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TCD Siemens T-56 Fault finding →
Circuit diagram
On this page, the circuit diagram of the
Siemens T-56 teleprinter connection device (TCD).
is discussed. Although the T-56 is a seemingly simple device,
its relay based circuits are extremely difficult to understand.
This is mainly caused by the fact that the T-56 can be configured
for a variety of operating modes, such as 2-wire, 4-wire,
single-current and double-current. Below are several edited
variants of the circuit diagram that should help to understand its many
functions.
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Below is the original circuit diagram, extracted from the
original 1962 manual. There are many relays that operate
a 'sea of contacts'. Some relays are controlled by the
push-buttons at the bottom edge of the
control panel.
In addition there is a
3-position MODE-selector with a complicated
array of contacts that affects the circuit in various ways, as discussed
in more detail below.
➤ Original circuit diagram
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The configuration of the MODE-selector is
shown at the bottom centre of the circuit diagram, and is marked
Betriebsartenschalter U1/U2.
It is a complex multi-deck switch array that is located at the left side
of the device, just below the control panel.
It has the following settings:
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Below is an extract from the original circuit diagram that shows
the configuration of the MODE-selector. It consists of two independent
sections: the upper part (U1) and the lower part (U2). The selector
is operated with a pivoting lever at the centre. If the lever is moved
down, only switch U1 moves up (U2 does not). Likewise, if the lever is moved up,
switch U2 moves down (U1 does not).
In the original diagram, the contacts of the MODE-selector are
shown in 4-wire double-current mode. This means that the switches
U1 and U2 are not engaged.
Note however, that some of the contacts are single-pole double-throw (SPDT),
such as sections II, III, VI and VII of U1.
Others are simple 'make' contacts (normally open), such as section IV and VIII
of U1.
The same is true for U2. In the circuit diagram, the contacts of U1 and U2
are scattered all over the place.
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The configuration of the push-buttons is shown at the bottom right of
the circuit diagram. Some of them are a quite complex mixture of
double-pole double-throw (DPDT), normally-open and normally-closed
contacts. In the diagram, the switches are shown in 'not engaged' position.
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The mains transformer (Tr1) is at the bottom right of the circuit
diagram. It has two secondary windings with a bridge rectifier each,
followed by two electrolytic capacitors. They produce the +60V (+TB)
and -60V (-TB) voltages for the line current. The current is limited
by two series connected lamps that act as a postitive temperature
coefficient resistor (PTC). The lamps are an early form of
a resettable fuse.
The +60V (+TB) and -60V (-TB) voltages are used for providing line current
when the teleprinter is used in local mode. This is discussed further below.
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The current meter (J) is visible at the bottom of the circuit diagram,
between the MODE-selector and the push-buttons. Is consists of a
50 mV meter and a double-pole 4-position rotary selector that switches
both sides of the meter. It allows the meter to measure the voltage
over a resistor, at four different places in the circuit. This is
a measure for the current through that resistor.
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| | German | English | Description |
| S | Sender | Transmitter | Transmission circuit current |
| E | Empfänger | Receiver | Reception circuit current |
| JT | Impuls Telegrafie | 2-wire double-current | Null setting (in 2 Dr 
mode) |
| Fs | Fernschreiber | Teleprinter | Teleprinter current (default) |
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Annotated circuit diagrams
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Relay and push-button contacts
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Below is an annotated circuit diagram of the T-56, in which the push-buttons
and the indicator lamps are marked with the colours of their caps. Use the
enlarge function of your browser to zoom in and view the circuit diagram in
more detail.
The function push-buttons are marked with coloured circular dots,
whilst the two polar relays (A and B) are marked with coloured diamonds.
Note that polar relay B (with blue diamonds) has four coil sections (!)
— located at different positions in the circuit diagram —
but just a single SPDT contact.
The contacts of both polar relays are connected to a filter section (FE1, FE2),
that is located between them. They are not shown in the original circuit diagram
and are not essential for a good understanding of the circuit.
Move the mouse over the diagram above to see the relationships between the
various relays and their (many) contacts. For each relay a different colour
is used. You may use this as a reference when reading the circuit description.
It may also be useful when reparing a broken T-56 unit.
