi—O—é
TP
GEN
TT GEN
MU-D-TCOl—O-9
Utilization
Module List
(Sheet
1)
MU—D-TCOl-0-9
Utilization
Module List
(Sheet
2)
BS-D-TCOl-O-IZ
Panel
Indicator
Drivers
BS—D-TCOI-O-lB
Maintenance
Control
Panel
BS-D-TCOl-O—l4
Error
Flags
BS-D—TCOI-O-I5
R/W
AMPS,
SP
GEN,
TEST CONN.
viii
5-18
5-19
5-21
1-2
i—3
2-5
2-6
2-7
2-9
2—1 2
2—1 5
2-19
3-17
3-26
3-38
3-40
5—1
5-3
5-7
6—1
6—5
6-7
6—9
6—H
6-13
6—1 5
6—1 7
6-19
6-21
6—23

CHAPTER
T
INTRODUCTION
This
manual,
together
with
the
referenced
documents,
provides
information
on
the
installation,
Operation,
and maintenance
of the
Type
TCOl
DECtape
Control,
manufactured
by
Digital
Equipment
Corporation, Maynard,
Massachusetts.
The TCOl buffers
and
controls
information transfers between
a
TU55
DECtape
Transport
and
its associated data
processor.
This document assumes
use
of the TCOl in
coniunction
with
either
a
DEC
PDP-8 or
PDP-8/I
Programmed
Data
Processor.
Except
where
specifically
noted
otherwise,
all
references
to the
PDP-8
also
apply
to
the
PDP-8/I.
The
level
of discussion
pro-
vided in this
document
assumes
a
prior
knowledge
and
understanding
of the
TU55
DECtape
Transport
and
the
particular processor
(PDP-8
or
PDP~8/I)
provided
with the
user's
system.
1.1 GENERAL DESCRIPTION
The
Type
TCOl
DECtape
control
is
used to buffer and control the
transfer
of
binary
data be-
tween
the PDP-8
processor
and
up
to
eight
Type
TU55
DECtape transports.
The
system
configuration
is
shown
on
Figure
l-l
.
TCOl
TU55
POP—B
PROCESSOR <:>
DECTAPE
(1:?)
DEC
TAPE
CONTROL
TRANSPORTS
L:
—.
UP
TO SEVEN
ADDITIONAL
—
TU55 TAPE
TRANSPORTS
Figure
l-l
TCOl
System
Configuration
The
TU55
DECtape transport
is
a
bidirectional device
consisting
of
a
magnetic
tape
transport
and solid state
logic.
During
both
input
and
output operations,
the TCOl
receives data and
control
information from
the
processor
and
generates
the
appropriate
signals
to the
selected
transport
to
execute
the
program-
med
commands.
Binary
information
is
transferred
between
the
tape
transport
and
the
computer
as one
l2-bit
computer
word
every
133—1/3
us.
In
writing,
the TCOl
disassembles the l2—bit
computer
word
into four
successive
3-bit words to be written on
tape.
During reading
the TCOl
assembles the four
successive 3-bit words into one 12—bit word for transfer
to
the
computer.
Transfers between
the com-
puter
and
the control
always
occur in
parallel
for a l2—bit
word. Data
transfers
use the
3—cycle
data-break
(high-speed
channel)
facility
of
the
computer.
As
the start
and
end
of
each
block are
detected,
the TCOl
generates
a
DECtape
control
flag
signal
(DTCF)
which
causes
a
program
in—
terrupt
in
the
computer.
The
program
interrupt
is
used
by
the
computer program
to
determine the
block number. When
it
determines
that
the
forthcoming
block
is the one
selected for a data
transfer,
it
selects the read or
write
control function. Each time a word
is assembled or
DECtape
is
ready
1-]

to
receive
a
word from
the
computer,
the
control
produces
a
data
flag signal
(DF)
to
request
a
data break.
Therefore,
when each
I2-bit
computer
word
is
assembled,
the
data break
initiates
a
transfer.
1.2
-
REFERENCED DOCUMENTS
The DEC
documents listed
in Table
l-l contain
material
which
supplements
the information
in
this
manual. These documents
may
be
obtained from
the
nearest
DEC
field
office
or from the
main
office.
