<s>
TXE	B-Device
,	O
(	O
telephone	O
exchange	O
electronic	O
)	O
was	O
a	O
family	O
of	O
telephone	O
exchanges	O
developed	O
by	O
the	O
British	O
General	O
Post	O
Office	O
(	O
GPO	O
)	O
,	O
designed	O
to	O
replace	O
the	O
ageing	O
Strowger	B-Device
switches	I-Device
.	O
</s>
<s>
When	O
World	O
War	O
II	O
ended	O
,	O
the	O
UK	O
telephone	O
exchange	O
suppliers	O
supported	O
the	O
GPO	O
's	O
decision	O
to	O
stay	O
with	O
Strowger	B-Device
until	O
a	O
viable	O
electronic	O
system	O
became	O
available	O
.	O
</s>
<s>
Suddenly	O
the	O
rules	O
had	O
changed	O
and	O
the	O
race	O
was	O
on	O
to	O
develop	O
an	O
electronic	O
telephone	O
exchange	O
that	O
could	O
operate	O
with	O
the	O
current	O
GPO	O
telephones	O
used	O
in	O
the	O
UK	O
,	O
including	O
shared	B-Architecture
service	I-Architecture
.	O
</s>
<s>
This	O
gave	O
him	O
the	O
confidence	O
during	O
the	O
war	O
to	O
build	O
the	O
world	O
's	O
first	O
digital	O
computer	O
,	O
called	O
Colossus	B-Device
,	O
at	O
Bletchley	O
Park	O
.	O
</s>
<s>
After	O
the	O
war	O
,	O
the	O
success	O
of	O
Colossus	B-Device
encouraged	O
him	O
to	O
contemplate	O
the	O
possibility	O
of	O
telephone	O
exchanges	O
each	O
using	O
tens	O
of	O
thousands	O
of	O
valves	O
.	O
</s>
<s>
He	O
was	O
told	O
that	O
this	O
was	O
impossible	O
and	O
he	O
could	O
not	O
say	O
he	O
had	O
already	O
done	O
it	O
with	O
Colossus	B-Device
because	O
he	O
was	O
bound	O
by	O
the	O
Official	O
Secrets	O
Act	O
.	O
</s>
<s>
However	O
,	O
a	O
fully	O
electronic	O
prototype	O
Time-Division	B-Protocol
Multiplex	I-Protocol
Model	O
Exchange	O
was	O
constructed	O
at	O
the	O
Post	O
Office	O
Research	O
Station	O
at	O
Dollis	O
Hill	O
and	O
then	O
an	O
experimental	O
TDM	B-Protocol
exchange	O
system	O
was	O
built	O
and	O
tested	O
at	O
Highgate	O
Wood	O
in	O
1962	O
,	O
but	O
it	O
was	O
found	O
to	O
be	O
beyond	O
the	O
technology	O
of	O
the	O
time	O
:	O
the	O
solid-state	O
switching	O
worked	O
well	O
,	O
but	O
the	O
analogue	O
transmission	O
(	O
which	O
had	O
worked	O
on	O
the	O
short	O
cable	O
runs	O
of	O
a	O
laboratory	O
model	O
at	O
Dollis	O
Hill	O
)	O
was	O
too	O
noisy	O
for	O
public	O
service	O
on	O
the	O
long	O
cable	O
runs	O
of	O
a	O
large	O
exchange	O
.	O
</s>
<s>
)	O
,	O
which	O
were	O
to	O
be	O
central	O
to	O
the	O
development	O
of	O
the	O
British	O
TXE	B-Device
exchanges	O
.	O
</s>
<s>
At	O
this	O
time	O
,	O
in	O
the	O
US	O
,	O
Bell	O
Labs	O
were	O
developing	O
a	O
system	O
based	O
on	O
electronically	O
controlled	O
reed	B-Algorithm
relays	I-Algorithm
,	O
and	O
this	O
looked	O
promising	O
.	O
</s>
<s>
One	O
of	O
Ericsson	O
's	O
marketing	O
points	O
for	O
crossbar	O
was	O
that	O
it	O
used	O
precious-metal	O
contacts	O
,	O
but	O
reed	B-Algorithm
relays	I-Algorithm
would	O
be	O
even	O
better	O
as	O
their	O
precious	O
metal	O
contacts	O
were	O
hermetically	O
sealed	O
.	O
</s>
<s>
The	O
manager	O
at	O
AEI	O
(	O
W	O
G	O
Patterson	O
)	O
decided	O
that	O
reed-electronic	O
space-division	O
switching	O
was	O
the	O
way	O
to	O
go	O
,	O
and	O
it	O
was	O
then	O
that	O
the	O