In the original circuit diagram, the position of the contacts of the
MODE-selector are shown in the
4-wire double-current mode. This is the
middle position of the selector, marked 4 Dr
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The table above is shown at the bottom left of the
original circuit diagram.
It shows the complex arrangement of the relay contacts. To under stand this,
we need to simplify the circuit diagram. This is done by merging the push-button
contact arrangements and the current meter with the existing circuit diagram.
All switches that are related to the MODE-selector have been coloured green,
so that they can easily be recognised and discriminated from the other switches.
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Simplified circuit diagrams
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For a better understanding of the circuit, we have simplified copies of
the diagram in such a way that the MODE-selector has been eliminated — as if it
is hard-wired for the selected mode. Below we will show the simplified for
each individual mode, starting with 2-wire double-current.
We will only discuss the circuit in detail for the 2-wire double-current mode.
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Below is a simplified circuit diagram in which the contacts of the
MODE-selector have been eliminated and the parts that are not used
(in this mode) have been erased.
It shows the device as if it was
hard-wired for 2-wire single-current operation.
Place the mouse over the image to show the original circuit diagram,
with all contacts of the MODE-selector in the 2 Dr
position.
Idle state
The exchange is connected and delivers a voltage of +60V at
terminal a1 and -60V at terminal b1. Current cannot flow, since C2
blocks the DC current path.
In addition, polar relay B is forced to the Z-position. 1
This goes from +60V,
through its contact, Wi29, Wi9 and then through one of its windings
13-12 (2000Ω) to GND. As soon as switch-over has occurred, there is
no voltage left on this winding (12-13), because polar relay A
winding (1-4-5-8) ...
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The 'Z' position (German: Zeichen) is also known as the 'mark' position.
In corresponds to the relay contact pointing to the front of the device.
This can be difficult to see, as the contacts are just 0.04 mm apart.
The other contact position is known as 'T' (German: Trennung), which is
known as the 'space' position.
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User wants to call
The user presses the AT (green) button. A 40mA current will now flow:
from terminal a1, following the path through resistor Wi1a, going through
cII2, then uhIII1, h1III1, through the teleprinter receiver relay (EM),
through uhII1 and then through LsVI,
through the Tape Reader contact and through LsII, through
the engaged ATI contact, through polar relay B windings 10-9 and 5-6 to the
b1 terminal. The exchange recognizes that we want something.
Next, relay C will engage because of the key press contact ATII.
Current goes from +60V through contact l1, via ATII through relay C (3-4)
to uII and then to -60V.
Relay C is a hold-relay: Contact cIII2 will keep it engaged.
Also, relay C results in the bridging of Wi24 and C2 because contact c:III1}
is engaged. Another result of this contact is that resistor Wi27 is switched
away to give polar relay B windings (10-9-5-6) its maximum sensitivity.
The 40mA will - again - force relay B into its Z-position. Another thing with
polar relay B is that contact cI1 ensures bias through the relay core via
+60V, contact cI1, through contact azI2, resistor Wi25 and relay B
windings (12-13). This is to partially compensate for the 40mA line current already
going through the other windings en so to ensure maximum sensitivity for relay B.
So now the AT-button can be released, and relay C will stay engaged.
The 40mA current from the exchange will stay.
Free line indicator
The exchange will give a 'free line' indication by cutting the loop current
of 40mA for 25 ms. Winding (12-13) of polar relay B now wins, and
the contact will go to the T-position.
Current will flow from +60V, through contact b of relay B, Wi29, cI2
(relay C is engaged), Wi10 to activate relay AZ (1-2), going to -60V.
Relay AZ is a bit of a mean beast having 2 seperate windings.
One of its own contacts azII will now keep AZ engaged: +60V going through
contact azII, through the 2nd AZ relay winding (3-4), contact cII1,
resistor Wi26 to -60V.
Next, contact azIII turns the green lamp on.
Another contact azI1 forces resistor Wi27 over polar relay B windings to
make this relay less sensitive to line changes. The same counts for contact
azI2, switching away Wi25 to disable polar relay B compensation current:
This is necessary, since we want to go and dial now to reach the other party.
Relay B may not change its position during the dial process.
So it is less sensitive now.
Dialling out to the other party
Contact nsi is the dial interruptor contact, i.e. a normally closed
contact that is periodically interrupted. Note that relay C still is
engaged, so contact cII2 is open, so enabling the nsi contact.