Digital
Equipment Corporation
I46
Main
Street
Maynard,
Massachusetts
Table
I-I
DEC Documents
Doc.
No.
Title
Description
C105
Digital
Logic
Handbook
Specifications
and
descriptions
Of
the
FLIP
CHIP
modules
and
a
simplified
explanation
of
selection
and use of these
modules in
various
applications.
CIOO
System
Modules
Specifications
and
descriptions
of
basic
system
modules and
power
supplies.
This
manual
pro-
vides a
simplified
explanation
of
selection and
use
of
these
modules
in
system
applications.
F—85
PDP-8
User Handbook
Programming
and
Operating
information for
the
computer,
including
brief instructions on
the
TCOI
DECtape
Control.
F—87
PDP-8
Maintenance
Manual
Complete
information
on the internal
operations
Digital
8-27-U
H -T
U55
C—800
DEC-8/I—HMAA
PDP-8
Programming
Manual
Type
TU55
DECtape
Transport
Maintenance Manual
Small
Computer
Handbook
PDP-8/I
Maintenance
Manual
of
PDP-8
logic,
memory
basic
in/out,
and
pro-
cessor
options.
Programs
for
PDP—8.
Complete
descriptions
of
DECtape
subroutines
designed
for
assembly
with
an
obiect
program,
the
DECtape library
system,
and
the
DECtape utility
routines.
Transport
drive
logic
and internal
Operations,
plus
preventive
and corrective maintenance instructions.
Handbook of basic functions
of
PDP-8/I
computer.
Complete
information on the
internal
operation
of
PDP-8/I
logic,
memory,
basic
in/out,
and
processor Options.
In addition to
the
documents
listed
in
Table
I-l,
a
complete
set
of
library
programs
are
available.
1.3
PHYSICAL
DESCRIPTION
The
Type
TCOI
DECtape
control
logic
is mounted
in three
FLIP CHIP
mounting
panels
which
can
be installed
together
with
up
to three TU55
DECtape transports
in a
standard
DEC cabinet.
For
de-
tailed
information on the
mounting
panels
and
cabinets,
refer
to the
hardware section
of
the
Digital
Logic
Handbook
(CI05).

The
standard
DEC
computer
cabinet is
constructed with
a
welded steel
frame and sheet
steel
covering.
Double
doors
on
the front
and
rear
are,
held
closed
by magnetic
latches.
Power
supplies
and
power
controls are mounted
inside the
rear
double
doors
on a
full-width
plenum
door
latched
by
a
Spring-
loaded
pin
at
the
top.
Module
mounting
panels
are
mounted
behind
the
double
door
in
front with
the
wiring
side
facing
outward. Fans
at
the
bottom
of
the
bay
draw
cooling
air
through
dust
filters,
pass
it
over the
electronic
components,
and exhaust it
through
the
wiring
and other
openings
in
the front
and
tap
of
the
cabinet. Four
casters
provide
mobile
support
for the
system.
The rear
plenum
door contains
blank
panels
in
the
Space
not
occupied
by
components.
The
TCOl control
and
DECtape
units
receive
power
from the
Type
779
Power
Supply
and
a
Type
834
Power
Control. These units
are
mounted near the
bottom of
the
plenum
door.
The cabinet has an
access
door
which
extends
8-3/4
in.
to
the
front
of
the
cabinet and
a
plenum
door which
extends
13-3/4
in.
to the
rear. At least a
3-ft
clearance
should be
allowed
at
both
front
and rear for
access and
maintenance.
Cabinets
may
be
bolted
together
at the
sides
to
form
contig-
uous
units.
1
.3.l
Equipment
Characteristics
A
summary
of
the characteristics
of the
TCOl
control
and
associated
equipment
is
listed in
Table
1-20
Table
1-2
Summary
of
Equipment
Characteristics
for the
TCOl
DECtape
Control
Equipment
Characteristics
TCOl
Control
TU55
Transport
Cabinet
Weight
TCOl Control
TU55
Tran5port
Cabinet
Power
Requirements
TCOl Control
TU55
TranSport
Cabinet
15-3/4
in.
high,
T9
in.
wide for
equipment
which
operates
up
to
eight
Type
TU55
transports
10-1/2
in.
high,
19-1/2
in.
wide,
9-3/4
in.
deep.