term	O
'	O
TXE'( Telephone	O
exchange	O
electronic	O
)	O
was	O
coined	O
,	O
even	O
although	O
the	O
reed	B-Algorithm
relays	I-Algorithm
themselves	O
were	O
not	O
regarded	O
as	O
electronic	O
components	O
.	O
</s>
<s>
STC	O
built	O
the	O
common	B-Device
control	I-Device
,	O
AEI	O
the	O
switching	O
and	O
scanners	O
,	O
line	O
scanning	O
and	O
test	O
console	O
,	O
and	O
AT&E	O
the	O
dialling	O
capturing	O
equipment	O
(	O
registers	O
)	O
and	O
the	O
incoming	O
and	O
outgoing	O
junctions	O
.	O
</s>
<s>
An	O
equipment	O
practice	O
was	O
needed	O
quickly	O
,	O
and	O
it	O
was	O
realised	O
that	O
a	O
matrix	O
of	O
reed	B-Algorithm
relays	I-Algorithm
would	O
be	O
about	O
the	O
same	O
size	O
as	O
a	O
crossbar	O
switch	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
equipment	O
practice	O
of	O
E	O
's	O
crossbar	O
system	O
was	O
adopted	O
for	O
the	O
TXE1	O
apart	O
from	O
the	O
common	B-Device
control	I-Device
,	O
which	O
had	O
its	O
own	O
equipment	O
practice	O
.	O
</s>
<s>
The	O
common	B-Device
control	I-Device
consisted	O
of	O
14	O
racks	O
and	O
made	O
up	O
a	O
complete	O
suite	O
of	O
the	O
exchange	O
.	O
</s>
<s>
One	O
of	O
the	O
functions	O
of	O
the	O
common	B-Device
control	I-Device
was	O
to	O
decide	O
which	O
was	O
the	O
best	O
connection	O
to	O
be	O
used	O
through	O
the	O
switching	O
network	O
,	O
and	O
this	O
part	O
was	O
called	O
the	O
route	O
choice	O
.	O
</s>
<s>
The	O
exchange	O
used	O
reed	B-Algorithm
relays	I-Algorithm
as	O
the	O
switching	O
medium	O
,	O
and	O
the	O
reeds	O
themselves	O
were	O
approximately	O
3	O
inches	O
in	O
length	O
and	O
the	O
only	O
ones	O
available	O
.	O
</s>
<s>
It	O
also	O
had	O
a	O
test	O
console	O
,	O
which	O
monitored	O
all	O
calls	O
on	O
a	O
digital	O
edge-lit	B-General_Concept
display	I-General_Concept
.	O
</s>
<s>
They	O
would	O
then	O
run	O
this	O
compass	O
along	O
outside	O
of	O
the	O
reed	B-Algorithm
relays	I-Algorithm
,	O
and	O
when	O
a	O
relay	O
was	O
operated	O
the	O
needle	O
would	O
move	O
back	O
to	O
north	O
.	O
</s>
<s>
PBX	B-Device
,	O
shared	B-Architecture
service	I-Architecture
,	O
incoming	O
calls	O
barred	O
(	O
ICB	O
)	O
,	O
temporary	O
out	O
of	O
service	O
(	O
TOS	O
)	O
etc	O
.	O
</s>
<s>
This	O
was	O
a	O
capacitor	B-General_Concept
store	I-General_Concept
,	O
and	O
it	O
held	O
information	O
on	O
a	O
thin	O
plastic	O
strip	O
,	O
into	O
which	O
could	O
be	O
inserted	O
up	O
to	O
10	O
little	O
copper	O
squares	O
which	O
had	O
a	O
capacitance	O
of	O
10pF	O
.	O
</s>
<s>
This	O
information	O
was	O
then	O
pulsed	O
by	O
the	O
common	B-Device
control	I-Device
translator	O
and	O
appropriate	O
action	O
taken	O
.	O
</s>
<s>
This	O
system	O
was	O
replaced	O
in	O
later	O
TXE	B-Device
exchanges	O
by	O
Dimond	B-General_Concept
rings	I-General_Concept
.	O
</s>
<s>
Incoming	O
registers	O
used	O
a	O
time-shared	O
electronic	O
dial	O
path	O
(	O
TDM	B-Protocol
)	O
to	O
transfer	O
pulsing	O
information	O
from	O
the	O
incoming	O
junction	O
to	O
the	O
incoming	O
register	O
.	O
</s>
<s>
These	O
were	O
provided	O
by	O