As soon as the number has been dialled, the exchange checks
for the other party's availability. As soon as it recognizes that it's free,
the exchange will change polarity on terminals a1 and b1:
a1 becomes -60V and b1 becomes +60V.
The current reverses to -40mA. Because of this, polar relay B switches
over to the T-position and will now engage relay L via +60V,
polar relay contact b, Wi29, Wi18, L (3-4) (6000Ω) Wi19 to -60V.
C9 and Wi17 form a delay circuit in parallel with relay L.
Relay L will turn on the red lamp via contact lII.
But relay L also breaks the circuit for relay C.
As soon as contact l1 switches over, the holding current for relay C
is gone and it will drop-off, also releasing relay AZ. The green lamp
goes off. The release of relay C also switches over contact cII1
so that resistor Wi27 is connected in parallel with polar relay windings
(10-9-5-6).
The -40mA line current still is there, since H1 is engaged by closing
contact I1. H1 closes contact h1III2, enabling the current to
flow backwards from terminal b2, to polar relay B (6-5-9-10)
parallel to resistor Wi27,
to contact ST1, contact uIII2, h1III2, going down and to LsIII
etc. Relay H1 also turns the motor on, with the h1I1 and h1I2 contacts.
The teleprinter now is ready for use.
End call on our side
The button ST (red) must be pressed for at least 2 seconds.
By pressing ST, contact ST1 above C2 is opened. The capacitor C2 first
charges. As soon as the other party breaks the connection as well,
the exchange will swap the polarity of the line. So a1 will
become +60V and b1 will become -60V. With this action, the complete
charge of the capacitor will pulse force polar relay B to switch
its contact b to the Z-position.
With C9 charged, relay L will be engaged for a while. But it will
drop-off due to the discharge of C9 anyway. As soon as relay L drops-off,
the red light will go off as well. Due to the fact that contact l1 is open,
relay H1 will also drop-off. Relay H1 contact h1III2 finally breaks the
line completely. Also, the motor is stopped by opening of contacts
h1I1 and h1I2.
The other party calls us
Normally, the line is open and +60V is on terminal a1 and -60V is at
terminal b1. Therefore C2 is charged with this voltage. As soon as the
other party calls, the polarity of the line is swapped.
The result is that C2 will pulse-discharge and drives polar relay B
coil (6-5-9-10). As a result, its contact swithes over to the T-position.
Relay L is engaged and this relay engages relay H1.
The opening of the h1III1 contact allows the teleprinter
send-contact to transmit characters. Also, by engaging the H1 relay,
both teleprinter motor contacts engage h1I1 and h1I2 contacts.
The motor turns on. Now the other party can send us messages,
and we even can send messages back if we want, but of course
one at a time, due to the simplex nature of teleprinter machines.
The other party finishes the session, and breaks the connection
As soon as the other end finishes the connection, the exchange
swaps current polarity. On terminal a1 +60V and on terminal b1 -60V will
be set. So the current will go the other way around. The polar relay B
is engaged through the immediate charge pulse from C2 and switches its
contact b to Z-position. Now the circuit around relay AZ and C2 does not
have +60V via resistor Wi29 anymore. C9 will discharge. and after a short
moment relay L will be disengaged. As a result, contact lI opens.
This disables the current through H1, so H1 drops-off as well and
the teleprinter's motor stops. Contact h1III2 finally breaks
the DC current in the line.
Local operation to compose messages before connection
The power supply of +60V and -60V inside the T-56 enables local
operation. This creates an in-house telex current loop, that allows
the user to type messages, punchtapes and read text from tapes.
To start local operation push-button Fs Loc must be pressed
(the yellow button).
Next, Relay U will be engaged: +60V goes through the l1 contact,
through diode Gr5, through the FsLocI contact, through the coil
of Relay U (3-4) and then to -60V. Because relay U is engaged,
it will latch itself by means of its own contact uI2. Relay U
lets contact uII switch over, so that relay C is unpowered
and can never switch the line. In addition, contact uIII1 of
relay U switches to -60V, and so relay UH is engaged as the Fs Loc
is pressed. Contact uhI2 also latches the UH relay.
Contact uhII2 switches on the yellow lamp inside the push-button.