Chassis
can be
extended
16-3/4
in.
beyond
the
mounting
surface for
maintenance
69—1/8
in.
high,
22-1/4
in.
wide,
27—1/6
in.
deep.
Will
hold
a
maximum of
one
TCOl
control and
three
TU55
tranSports
30 lb
65
lb
(rack
mounted)
555
lb
(with
maximum
equipment
mounted)
115V,
60
c/s,
4A
A
Type
834
Power
Control and
a
Type
729 Power
Supply
are
included
with
the
Type
TCOl
Control
(N9M
transformer
used for 50
c/s)
ll5V
:ElO°/o,
60
c/s,
2A
maximum,
l.5A
idle
115V,
60
c/s
source
capable
of
delivering
20
A
Tape
Characteristics
and
Density
a.
260 ft
of
0.75
in.,
l.0
Mylar
tape
per
3.5 in. reel
b.
350 =l=55 lines
per
inch
c.
849,
036
usable lines
per
tape
d.
60 lines
of
control
information
1-3

Table
l-2
(Cont)
Summary
of
Equipment
Characteristics
for
the
TCOI
DECtape
Control
Equipment
Characteristics
e.
4096 is
the
maximum
number
of
addressable blocks
per
reel
f.
Number of
words in
a
block must
be
divisible
by
3
_
212112
9'
NB—N
+15—
2
NB=decimal
number
of
blocks
W
NW=number
of
words
per
block
h.
Capacity
for
I90,000
l2-bit
words
in
blocks of I29
words
Word Transfer
Rate
a.
One
tape
line is read
or
written
every
33-l/3
ps
and
one
12-bit word
is read
and
assembled
or
disassembled
and written in
l33-i/3
ps.
b.
In reverse
direction,
the
transfer
rate varies
by
20%
as
the
effective
reel
diameter
changes.
c. Transfers
require
l .2%
of PDP-8
cycles
after the
initial 200—ms
start
time.
Addressing
a. Mark
and
timing
tracks
allow
searching
for
a
particular
block.
Time
a.
Start
time
is
<375
ms,
stop
time
is
<375
ms,
turn—around
time is <375
ms.
b.
Start
and
stop
distances
are
approximately
8
in.
Input
Signals
to
Transport
from Control
Commands
FORWARD
N I I I t I I
REVERSE
orma
y
comp
emen
ary
eve
5
GO
I
N II I l I
STOP
orma
y comp ementary
eve
5
ALL
HALT
(stops transport
when
computer
halts)
Unit
Select
Select
unit l
through
select
8
Information
Analog
write
signals
to
the
recording
head
Output
Signal
from
Tran5port
to
Control
Control
WRITE
ENABLE
(ground
level
assertion)
Information
Analog
read
signals
from
the
recording
head
Environmental
Conditions
Thermal
Dissipation
2l50
Btu/hr
Operating
Temperature
50°
-
95°F ambient
Humidity
10%
-
90% relative
humidity
1—4

NOTE
The
magnetic
tape
manufacturer
recommends 40%
-
60%
rela-
tive
humidity
and
60°
-
80°F as
an
acceptable
operating
en-
vironment
for
DECtape.
_
I.3.2
Electrical
Requirements
A cable rated
at
II5V,
60
c/s,
30A furnishes
power
for the
TCOI
DECtape
Control.
This
cable
is
terminated
by
a
Hubbell
Twist
Lock
plug
rated
at
30A,
I25
Vac.
Signals
between
the
TCOI
DECtape
Control
and-the
computer
are the
standard
voltage
levels
for
the
DEC
FLIP
CHIP
modules,
as stated
in
the
Digital Logic
Handbook
(CIO5).
Command
signals
be-
tween
the
TCOI
and
the
tape tranSport
are also standard
FLIP
CHIP
levels
except
for the
SINGLE
UNIT
signal
which is
a
dc level
between
0
and -9V.
During
writing,
the information carries 200
ma in
either
direction with
a
20V
peak—to-peak
waveform,
symmetrical
with
respect
to
ground.
When
reading,
the
peak-to-peak
head-voltage
waveform is between
8
and
I2
mV
when
the
tape
is
up
to
speed.
The
inter-
nal
logic
for the
TCOI
consists
of
DEC FLIP CHIP
modules.