lead	B-Device
–	I-Device
acid	I-Device
batteries	I-Device
charged	O
from	O
the	O
mains	O
supply	O
,	O
backed	O
up	O
by	O
a	O
diesel	O
generator	O
.	O
</s>
<s>
Most	O
of	O
these	O
were	O
caused	O
in	O
the	O
common	B-Device
control	I-Device
area	O
.	O
</s>
<s>
The	O
common	B-Device
control	I-Device
equipment	O
was	O
divided	O
into	O
functional	O
units	O
,	O
and	O
each	O
unit	O
was	O
duplicated	O
,	O
an	O
A	O
side	O
and	O
a	O
B	O
side	O
,	O
and	O
each	O
section	O
was	O
isolated	O
by	O
means	O
of	O
reed	B-Algorithm
relays	I-Algorithm
.	O
</s>
<s>
The	O
changeover	O
of	O
the	O
relays	O
was	O
controlled	O
by	O
a	O
series	O
of	O
reed	B-Algorithm
relays	I-Algorithm
whose	O
reed	O
inserts	O
were	O
wetted	O
with	O
mercury	O
.	O
</s>
<s>
This	O
happened	O
over	O
all	O
sections	O
of	O
the	O
electronic	O
equipment	O
in	O
the	O
common	B-Device
control	I-Device
area	O
.	O
</s>
<s>
These	O
units	O
,	O
unlike	O
the	O
common	B-Device
control	I-Device
,	O
were	O
hard-wired	O
.	O
</s>
<s>
However	O
,	O
they	O
were	O
hard-wired	O
to	O
the	O
common	B-Device
control	I-Device
translator	O
.	O
</s>
<s>
It	O
was	O
therefore	O
mainly	O
used	O
to	O
replace	O
the	O
larger	O
rural	O
Strowger	B-Device
exchanges	I-Device
–	O
usually	O
UAX13s	O
.	O
</s>
<s>
Because	O
of	O
their	O
common	B-Device
control	I-Device
design	O
,	O
isolation	O
(	O
inability	O
of	O
the	O
exchange	O
to	O
set	O
up	O
calls	O
)	O
of	O
the	O
entire	O
exchange	O
was	O
always	O
a	O
possibility	O
and	O
very	O
occasionally	O
happened	O
.	O
</s>
<s>
This	O
potential	O
weakness	O
had	O
been	O
at	O
least	O
partially	O
recognised	O
in	O
the	O
design	O
of	O
the	O
exchange	O
type	O
,	O
so	O
the	O
most	O
critical	O
common	B-Device
control	I-Device
units	O
were	O
split	O
into	O
three	O
sections	O
and	O
each	O
section	O
was	O
duplicated	O
into	O
an	O
A-side	O
and	O
a	O
B-side	O
.	O
</s>
<s>
Unlike	O
the	O
preceding	O
rural	O
Strowger	B-Device
exchanges	I-Device
(	O
UAX	O
13s	O
and	O
smaller	O
)	O
the	O
TXE2s	O
were	O
equipped	O
with	O
an	O
uninterruptible	O
power	O
supply	O
with	O
auto-starting	O
diesel	O
generators	O
.	O
</s>
<s>
If	O
more	O
than	O
eight	O
call	O
failures	O
were	O
detected	O
in	O
less	O
than	O
8	O
minutes	O
,	O
then	O
the	O
critical	O
common	B-Device
control	I-Device
units	O
would	O
be	O
forced	O
to	O
change	O
from	O
the	O
side	O
in	O
service	O
(	O
A	O
or	O
B	O
)	O
to	O
the	O
other	O
side	O
,	O
the	O
automatic	O
8-minute	O
changeover	O
would	O
be	O
suspended	O
and	O
a	O
prompt	O
alarm	O
would	O
be	O
sent	O
out	O
.	O
</s>
<s>
The	O
common	B-Device
control	I-Device
equipment	O
consisted	O
of	O
B	O
-	O
and	O
C-switch	O
selectors	O
,	O
supervisory	O
selectors	O
(	O
a	O
supervisory	O
relay	O
set	O
stayed	O
in-circuit	O
throughout	O
each	O
call	O
)	O
,	O
Register	O
Selectors	O
,	O
Registers	O
and	O
Call	O
Control	O
.	O
</s>
<s>
If	O
the	O
A-switch	O
group	O
contained	O
many	O
busy	O
PBX	B-Device
lines	O
,	O
then	O
the	O
number	O
of	O
customers	O
might	O
be	O
reduced	O
to	O
75	O
.	O
</s>
<s>
The	O
choice	O
thus	O
went	O
