IBM 3803 User manual

GA32-0021 -1
Systems

Preface
This manual provides reference information about the IBM 3803 Model 2 Tape
Control and 3420 Models 3 - 8 Tape Units.
This manual requires no prerequisite reading; however, it is assumed the reader
has a basic knowledge of stored program computers.
Second Edition (March 197 4)
This is a major revision of, and obsoletes GA32-0021-0. Changes are periodically made to the
information herein; any such changes will be reported in subsequent revisions or Technical
Newsletters.
All specifications in this manual are nominal unless otherwise indicated.
Requests for copies of IBM publications should be made to your IBM representative or to your
local IBM Branch Office.
This manual has been prepared by the IBM General Products Division, Maintainability and
Testing, Department 26T, P. 0. Box 1900, Boulder, Colorado, 80302. A form is provided at
the back of this publication for the reader's comments. If the form has been removed,
comments may be sent to the above address.
© Copyright International Business Machines Corporation 1973, 1974

Contents
Section 1: Introduction 9
Basic Subsystem . . . 9
3420 Tape Unit . . . 9
Automatic Threading 9
IBM Easy Load Cartridge 9
Tape Transport . . . 10
Read Back Checking 10
Full-width Erasure 10
File Protection . . . 10
Rewinding . . . . . 10
3420 Models 4, 6, and 8 10
Recording Methods 11 ·
6250 BPI . . . . . . . 11
1600 BPI . . . . . . . 11
800 BPI, Nine-Track NRZI 11
Seven-Track . . . . . 11
Interblock Gaps . . . . . 11
Magnetic Tape and Reels . . 11
3803 Model 2 Tape Control 13
General Characteristics 13
Addressing . . . . . . 13
Metering . . . . . . . 13
Enable/Disable Switch . 14
Power On/ Off Sequencing 15
3803 Model 2 Features 15
Nine-Track NRZI . . 15
Seven-Track . . . . 15
Two-Channel Switch 16
Device Switch 16
2 Control Switch 17
3 Control Switch 17
4 Control Switch 17
Section 2: Commands and Instructions 21
Commands . . . 21
Burst Commands 23
Write . . . . 23
Read Forward 23
Read Backward 23
Sense . . . . 23
Sense Reserve 23
Sense Release 24
Request Track-In-Error (Request TIE) 24
Loop Write-To-Read (LWR) 24
Set Diagnose . . . . . . 24
Motion Control Commands . 24
Rewind (REW) . . . . 25
Rewind/Unload (RUN) 25
Erase Gap (ERG) . . . 25
Write Tape Mark (WTM) . 25
Backspace Block (BSB) 25
Backspace File (BSF) 26
Contents 3

Forward Space Block (FSB) 26
Forward Space File (FSF) 26
Data Security Erase (DSE) . 26
Non-Motion Control Commands 27
No-Operation (NOP) 27
Mode Set 1 (MS 1) . . . . 27
Mode Set 2 (MS 2) . . . . 27
Diagnostic Mode Set (DMS) 27
1/0 Instructions 28
Test 1/0 . . . . . . . . . 28
Halt 1/0 . . . . . . . . . 28
Section 3: Subsystem Operator Functions 29
Inserting Tape Reel in Cartridge 29
Load Procedure . . . . . . . . . 29
Unload Procedure . . . . . . . . 30
Operating Procedures After Failure 30
Tape Fails to Thread (With Cartridge) . 30
Tape Fails to Thread (Without Cartridge) 30
End of Tape Comes Off Machine Reel Hub
as Tape Loads in Columns . . . . . . . 30
Tape Unit Fails to Sense EOT Marker (Tape End Comes Off Reel) 30
Tape Threads Successfully But Fails to Load in Columns 31
Window Fails to Open After Unload Operation . . . . 31
Channel Fails to Select Tape Unit (Device Switching or
Two-Channel Switch Feature Installed) 31
3420 Indicators and Pushbuttons 32
3803 Operator's Panel . . . 33
Toggle Switches . . . . . 33
Enable/Disable Switch(es) 34
Power On/Off Procedures 34
Power Off . . . . . . . 34
Power On . . . . . . . 35
Section 4: Status and Sense Information 37
Status Byte . . . . . . . . . 37
Sense Bytes . . . . . . . . . 39
Sense Byte 0 (Unit Check) . 39
Sense Byte 1 . . . . . . . 40
Sense Byte 2 (Track In Error) 41
Sense Byte 3 (Data and Equipment Checks) 42
Sense Byte 4 . . . . . . 43
Sense Byte 5 . . . . . . . . . . . . . . 43
Sense Byte 6 (Tape Unit) . . . . . . . . 44
Sense Byte 7 (Tape Unit Ready-Drop Source) 44
Sense Byte 8 (Microprogram Detected Errors) 45
Sense Byte 9 . . . . . . . . . . . . 45
Sense Byte 10 . . . . . . . . . . . 46
Sense Byte 11 (MAL 1 ROS/MP Errors) 46
Sense Byte 12 (MAL 2 ROS/MP Errors) 47
Sense Byte 13 . . . . . . . . . . . 47
Sense Byte 13, Bits 2-7 and
Sense Byte 14 (Tape Control Identification) 47
Sense Byte 15 and 16 (Tape Unit Identification -Serial Number) 47
Sense Byte 17 (Tape Control Features) 48
Sense Byte 18 48
Sense Byte 19 48
Sense Byte 20 48
4 3803-2/3420 Subsystem Description .