All internal
signals
are standard
FLIP
CHIP
levels and
pulses, except
those
for
the
read/write
amplifiers
and
the
SINGLE UNIT
signals.
I-5

CHAPTER
2
OPERATION
AND PROGRAMMING
This
chapter
contains
the information
required
for the
operation
and
programming
of
the
TCOI
DECtape
Control unit.
Included
in this
chapter
is
a
description
of the
format
of information
on
the
DECtape
magnetic
tape,
and
the
modes
of
Operation
used
in
the
programming
of
TCOI
Operations.
The
general
operating
information
for the
TU55
DECtape
tranSport
is
contained
in
the
TU55
maintenance
manual
listed in Table
I-I
.
2.
i
DECTAPE
FORMAT
The
format
of
the
DECtape
(Figure
2-1)
used in-the
DECtape tranSports
provides
ten
tracks,
of
which three
pairs
of tracks are available
for
data and
two
pairs
for
timing
and mark
information. A
IO-track
recording
head
reads and
writes
the
five
duplexed
channels of the
DECtape.
Duplication
of
each track
by
nonadiacent
read/write
heads,
wired
in
series,
eliminates
most
dropouts
due
to
noise and
dust,
and minimizes
skew
problems.
The
location of the
redundant
tracks eliminates most
cross
talk
be-
tween
tracks. In
addition,
the
location of
the
timing
tracks
along
the
edges
of the
tape permits
strobing
on
the
analog
sum of the
timing-track signals
and
reading
of
the data
tracks,
when
they
are
in
the most
favorable
position.
The
location of the
data
tracks
in
the
middle
of
the
tape
also minimizes the
effects
of
skew.
mm TRACK I
.g
“T
MARK
TRACK
l
INFORMATION
TRACK
I
INFORMATION TRACK 2
INFORMATION
TRACK
3
u
INFORMATION
TRACK
IA
3/4
(Same
as
IT
I)
INFORMATION
TRACK 2A
(Same
as IT
2)
IN FORMATION TRACK 3A
REDUNDANT
(Same
as IT
3)
TRACKS
MARK
TRACK IA
(Same
as MT
I)
TIMING
TRACK
IA
(Same
as TT
I)
Figure
2—]
DECtape
Track Allocation
Data
is
recorded
by
the
Manchester
method
in
which
a
prerecorded
timing
track
synchronizes
read/write
operations.
When
writing
on
the
tape,
the
write
amplifiers
supply
the
maximum
current
in
either one
direction or the other
(non-return
to
zero,
NRZ).
To Write
a
pulse,
the
polarity
of the
write
current
is
reversed. The
timing
track
is
prerecorded
with
alternate
positive
and
negative
transitions
at
fixed
time
intervals. The
negative
transition is
used
during
writing
to
load
the write
buffer
and
during
2-]

reading
to shift data. The
positive
transition is
used
during
both
reading
and
writing.
During
writing,
this
transistion
is
a
signal
to
switch
the
polarity
of
the
write
current
in all
write heads.
If a
ZERO
is
being
written,
the
current,
which
starts out
positive
for
writing
ZEROs,
is switched to
negative
resulting
in
a
negative
transition.
If
a
ONE is
being
written,
the
current
starts
out
negative
and
generates
a
positive
transition
when
switched
to
positive.
During reading,
the
positive
transition of
the
timing
track
is
a
signal
to
strobe
the
data and mark track
read—amplifier
outputs
into the
read buffer.
If
a
positive
transition is sensed
at strobe
time,
0
ONE
is
placed
in
the
buffer;
otherwise
a
ZERO
is strobed in.
Because the
strobe
is
a
relatively
narrow
pulse,
the
system
is
not
affected
by
noise
outside the
strobe
time.
At
strobe
time,
all
data
signals
are
negative
pulses representing
ZEROs or
positive pulses
representing
ONEs.
These
pulses
are
all
at
their
peaks.
To
have
any
effect,
a
noise
pulse
must be
large
enough
to
reverse the
polarity
of a data
pulse.
Data
can
be written
over
previously
written data
because
the
timing
is
controlled
by
the
timing
track
that
is
written on
the
tape.
Information is
stored
on the
tape
in
block form
(Figure
2-2).