to	O
the	O
Dimond	B-General_Concept
ring	I-General_Concept
type	O
of	O
memory	O
,	O
named	O
after	O
T	O
.	O
L	O
.	O
Dimond	O
of	O
Bell	O
Laboratories	O
,	O
who	O
invented	O
in	O
1945	O
.	O
</s>
<s>
This	O
was	O
a	O
considerable	O
innovation	O
in	O
British	O
exchanges	O
since	O
in	O
Strowger	B-Device
exchanges	I-Device
the	O
directory	O
and	O
equipment	O
numbers	O
had	O
to	O
be	O
the	O
same	O
.	O
</s>
<s>
The	O
switching	O
in	O
TXE2s	O
was	O
carried	O
out	O
by	O
reed	B-Algorithm
relays	I-Algorithm
and	O
a	O
typical	O
TXE2	O
contained	O
about	O
100,000	O
reeds	O
.	O
</s>
<s>
Switching	O
with	O
these	O
reeds	O
held	O
out	O
the	O
prospect	O
of	O
much	O
greater	O
reliability	O
compared	O
with	O
the	O
Strowger	B-Device
system	I-Device
,	O
where	O
switching	O
was	O
carried	O
out	O
by	O
base	O
metal	O
wipers	O
moving	O
through	O
banks	O
of	O
metal	O
contacts	O
.	O
</s>
<s>
The	O
Strowger	B-Device
switches	I-Device
required	O
routines	O
to	O
be	O
carried	O
out	O
on	O
them	O
to	O
clean	O
the	O
banks	O
:	O
they	O
also	O
required	O
oiling	O
and	O
occasional	O
adjustment	O
.	O
</s>
<s>
Reed	B-Algorithm
relays	I-Algorithm
required	O
none	O
of	O
this	O
.	O
</s>
<s>
As	O
the	O
first	O
steps	O
in	O
dealing	O
with	O
the	O
new	O
call	O
the	O
MCU	O
recorded	O
the	O
directory	O
number	O
and	O
class	O
of	O
service	O
(	O
shared	B-Architecture
service	I-Architecture
,	O
PABX	O
line	O
,	O
incoming	O
junction	O
register	O
,	O
TOS	O
and	O
so	O
on	O
)	O
information	O
offered	O
by	O
the	O
Cyclic	O
Store	O
and	O
allocates	O
one	O
of	O
its	O
associated	O
group	O
of	O
up	O
to	O
30	O
registers	O
.	O
</s>
<s>
The	O
switching	O
network	O
was	O
composed	O
of	O
reed	B-Algorithm
relay	I-Algorithm
cross-points	O
arranged	O
to	O
give	O
three	O
(	O
A	O
,	O
B	O
and	O
C	O
)	O
stages	O
of	O
switching	O
on	O
either	O
side	O
of	O
a	O
number	O
of	O
linking	O
circuits	O
.	O
</s>
<s>
"	O
X	O
"	O
and	O
"	O
Y	O
"	O
phases	O
were	O
only	O
required	O
to	O
support	O
the	O
metering	O
for	O
shared	B-Architecture
service	I-Architecture
subscribers	O
,	O
which	O
has	O
long	O
since	O
disappeared	O
thankfully	O
.	O
</s>
<s>
The	O
TXE	B-Device
3	O
model	O
gave	O
satisfactory	O
service	O
and	O
the	O
experience	O
gained	O
from	O
the	O
model	O
confirmed	O
the	O
validity	O
of	O
the	O
basic	O
design	O
and	O
led	O
to	O
the	O
development	O
of	O
the	O
TXE4	O
.	O
</s>
<s>
It	O
was	O
built	O
at	O
the	O
STC	O
Southgate	O
factory	O
in	O
north	O
London	O
and	O
used	O
reed	B-Algorithm
relays	I-Algorithm
as	O
the	O
switching	O
medium	O
which	O
proved	O
reliable	O
in	O
service	O
.	O
</s>
<s>
It	O
had	O
a	O
programmable	O
common	B-Device
control	I-Device
called	O
the	O
Main	O
Control	O
Unit	O
(	O
MCU	O
)	O
and	O
each	O
exchange	O
had	O
at	O
least	O
three	O
MCUs	O
for	O
security	O
and	O
a	O
maximum	O
of	O
twenty	O
,	O
but	O
in	O
theory	O
,	O
could	O
operate	O
with	O
just	O
one	O
.	O
</s>
<s>
The	O
switching	O
was	O
via	O
reed	B-Algorithm
relays	I-Algorithm
and	O
multi-stage	O
like	O
the	O
TXE1	O
.	O
</s>
<s>
Subscriber	O