Sense Byte 21 (Thread and Load Diagnostics) . . .
Sense Byte 22 (Field Replaceable Unit Identification)
Sense Byte 23 (Field Replaceable Unit Identification)
Section 5: Error Recovery Procedures
Tape Cleaner Sequence
Operator Messages . . . . . . .
Action Requirements . . . . . . . .
Terminal Action la (With Operator Option)
Terminal Action lb (Without Operator Option)
Action II (Unit Check) . . . . .
Action III (Intervention Required) . . . . .
Action IV (Bus Out Check) . . . . .
Action V (Data Check On Read or Read Backward)
Action VI (Data Check On Write and Write Tape Mark)
Action VII (Overrun, Channel Data Check Chaining Check)
Action VIII (Data Check on Control) . . . . . . . . .
Action IX (Not Capable) . . . . . . . . . . . . . .
Action X (Equipment Check, No Previous Sense Bits On)
Action XI (Data Security Erase) . . . .
Action XII (Load /Point, Program Check,
Protection Check, Incorrect Length) . .
Action XIII (Command Reject, Data Converter Check)
Action XIV (ID Burst Check on Write and Write Tape Mark)
Supplementary Information
Index
49
49
49
51
51
52
52
52
52
52
52
53
53
54
55
55
55
55
55
55
55
55
56
57
Contents 5

Figures
Figure 1. Subsystem Characteristics . . . . . .
Figure 2. 3803 Model 2 and 3420 Configurations
Figure 3. Density Feature Combinations
Figure 4. Two-Channel Switch . . . . . . . .
Figure 5. Switching Configurations (Part 1 of 2)
Figure 5. Switching Configurations (Part 2 of 2)
Figure 6. 3803-2/3420 Commands . . . .
Figure 7. Inserting Tape Reel In Cartridge
Figure 8. Load Procedure . . ..... .
Figure 9. 3420 Indicators and Pushbuttons
Figure 10. 3803 Operator's Panel .....
6 3803-2/3420 Subsystem Description
12
14
15
16
18
19
22.
29
29
32
34

Abbreviations
BCD binary coded decimal LWR
loop write to read
BOT beginning of tape meter
m
BPI bytes per inch millimeter
mm
BSB backspace block millisecond
ms
BSF backspace file
MS 1 Mode Set 1
ccw channel command word MS 2 Mode Set 2
CE
cust9mer engineer
MTE multi-track error
ChE channel end
NOP no operation
cm centimeter
NRZI non-return to zero, IBM
CPU central processing unit
csw channel status word
OS
operating system
CUE
control unit end
PE phase encoded
DMS
diagnostic mode set rewind
DSE data security erase
REW
ROS
read-only storage
EBCDIC extended binary coded
RUN rewind/unload
decimal interchange code R/W read/write
EOT
end of tape secondsec
ERG
erase gap
SIO start I/ 0 instruction
FRU field replaceable unit
Sw switch
FSB forward space block Two-Channel Switch
FSF forward space file
TCS
TI
tape indicate
hex hexadecimal TIE track in error
IBG
interblock gap
TM
tape mark
ID identification
UE
unit exception
IF interface usec microsecond
1/0
input/ output
YRC vertical redundancy check
ips inches per second
Kb kilobyte
WTM write tape mark
Kd kilodigit
Abbreviations 7