Block
length
is
flexible and deter-
mined
by
information
on
the
mark—track.
A
complete
reel
of
tape
(849,036
lines)
can
be divided
into
any
I
number
of
blocks
up
to 4096.
A
uniform block
length
can
be
established
over tlhe
entire
length
of a
reel
of
tape
by
a
program
which
writes mark and
timing
information
at
specific
locations.
However,
the
ability
to write
variable-length
blocks is
useful
for
certain
formats,
for
example,
where
small
blocks
containing
index or
tag
information
need
to be alternated with
the
large
blocks
of data.
j:
ONE
COMPLETE
REEL
—
260
FT.
4096 BLOCKS
j
-:ZONE
B'Locx“-.ELE>cK
iambic.
aLocki
aLo'ck' BLOCK BLOCK
I'eLoéK.
BL'CJ'CKE
swcx-
BLOCK:.-5LOOf<"'f_BLdCK’.II'BLQCKfI-UEL‘QQ
Q. m
r
I
\
-————«-——M
ONE
BLonK.
as man wono LOCATIONS
TIMING
TRACK
MARK
TRACK
f
-
’
2%
s
sss
sss‘s
- ,
0‘
=-
_ .7
-
.
$732M
z
ui
'3
ggé
la
gig
HIE
In
g
mi
112
E
113- m-
g
u.
TRACKS
“1 Wu. “1'
Lu I Lu
afigégé
essssssss-s
55553
5\'
\
DATA
l
DATA
WORDS
CONTROL WORDS
—|29Io
DATA WORD LOCATIONS CONTROL WORDS
WORDS
Figure
2-2
DECtape
Control and
Data
Word
Assignments
Each
block contains
data
and control
words
as
shown on
Figure
2-3
which
are
assembled
by
the
TCOl:
DECtape
Control. Control
words
separate
the
data
portions
of
adiacent
blocks
and
record ad-
dress
and
checking
information.
Although
control
words
usually
occupy
six
lines,
only
the
last four lines
are
used.
Data
words contain
stored information
and
occupy
four
lines
on
tape
(l2
bits).
To
maintain
2—2

compatibility
with the mark-track
format,
data words are
recorded
in
lZ-line
segments
(which
is
the
lowest
common
multiple
of 6-line
marks and 4-li-ne
data
words)
correSponding
to
three 12-bit
data
words.
Therefore
the
number
of
words
per
block
must be
evenly
divisable
by
3.
Line Line
Llna Line Line Llne
Line Line Line Line
Line
Line Line
Line Line Line Line Line Line Line Llne
Line Line Llne
I 2 3
4
5 6
1 2 3 4
5_
6
l 2 3
4
t
2
3 4 I 2 3
4
TlMlNG TRACK
MARK
TRACK
A:
.1:
5
l
.l”
INFORM-
ATION
2
TRACKS
_
REDUNDANT
TRACKS
NOT
SHOWN
_
"O
3
4
lines
(l2
bits
4
Ilnes
(l2
bits)
4
lines
(t2
bits)
6 lines
(IR
bits)
6 lines
()8
bits)
Figure
2-3
DECtape
Recording
Format
Block
numbers
normally
occur
in
sequence
from
I
to
n. There
is one
block
numbered
0
and
one block
n+l
.
Programs
are entered
with a
statement
of
the first block number to be
used and
the
total
number of blocks to be read
or
written.
The
maximum number
of
blocks
is
determined
by
the
following
equation.
NB
=
£91435
-2
_NB
=
decimal
number
of
blocks
W
NW
=
decimal
number
of
words
per
block
(NW
must
be divisible
by
3.)
2.]
.l
Mark-Track
Format
The
mark
track
contains
six-bit
serially
stored codes which
initiate controls
to
raise
flags
in
the
program, request
data
breaks,
detect block mark
numbering
and block
ends,
and
protect
control
portions
of the
tape
(Figure
2-4).
The
DECtape
control unit
automatically
identifies these
codes. The
mark track also
provides
for automatic bidirectional
compatibility,
variable block
formatting,
and
end-
of-tape
sensing.
In all
tape processing
Functions
except
the
recording
of the
timing
and
mark
tracks,
a
single
mark track bit is read
from
each
line
of
tape
regardless
of
whether
the
information is
being
read or
writ-
ten into the
data tracks.