information	O
was	O
programmed	O
into	O
the	O
exchange	O
in	O
racks	O
called	O
cyclic	O
stores	O
which	O
used	O
PTFE	O
wire	O
threaded	O
through	O
magnetic	O
cores	O
known	O
as	O
'	O
Dimond	B-General_Concept
ring	I-General_Concept
 '	O
(	O
see	O
TXE2	O
section	O
for	O
more	O
information	O
)	O
.	O
</s>
<s>
PBX	B-Device
,	O
coin	O
collecting	O
box	O
(	O
CCB	O
)	O
or	O
single	O
line	O
,	O
followed	O
by	O
the	O
directory	O
number	O
.	O
</s>
<s>
When	O
a	O
subscriber	O
lifted	O
their	O
handset	O
it	O
sent	O
a	O
pulse	O
down	O
this	O
wire	O
,	O
which	O
was	O
picked	O
up	O
by	O
a	O
156ms	O
scanner	O
,	O
which	O
set	O
up	O
a	O
path	O
through	O
the	O
reed	B-Algorithm
relays	I-Algorithm
to	O
a	O
register	O
.	O
</s>
<s>
TXE4A	O
was	O
the	O
last	O
in	O
the	O
line	O
of	O
TXE	B-Device
exchanges	O
and	O
an	O
improved	O
version	O
of	O
the	O
TXE4	O
.	O
</s>
<s>
It	O
had	O
the	O
same	O
switching	O
as	O
TXE4	O
but	O
an	O
redesigned	O
common	B-Device
control	I-Device
,	O
using	O
integrated	O
circuits	O
(	O
including	O
microprocessors	O
)	O
to	O
achieve	O
significant	O
size	O
and	O
cost	O
reductions	O
.	O
</s>
<s>
TXE4A	O
dispensed	O
with	O
Dimond	B-General_Concept
rings	I-General_Concept
and	O
used	O
solid	O
state	O
memory	O
.	O
</s>
<s>
customer	O
information	O
to	O
be	O
made	O
by	O
keyboard	O
instead	O
of	O
by	O
manually	O
threading	O
jumpers	O
through	O
Dimond	B-General_Concept
rings	I-General_Concept
.	O
</s>
<s>
The	O
enhancement	O
software	O
was	O
implemented	O
on	O
Intel	O
8080	O
,	O
286	O
and	O
386	O
processors	O
running	O
on	O
RMX	B-Operating_System
.	O
</s>
<s>
TXE6	O
was	O
an	O
electronic	O
common	B-Device
control	I-Device
exchange	O
that	O
was	O
designed	O
to	O
extend	O
Strowger	B-Device
exchanges	I-Device
,	O
and	O
known	O
as	O
the	O
Electronic	O
Reed	O
Selector	O
System	O
or	O
Reed	O
Group	O
Selector	O
(	O
RGS	O
)	O
.	O
</s>
<s>
The	O
TXE6	O
went	O
into	O
service	O
on	O
the	O
night	O
of	O
18	O
August	O
1971	O
and	O
proved	O
very	O
reliable	O
until	O
withdrawn	O
from	O
service	O
in	O
1977	O
,	O
when	O
a	O
TXE	B-Device
4	O
exchange	O
took	O
over	O
from	O
the	O
TXE	B-Device
1	O
and	O
three	O
TXE2	O
exchanges	O
.	O
</s>
<s>
unit	O
was	O
the	O
interface	O
,	O
converting	O
information	O
from	O
dialled	O
pulses	O
,	O
in	O
Strowger	B-Device
form	O
,	O
to	O
fast	O
parallel	O
signal	O
conditions	O
for	O
the	O
reed	O
group-selector	O
registers	O
.	O
</s>
<s>
DateAction1956JERC	O
formed1959Prototype	O
TDM	B-Protocol
Model	O
Exchange	O
Dollis	O
Hill1962Highgate	O
Wood	O
Telephone	O
Exchange1963JERC	O
agrees	O
to	O
return	O
TDM	B-Protocol
to	O
research	O
and	O
concentrate	O
development	O
on	O
Reed	O
SystemsTXE1	O
development	O
startedPlessey	O
started	O
development	O
of	O
TXE2	O
1964TXE3	O
design	O
initiated	O
1965TXE2	O
Field	O
Trial	O
started	O
at	O
Peterborough	O
1966First	O
TXE2	O
in	O
service	O
at	O
Ambergate1968First	O
and	O
only	O
TXE1	O
opens	O
at	O
Leighton	O
Buzzard	O
TXE3	O
trial	O
started	O
,	O
then	O
abandoned	O
by	O
GEC	O
for	O
commercial	O
reasons	O
.	O
</s>