Section 1: Introduction
Basic Subsystem
3420 Tape Unit
Automatic Threading
The IBM 3803-2/3420 Magnetic Tape Subsystem consists of an IBM 3803 Model
2 Tape Control and one or more IBM 3420 Magnetic Tape Units. The 3420
Tape Units are available in six models with tape speeds of 75, 125, and 200
inches per second (190,5/317,5/508 cm/sec) for Models 3/4, 5/6, and 7/8,
respectively.
The 3803 Model 2 operates at 6250 BPI and 1600 BPI densities.
A ~803 Tape Control without any switching features controls up to eight 3420
Tape Units (1 x 8 configuration).
The 3803 command set, status responses, and basic sense data are compatible
with 2400-series tape subsystems. However, there are some minor programming
differences. For example:
1. The number of sense bytes and contents of those bytes differs from those
used by 2400-series subsystems.
2. All commands not listed in Figure 6 except Mode Set type set COMMAND
REJECT (sense byte 0, bit 0).
3. A sense command must be issued after a condition is encountered which sets
UNIT CHECK in the unit status byte.
In most instances, non-time dependent programs that operate successfully on
2400-series tape units will operate correctly on a 3803/3420 subsystem with the
appropriate features.
Information presented in this section applies to all models of the tape unit.
When the operator places a file reel or a cartridge on the reel hub and presses
LOAD-REWIND, the power window closes, the automatic reel latch secures the
file reel to the hub, and tape is threaded, loaded into the columns, and positioned
at load point without further operator action.
IBM Easy Load Cartridge
When used with a solid-flange tape reel (standard IBM 10.5 inch), the IBM Easy
Load Cartridge reduces tape handling and helps prevent contamination or
physical damage.
During a load operation, if the first threading sequence is unsuccessful, tape is
rewound into the cartridge and another attempt is made.
/
Section 1: Introduction 9

Tape Transport
Read Back Checking
Full-Width Erasure
File Protection
Rewinding
3420 Models 4, 6, and 8
A single, direct-drive capstan moves tape forward or backward. Air bearings
reduce friction and tape wear since the oxide (recording) tape surface contacts
only the read/ write head, the erase head, and the tape cleaner. Short, tapered
vacuum columns greatly reduce tape inertia when starting and stopping tape. The
tapered columns and single-drive capstan start and stop tape quickly and
smoothly.
A two-gap read/ write head allows read back checking during a write operation.
Moving forward tape passes first the write gap, then the read gap.
An erase head applies a strong magnetic field that erases the entire width of tape
during write operations. Full-width erasure prevents interchangeability problems
when tape is written on one tape unit and read on another; it also reduces the
chances of leaving extraneous bits in interblock gaps or skip areas.
During a write, write tape mark, or erase operation, the tape unit monitors the
erase head operation. On a 3420 Model 4, 6, or 8 an erase head failure drops
tape unit ready status and halts tape motion. On a 3420 Model 3, 5, or 7 an
erase head failure sets UNIT CHECK, but does not drop ready status.
A write enable ring must be present in the file reel when writing. To avoid
destroying information on tape, remove the write enable ring. A reel without the
ring is "file-protected." The FILE PROTECT indicator remains on when a reel
without a file protect ring is mounted and no writing can occur.
Tape remains in the vacuum columns during rewind operations. Rewind ends
when a photocell senses a beginning-of-tape (load point) reflective marker on
tape.
During a rewind/unload operation, tape is rewound completely onto the file reel.
The tape unit is left in unloaded status, with the tape reel latch unlocked and the
window open, allowing the operator to remove the file reel.
The 3420 Models 4, 6, and 8 have the following added capabilities and features:
• 6250 BPI capability.
• 0.3-inch IBG.
Nominal data rates of 1250, 780, or 470 Kb/sec at 6250 BPI.
• Improved access times.
• Prec:1e motion control of the media to achieve faster data access times.
6250 BPI or 6250/1600 BPI density features.
• Automatic read amplification.
New tape cleaning mechanism.
3420 Models 3, 5, and 7 can be converted to 3420 Models 4, 6, and 8.
10 3803-2/3420 Subsystem Description