Engaging Relay UH ensures that contacts uhII1 and uhIII1
are closed, so that local current can flow through the teleprinter:
+60V goes to Wi3 through the LsIV contact, going up via the
engaged uhIII1 contact to terminal a of the teleprinter.
Terminal C of the teleprinter goes via engaged contact uhII1
via coil Dr4 to GND. The contacts of relay UH enable relay H1
so that the teleprinter motor runs through the now closed contacts
h1I1 and h1I2. Next, the H1 relay releases the h1III1 contact
so that the teleprinter transmit contact is free for transmission.
The tape reader (Ls) can be used for checking a previously punched
tape. This is done by pressing the orange Ls button.
Since this is a toggle switch, all Ls
contacts will switch over (SPDT) or engage (n.o.). So LsIV
will disengage, but LsII will engage: the +60V now goes through
the tape reader contact before it goes to contact LsVI, then
to the engaged uhIII1 contact, and finally via the teleprinter
transmit contact to terminal a.
Finish Local operation
As soon as the user is ready with the tapes, the T-56 must be set
back to normal online operation.
This is done by pressing the Fs Lin button.
What then happens is that the FsLinI contact breaks the current
through relay UH. As a result, it will no longer be latched.
The yellow lamp will also go off. Next, the uhI1 contact drops-off
and relat H1 switches off as well. Contacts h1I1 and H1I2
will now open and the teleprinter motor is turned off.
Next, H1 relay contact h1III1 shorts the teleprinter's
transmit contact. From the disengaged UH relay, contacts uhIII1
and uhIII2 will reconnect the teleprinter to the line.
While pressing button Fs Lin, its FsLinII contact shorts
relay U momentarily, until the button is released.
Relay U will remain engaged for ~45 ms while the Fs Loc button
is pressed, due to the stored magnetic energy in its core
(letting current flow due to a short circuit, delays the decay of the
magnetic field). This is done to ensure that contact h1III2
is opened first before uIII2 is released.
Otherwise it would be like setting up a new call, and this is not
what we want here.
Answer the line while in local operation
Imagine the situation where the user is using the device in Local
mode – for example to check a punched tape – when a call comes in.
In that case, the T-56 is in local mode and both the yellow lamp
(Fs Loc) and the organge lamp (Ls Loc) are on.
In this situation relay UH is engaged.
Contact uhIII2 lets the line current flow from terminal a1,
through Wi1a, then through contact uhIII2 and through coil DR1.
LSII and LSIII are closed, so via Wi24 capacitor C2 can be charged
via terminal b1.
When a new call comes in, the charge in C2 will flip polar relay B,
and the rest goes as if the line is setup by the user.
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Below is a simplified circuit diagram in which the contacts of the
MODE-selector have been eliminated and the parts that are not used
(in this mode) have been erased.
It shows the device as if it was
hard-wired for 4-wire double-current operation.
Place the mouse over the image to show the original circuit diagram,
with all contacts of the MODE-selector in the 4 Dr
position.
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Below is a simplified circuit diagram in which the contacts of the
MODE-selector have been eliminated and the parts that are not used
(in this mode) have been erased.
It shows the device as if it was
hard-wired for 2-wire single-current operation.
Place the mouse over the image to show the original circuit diagram,
with all contacts of the MODE-selector in the 2 Dr
position.
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The T-56 contains a hybrid circuit to convert the 2-wire
teleprinter signals, to the 4-wire domain, with full echo cancellation.
The hybrid is not trival however, as it is implemented with
two polar relays,
with multiple coils each. Various sub-circuit contribute to the currents
through these coils. The hybrid is scattered all over the circuit diagram,
as a result of which it is not easily recognised.
In the above block diagram of the T-56, the hybrid is in position (1).
In addition to the 2/4-wire hybrid described above, the T-56 contains
another hybrid, which converts the 4-wire signals to a single 2-wire
subscriber line (3). Again, this second hybrid is not easily recognised in
the circuit diagram, as it is integrated with the primary hybrid and with
the rest of the circuit. For a good understanding of the working of
a hybrid built with polar relays, read this article.
➤ Detailed description of a hybrid circuit
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© Crypto Museum. Created: Tuesday 03 September 2024. Last changed: Wednesday, 11 September 2024 - 19:13 CET.
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