Each
tape
line in
both the
information
and
mark
tracks
is
positioned
at
the
center of the
timing
track as
shown on
Figure
2-3. The
mark track
code is
contained
within
six
lines of
the mark
track.
A
change
of
polarization
on
tape
read in one
direction
produces
a
pulse
opposite
in
polarity
to that
produced
by
the same
change
read in
the
opposite
direction.
Consequently,
a
mark
code
read
in
reverse
has the
order of
bits reversed and the
bits
complemented.
2-3

4
Forward dIrIction
ol
lupu
motlon_
ONE
BLOCK
96.0
lB-BIT WORD LOCATIONS
'
I
I
I
|
'
I
| I29“,
IZ-BIT DATA
worm LOCATIONS
:
I
-
I
I
l
I
I
I
REVERSE REVERSE REVERSE
[EXPAND
BIDCK REVERSE
CHECK REVERSE PRE~ PRE- CHECK REVERSE BLOCK EXPAND
I
CODE
MARK
GUARD LOCK SUM FINAL
FINAL
DATA DATA DATA
DATA
DATA FINAL FINAL SUM LOCK GUARD
MARK
CODE
I
:‘M’TQE
Eff
M -G-
g:
"5c;
-r' -F"
o o
__o
o“;
D
5-.qu
PERMITS
EXPANSION OF
f f f f
.
f
f
f
f f T
PERMITS EXPANSION 0F BLOCK
BLOCK
NUMBER
0|
De
03 04
I35 0125 Dias Dm DIZB 0129
NUMBER
SIGNIFIEO
START
OF BLOCK AND
ALLOWS COMPUTER
PROGRAM TO BLOCK NUMBER
A5 REVERSE DIRECTION
IDENTIFY. BLOCKS
PROVIDES:
WRITE PROTECTION
IN
REVERSE
—"
SAME
FUNCTION
AS
REVERSE
LOCK
DIRECTION
AND SYMMETRY
PROTECTS TAPE
IN
EVENT OF MARK CHANNEL
ERRORS
NOT
USED
IN
PDP‘B CONFIGURATIONS
BUT
PROVIDED TO
ALLOW COMPATIBILITY WITH
OTHER COMPUTERS
PROVIDES AUTOMATIC ERROR DETECTION
--
AND END OF BLOCK DETECTION WHEN READING
PROVIDES, SYMMETRICAL ERROR PROTECTION
IN
BOTH
IDIRECTIONS
IDENTIFIES FINAL
DATA WORD AND REQUESTS
CHECKSUM
SAME AS FINAL IN REVERSE DIRECTION
(FIRST
DATA
WORD)
NOT USED BUT PROVIDED TO ALLOW
TAPE
——
COMPATIBILITY
WITH
OTHER SYSTEMS
SAME AS PRE-FINAL IN REVERSE DIRECTION
(SECOND DATA
WORDI
ADDITIONAL DATA WORDS
SUCCESSIVE
DATA
WORDS
NOTE:
END MARKS WHICH IDENTIFY THE PHYSICAL ENDS
OF THE
TAPEl
ARE THE
ONLY MARKS
NOT SHOWN.
Coda
tuncIionI
listed
apply only
In
tho
dlucllon
Indlcotod,
Figure
2-4
DECtape
Mark Track
Format
For
example,
the
mark
read forward as IOOIOI is
read
as
OIOI IO
in
reverse. This
carrespond-
ence
is
termed the
complement
obverse or
the
complement
image.
Every
6-bit
code
has
one
and
only
one
complement
obverse which is
constructed
by
complementing
all bits and
reversing
their order.
Therefore,
the
complement
obverse
of
the
complement
obverse is the
original
code
itself. In
octal nota-
tion,
the
complement
obverse
of
any
pair
of
digits
is
constructed
by
reversing
the
order
of
digits,
then
performing
the
Following
transformation
on
each:
O—>7 I—-'-3
2-—>5 3—>I
4—>6 5—a-2
6—D4
7-->O
The transformations
indicate
that
there
are
eight
octal
codes
which
are their own
complement
obverses:
07,
I3, 25, 3I,
46,
52, 64,
and
70.