Recording Methods
6250 BPI
1600 BPI
At 6250 BPI (246 bytes/mm), data is recorded in nine parallel tracks on tape.
6250 BPI is a basic density on the 3803 Model 2 and 3420 Models 4, 6, and 8.
In 1600 BPI (63 bytes/mm) mode, 1600 bytes per inch (63 bytes/mm) are
recorded in nine parallel tracks on tape. The data format uses eight of the nine
bits for data, the ninth is a parity bit. Data is recorded in odd parity. The eight
bits of one byte can represent an alphabetic character, zoned decimal digit, two
decimal digits (packed), a special character, or eight binary bits.
1600 BPI is a basic density on the 3803 Model 2 and on 3420 Models 3, 5, and
7 and a feature on 3420 Models 4, 6, and 8.
800 BPI, Nine-Track, NRZI
Seven- Track
lnterblock Gaps
Data representation is the same as for 1600 BPI PE. For nine-track NRZI
operation, the dual density feature on a Model 3, 5, or 7 tape unit and the
nine-track NRZI feature on a 3803 are required.
Nine-track NRZI is a feature on the 3803 Model 2 and on 3420 Models 3, 5,
and 7.
Data is recorded at 200, 556, or 800 BPI in seven tracks across the width of the
tape. The data format uses six of the seven bits for data and the seventh bit is a
parity bit. Data may be in either even or odd parity. The six bits of one
character can represent a BCD character or six binary bits.
For seven-track operations, a seven-track feature on a 3420 Model 3, 5, or 7 and
on a 3803 is required.
Interblock gaps (IBGs) are as follows:
6250 BPI: 0.3 inch (7,6 mm) nominal.
800/1600 BPI Nine-track: 0.6 incb (15,2 mm) nominal; 0.5 inch (12,7 mm)
minimum.
200/556/800 BPI Seven-track: 0.75 inch (19,05 mm) .nominal; 0.68 inch (17,27
mm) minimum.
Magnetic Tape and Reels
Most tape volumes which operate satisfactorily on 3420 Models 3, 5, and 7 will
operate with equal or better reliability for an equivalent number of bytes
transferred on 3420 Models 4, 6, and 8. Tape must conform to IBM Tape
Specifications, GA32-0006-4.
Section 1: Introduction 11

3420 Model
3 4 5 6 7 8
Tape Speed Read or Write
(ips) 75 75 125 125 200 200
(cm/sec) 190,5 190,5 317,5 317,5 508 508
Access Times (ms)
Read, nominal*
6250 BPI 2.3 1.6 1.1
1600 BPI
4.0 4.0 2.9 2.6 2.0 1.65
Write, nominal*
6250 BPI
2.1 1.5 0.95
1600 BPI
4.0 3.0 2.9 2.0 2.0 1.28
Forward Start (ms)
Time nominal** 1.8 1.4 1.4 1 .1
1.3 0.8
Data Rates
(Kb/sec;Kd/sec) 1250/
6250 BPI 470/940 780/1560 2500
1600 BPI
120/240 120/240 200/400 200/400
320/640 320/640
800 BPI 9-track
60/120 100/200
160/320
800 BPI 7-track 60 100 160
556 BPI 7-track 41.7 69.5 111.2
200 BPI 7-track 15.0 25.0 40.0
Passing Times per Byte
(usec)
6250 BPI 2.133 1.28 0.80
1600 BPI
8.3 8.3 5.0 5.0
3.1
3'.1
800 BPI
16.7 10.0 6.2
556 BPI
24.0 14.4 9.0
200 BPI
66.7 40.0 25.0
Passing Times I BG (ms)
6250 BPI 4.0 2.4 1.5
9-track PE & NRZI 8.0 8.0 4.8 4.8 3.0 3.0
7-track 10.0 6.0 3.75
Rewind Time (sec)
(2400-foot reel) 60 60 60 60 45 45
Rewind-Unload Time
2400-foot reel, sec 66 66 66 66
51 51
Load Operation, approx.
time to 'tape unit ready'
after reel/cartridge is
mounted and LOAD
REWIND is pressed in
sec. 10
10 10 10 7 7
* Read access time is the interval from initiation of a Forward Read command given to the
tape control when tape is not at load point, until the first data byte is read when tape is
brought up to speed from stopped status.
Write access time is the interval from the issuance of a Move command given to the tape
unit when tape is not at load point, until the first data byte is written on tape when tape is
brought up to speed from stopped status.
** Start time is the interval from the issuance of a Move command to the tape unit, until tape
attains 90% of specified velocity.
· Figure 1. Subsystem Characteristics
12 3803-2/3420 Subsystem Desciiption

3803 Model 2 Tape Control
General Characteristics
Addressing
Metering
The 3803 Model 2 tape control connects to the I/O interface of an IBM
System/360 Model 50 and above (by RPQ only) or to an IBM System/370
Model 135 and above. The tape control consists of a CE panel, two
microprogram control sections, a read section, a write section, and a channel
buffer section.
Note: "I/O Interface" refers to a set of lines over which the tape control and
system channel exchange control and data signals. Interface lines and operations
are described in IBM System/360 and System/ 370 I/O Interface, Channel to
Tape Control, Original Equipment Manufacturers' Information, GA22-6974.
The 3803 may exceed an interface signal sequence of 32 microseconds, and may
produce a worst case interface signal sequence of up to 50 microseconds on some
instructions when in seven-track mode with the two-channel switch feature
installed.
The·3803 Model 2 operates at 6250 or 1600 BPI. The 3803 Model 2 with
appropriate features can process nine-track, 800 BPI NRZI and seven-track,
200/556/800 BPI NRZI tape on those 3420 Model 3, 5, and 7 tape units having
the companion NRZI feature. (See Figure 2.)
All data transfers are in burst mode. The tape control executes one command on
one tape unit at a time. The tape control parity checks each data byte
transferred between the system and a tape unit. On write operations, bus out
parity is checked and parity is corrected, if necessary, before the byte is sent to
the tape unit. On read operations, parity of each byte is checked and corrected,
if necessary and possible, before the byte is placed on the I/O interface. On
sense operations, correct parity is supplied for each byte. Parity is also checked
on command bytes.
I/O commands issued by the channel are executed with microprograms resident in
two independent read-only storage (ROS) units. One ROS unit controls
communication lines to the channel, while the other ROS unit controls
communication lines to the tape unit.
Every,tape unit has a unique device address consisting of a channel address, a
tape control address, and a tape unit address. Pluggable jumpers assign the tape
control address when the system is installed. The tape control contains separate
device interface connectors for each tape unit address. A tape unit's address is
determined by the tape control connector to which it is attached. There is no
address decoding at the tape unit or device interface level.
A usage meter is incorporated in the tape control and in each tap~ unit. The tape
control's usage meter records elapsed time whenever the channel's METERING
line is active and the tape control is in online status (enabled). A tape unit's
usage meter records elapsed time when the tape control METERING OUT line is
active, tape unit is loaded, and the tape is not at load point. METERING IN is
used by the central processing unit (CPU) metering circuits; this line is active
from the time a command is accepted by the tape control until Device End is
generated for that command. See IBM System/360 and System/370 1/0
Interface: Channel to Tape Control OEM/, GA22-6974.
Section 1: Introduction 13

Operation with Model 4/6/8 Tape Units (6250 or 1600 BPI
Mode) and Models 3/5/7 1600 BPI Tape Units
Operation with Model 4/6/8 Tape Units (6250 or 1600 BPI
Mode) and Model 3/5/7 Tape Units (1600 BPI PE and
800 BPI NRZI Modes)
3803 Model 2
Tape Control
3803 Model 2
Tape Control
3420
Mod
4
3420
Mod
6
3420
Mod
8
6250 or 6250/ 1600
Mod
4
Mod
6
6250 or 6250/ 1600
3420
Mod
3
3420
Mod
5
3420
Mod
7
3420
Mod
4
3420
Mod
q
Nine Track NRZI Feature
3420
Mod
8
3420
Mod
3
3420
Mod
5
3420
Mod
7
Single Density (1600)
6250 or 6250/ 1600
Dual Density (1600/800)
Operation with 3420 Model 4/6/8 Tape Units (6250 or
6250/ 1600 BPI Modes) and Nine and Seven Track Tape Units
(Nine Track 1600 BPI PE and Nine Track 1600/800 BPI and
Seven Track 200/556/800 BPI N RZI Modes)
3420
Mod
3
3803 Model 2
Tape Control
Seven Track Feature
(Nine Track NRZI Feature Prerequisite)
3420
Mod
5
3420
Mod
7
3420
Mod
3
3420
Mod
5
3420
Mod
7
Single Density (1600) Dual Density (1600/800)
MAXIMUM OF 8 TAPE UNITS PER TAPE CONTROL
For 3420 Model 8 Power Requirements see IBM S/360 Physical
Planning Manual, GC22-6820.
Seven Track
Figure 2. 3803 Model 2 and 3420 Configurations
Enable/Disable Switch
This switch allows the tape control and tape units to be put on or taken off line
so a field engineer can use the CE panel switches and indicators to diagnose
errors. Whenever the tape control is in offline status (disabled), the usage meters
in the tape control and all attached t~pe units are prevented from running. When
the Two-Channel Switch feature is installed, a second Enable/Disable switch is
provided on the 3803.
14 3803-2/3420 Subsystem Description