All other
possible
combinations
of
two
octal
digits
(there
are
56)
are
different
from their
complement
obverses. As shown
in
Table
2-I,
the
complement
obverse of
any
mark
is
designated
by
the minus
sign (e.g.,
mark (3
=
5I
has
the
complement
obverse -G
=
32).
Since the
DECtape
system
allows
reading
and
writing
in both
directions
of
tape
motion,
the
mark—track
must be
coded to
present
the
same information
when
entering
a
block from
either direction.
The marks at the
end
of
a
block
are
the
complement
obverses
of
the marks at
the
beginning,
in
reverse
order.
For
example,
if
the control
reads the marks
E, M,
-G
as
the
first
three marks
beginning
a
block
in
forward
motion,
then it
will
read
(3, -M,
-
E,
in that
order,
as
the last
three marks
of
the
same
block.
In
reverse
motion, however,
the
control sees the
complement
obverse of the contents of the
mark
track;
thus
the
first
information,
when
reading
the
block
in
reverse,
is
-(—-
E),
-(-M),
-(G),
which is
identical
to
E, M,
-G.
All
marks used
in
the standard
DECtape
format
are listed
in
Table
2-]
.
Only
ten
valid codes
exist
even
though
a
given
code
may
have different
designations.
Some
of
these marks are
not
required
for
the
operation
of
the
Type
TCOI
DECtape
Control.
2-4

Table
2-]
Mark-
Track
Coding
Mark
Octal
Code
Function
C
(Check)
73
Signifies
the
end
of a
mark
frame
whose
first two
lines
were the
forward
parity
check
group.
-C
(Reverse
Check)
l0
‘
Signifies
that
the 6-bit
reverse
longitudinal
parity
check
group
is
contained
in
the
control
unit
read/
write
buffer
and that
the
beginning
of
the data
por—
tion of a
block is in
the
forward
direction.
D,
-D
(Data)
In
both
forward
and
reverse
tape
motion,
the
data
mark
occupies
all
mark
frames in
the
data
portion
of
the
‘block
except
for
the
final
and
prefinal
marks. The
number of
data
marks is
limited
only
by
the
length
of
tape.
E,
-E
(Extension)
25
i
The first
and last
mark of
every
block
(no-op
mark).
End
(Forward
End)
22
Indicates
the
end zone
of
tape
in
forward
direction.
The
forward
end mark is
positioned
approximately
l0
ft
from.
actual
tape
end.
~END
(Reverse
End)
55
Indicates
the
end zone of
tape
in
reverse
direction.
The
reverse end
mark
is
positioned
approximately
10 ft
from
end
of
tape.
F
(Final)
73
Signifies
that
the last
word
read from the
data
portion
of
the
block
is
in
the
read/write
buffer
and data
buffer.
Signals
that
the next
frame
begins
with
the
6-bit
forward
longitudinal
parity
check
group.
-F
(Reverse
Final)
10
-
Signifies
that
the last
word
read from
the
block,
in
the
reverse
direction,
is
in
the
read/write
buffer
and
data
buffer.
G
(Guard)
51
Performs
same
function
as
-L
(reverse
lock).
-G
(Reverse
Guard)
32
Not
used.
L
(Lock)
lO
Indicates
the first of
four octal
l0
marks.
-L
(Reverse
Lock)
73
Protects
subsequent
records in
the
event of
mark-track
errors.
M
(Forward
Block)
26
Signifies
the
start of a
block
and'indicates that
the
block
number is
contained
in
the
TCOl
control.
-M
(Reverse
Block)
45
Not
used.
P
(Prefinal)
73
In the
forward
tape
direction,
the
prefinal
mark is the
next to
last mark in
the
data
portion
of
a
block. It
is
the first of
four
marks
using
octal
code
73.
-P
(Reverse
Prefinal)
10
In
the
reverse
tape
direction,
signifies
the next to
last
mark
in
the data
portion
of
a
block.
The standard
mark—track
uses the
serial code of 6-bit
characters to
divide the
tape
into
words.
Codes are written on
the
mark track
opposite
word
locations to
identify
the
type
of
information
stored
at
that
location on
tape.
Block addresses
are
written
for
both
forward
and
reverse
directions
and
identified
by
two
types
of
mark
codes.
A checksum is
written at each
end of
the
block.
The
hardware
computed
2-5