Power On/Off Sequencing
3803 Model 2 Features
Nine-Track NRZI
Seven- Trac.k
System power interlock circuits control normal power on/ power off sequencing
for the 3803/3420 tape subsystem. However, maintenance activities may
necessitate dropping power in the tape control.
Features available on a 3803 Model 2 are Nine-Track NRZI, Seven-Track,
Two-Channel Switch, and Device Switch. See Figures 3, 4, and 5.
The nine-track NRZI feature, available on the 3803 Model 2, permits operation in
nine-track NRZI mode. Nine-track NRZI operation requires a 3420 Model 3, 5, or
7 tape unit with the dual density feature. Operation of this feature is similar to
that of the 3803-1 with the dual density feature.
The seven-track feature permits operation in seven-track NRZI mode in addition
to the nine-track operation provided by the nine-track NRZI feature. Seven-track
operation with a 3803 Model 2 is at 800/556/200 BPI. The seven-track feature
contains both the data translator and data converter for seven-track operations.
Their operation is similar to that of the 3803 Model 1 with the seven-track
feature. For seven-track operation, the seven-track feature on a 3420 Model 3,
5, or 7 and on the 3803 Model 2 are required. The nine-track NRZI feature is a
prerequisite for the seven-track feature on the 3803 Model 2.
Writing a tape with the translator on causes eight-bit bytes from the 1/0 interface
to be written on tape as six-bit BCD characters; reading such a tape causes six-bit
BCD characters to be translated into their EBCDIC equivalents. When using the
translator, data rates are not changed and there are no changes in the tape unit's
operation.
Writing a tape with the data converter on causes four tape characters (24 bits) to
be written for every three storage bytes (24 bits); reading such a tape reverses
the process by converting four tape characters into three storage bytes. Data
conversion reduces the data transfer rate to 75% of the operating rate with data
converter off.
Density (BPI) 3803-2
3420-3/5/7 3420-4/6/8
(Note 1) (Note 4) (Notes 2, 4)
(Note 3)
6250, 9-Track Standard
N/A
6250 Feature
1600, 9-Track Standard 1600 Feature
6250/1600 Feature
800, 9-Track 9-Track NRZI Feature Dual Density Feature
N/A
800, 7-Track 7-Track Feature 7-Track Feature
N/A
556, 7-Track 7-Track Feature 7-Track Feature
N/A
200, 7-Track 7-Track Feature 7-Track Feature
N/A
Notes:
1. Density must be specified for each nine-track 3420 tape unit.
2. 3420-3/5/7 can be attached to 3_803-1 or 3803-2.
3. 3420-4/6/8 can be attached to 3803-2 only.
4. 9-Track NRZI feature is a prerequisite for 7-Track feature on 3803-2.
Figure 3. Density Feature Combinations
Section 1: Introduction 15

Two-Channel Switch
Device Switch
This feature permits the 3803 Tape Control to be accessed by a second channel.
(See Figure 4.) Alternate path switching between two channels attached to the
same system is under program control. The two-channel switch can also be used
to share a tape control between two systems. Unless the two systems are
software coupled, the tape control should be offline to one of the systems at all
times. Partitioning between channels attached to two different systems is done
manually.
Two-Channel Switch Feature
Channel 1 Channel 2
Figure 4. Two-Channel Switch
Two Systems Sharing a Tape Control
Through the Use of a Two-Channel
Switch
0--------------7
Tape Units
Tape subsystem configuration flexibility is provided by field installable switching
features that allow up to 16 tape units to be switched among two to four tape
controls. 3803 Models 1 and 2 can be mixed in a switching configuration;
however, attempting to access a 3420 Model 4, 6, or 8 through a 3803 Model 1
is not allowed and will produce unpredictable results.
The three device switching features available with the tape subsystem are:
2 Control Switch (2 x 8 and 2 x 16 configurations)
• 3 Control Switch (3 x 8 and 3 x 16 configurations)
• 4 Control Switch (4 x 8 and 4 x 16 configurations)
The location of the device switches depends on the configuration desired. For
example: In a 2 x 8, 3 x 8, or 4 x 8 configuration, the switching feature is
required only on Tape Control 1, while in the 2 x 16, 3 x 16, or 4 x 16
configurations the switching feature is required on Tape Controls 1 and 2 (see
Figure 5).
Each device switch feature requires a Communicator. A Communicator sends
tape unit selection and device interface signals to one of two device switches,
depending on whether tape units 0-7 or 8-F are being addressed.
The Communicator replaces the selection logic circuits and associated device
interface cabling in the basic tape control with different logic circuits and cabling
to the device switches.
16 3803-2/3420 Subsystem Description
<

2 Control Switch
3 Control Switch
4 Control Switch
The Communicator must be installed in all tape controls to be switched. Using a
combination of the Communicator and the 2, 3, or 4 Control Switch special
features, two, three, or four interconnected tape controls can address a maximum
of 16 tape units. Figure 5 shows some possible switching configurations.
The 2 Control Switch is a 2 x 8 configuration of hardware switching logic. Tape
Units 0-7 (attached to Tape Control 1 in part 1 of Figure 5) can be accessed by
the Communicator in Tape Control 2 as well as the Communicator in Tape
Control 1. A 2 x 16 configuration is obtained by installing a 2 Control Switch in
Tape Controls 1 and 2, allowing the Communicators in each tape control to
access its own eight 3420s as well as the 3420s of the other tape controL (See
part 2 of Figure 5).
A 3 x 16 configuration is obtained by installing a 3 Control Switch in Tape
Controls 1 and 2. A third tape control must be added to the configuration. Tape
Control 3 does not contain any switch hardware or attach any tape units, but
does contain a Communicator (see part 2 of Figure 5).
A 4 x 16 configuration is obtained by installing a 4 Control Switch in Tape
Controls 1 and 2. Two more tape controls must be added to the configuratjon.
Tape Controls 3 and 4 do not contain any switch hardware or attach any tape
units, but each contains a Communicator (see part 2 of Figure 5).
The 3 Control Switch and the 4 Control Switch are expansions of the 2 Control
Switch. They allow access to the eight attached tape units by the additional
Communicators.
In all of the configurations, the device switch is logically invisible except for a
'busy' response to initial selection when the addressed tape unit is being used by
another tape control. All device switch features contain priority logic to;
systematically allocate a tape unit in the event of conflicting tape control
requests.
Section 1: Introduction 17

1x 8 Configuration
0--------------7
Tape Units
Notes:
Tape Control
Number 1
0--------------7
Tape Units
• A maximum of 8 tape units and 4 tape controls.
Tape Control
Number 1
0--------------7
Tape Units
3 X 8 Configuration
0--------------7
Tape Units
4 X 8 Configuration
• Tape units attach only to tape controls with switch features.
• Any or all tape controls may have a two-channel switch feature.
Figure 5. Switching Configurations (Part 1 of 2)
18 3803-2/3420 Subsystem Description
2 X 8 Configuration
Tape Control
Number 2
Tape Control
Number 3

2 x 16 Configuration
Tape Control
Number 3
Communicator
Notes:
0--------------7
Tape Units
Tape Units
0--------------7
Tape Units
• A maximum of 16 tape units and 4 tape controls.
3 x 16 Configuration
Tape Control
Number 2
4 x 16 Configuration
• Tape units attach only to tape controls with switch features.
• Any or all tape controls may have a two-channel switch feature.
Figure 5. Switching Configurations (Part 2 of 2)
Tape Control
Number 2
8--------------F
Tape Units
Tape Units
Tape Control
Number 2
8--------------F
Tape Units
Tape Control
Number 3
Tape Control
Number 4
Section 1: Introduction 19