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Telex hybrid circuit
A telex hybrid is an important part of a
teleprinter connection device (TCD).
It can combine and split two individual signals without one 'seeing' the other.
This is done to avoid delayed echos on long haul telegraphy lines. It fully
separates the two-way traffic — the transmission path and the reception path —
in the same way as the hybrid circuit in a telephone set
does for speech.
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Equipment with built-in hybrid on this website
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A hybrid, or fork circuit, is a little known circuit. In
this article, we will try to explain how it works. The function of a hybrid is
to merge the transmit (TX) and receive (RX) signals into a combined signal.
This combined signal is then carried to the other end, where another hybrid is used
to separate the individual TX and RX signals again.
In a block diagram, the hybrids look like this:
Generally speaking, there are two types of hybrid:
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- Analogue audio hybrid
This type of hybrid is used in old analogue telephone sets
on the POTS telephone network, to avoid echos. It merges
the signals from microphone and speaker in such a way that
a caller doesn't hear an echo of his own voice.
- Digital current hybrid
This type of hybrid circuit is used on telegraphy (telex) networks. It does the
same as an analogue hybrid, but then for digital current-switched signals.
It is used to avoid endless character-loops resulting from delayed
echos in the network. This type is described here.
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Simple teleprinter circuit
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Each teleprinter has a so-called transmit contact (TC) and a
receive magnet (RM).
In German literature these are known as Sende-Kontakt (SK) and
Empfangst-Magnet (EM) respectively.
The TC is opened and closed in quick succession,
according to the binary pattern of the selected character. At the other end
of the line,
the current from the TC activates the receive relay (RM) in the same
pattern. This is then used to select the same character and print it
on paper. In most cases, the TC and RM at either side are connected in
series, as shown in the diagram below.
The current flows through the TC and RM of both machines
simultaneously. As the TC is a normally-closed contact, this means
that in rest, the current flows through all parts. When the user presses
a letter on the keyboard, the binary pattern of that letter (with the addition
of start and stop bits) is used to interrupt the TC. Anything typed on
the teleprinter at the left, appears simultaneously on both machines.
The same happens when typing on the other machine.
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A letter typed on they board of a teleprinter is immediately
reproduced on the built-in printer, as in most situations the
transmit contact (TC) and receive magnet (RM) are connected
in series. This is known as a local echo.
It is also transmitted to the other end of the line,
where reproduced on the remote printer. In practice, the
remote reproduction of the character, may cause an echo on the
line, that may then cause it to be printed again on the local
printer. On short communication lines, this should normally
not be a problem, as the remote echo coincides with the local
echo.
On longer lines however, there might be a considerable delay,
as a result of which the remote echo comes later than the
local echo, and interferes with the local signal.
This introduces 'false bits', as a result of which
the wrong character will be printed on paper.
In addition,
it might caused another echo, which is then transmitted, and
so on. And this is precisely why we need to connect a
Teleprinter Connection Device (TCD)
between our teleprinter and the line.
In practice, signal delays between 1 ms and 50 ms were quite common.
A TCD contains a hybrid that is designed to suppress delayed
echos on long telex lines.
The delay depends on the route of the telex signal, the length of
the line and all patching, relaying and multiplexing equipment on
the way. In the example above, two teleprinters are connected over
a 4-wire double-current telex line.
We took the 4-wire double-current configuration as it is explained
more easily.
The diagram above shows a random example of a more complex route from the
subscriber to the exchange. In this example the signal runs over a combination
of 2-wire and 4-wire double current longe distance lines, and partly over a
time- or division-multiplexed long distance line.
As you an probably imagine, each of these sub-paths adds to the overall delay.
➤ Other configurations
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In old telex equipment and TCDs, polar relays
– also known as latching relays – are often used as part of their
circuitry. A polar relay has a minimum of two coils, one for each of its
two possible states. Driving coil A (Ia) manipulates the relay's armature
to the left, and closes contact (a). The C-shaped
core of the polar relay is mildly pre-magnetised, so that the armature
sticks in the last state once the current through the coil is removed.
Briefly driving a current through coil (B), moves the armature to the
right and closes contact (b). This is illustrated in this diagram:
Instead of using two separate coils, the above can also be realised
with a single coil, or with two series-connected coils with a centre contact.
In that case, all we have to do, is reverse the
current through the coil to force the armature to its alternate position.
But there is another property that is often overseen.
If you drive some bias current through coil A (Ia), it generates a
small magnetic field that keeps the armature in position (a).
But if you drive more current through coil B (Ib), the
magnetic field of coil B will cancel out the magnetic field of coil A
and will eventually 'win', 1 causing the relay to
switch over to position (b).
This property of the polar relay — i.e. the 'fighting currents' —
is used in old equipment to create a hybrid.
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Trls-64a polar relay with seven coils
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To complicate things, some polar relays have more than two individual
coil windings. A good example is the
Siemens Trls-64a polar relay,
that is used in the Siemens T-56 TCD. It has seven
individual windings (w1-w7), two of which are connected in series (w6, w7).
The diagram above shows the Trls-64a. In reality, the seven individual
windings are wound onto a single spool.
The windings can be used for al kinds of features, but the most important
function of the T-rls-64a in the T-65 TCD, is to offer
the hybrid functionality in all available
line configurations (modes).
➤ More about the older Trls-43a polar relay
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In old Siemens service documentation, they talk about Ampere-windings
to express the force generated in an electromagnet. It depends on the
current as well as the number of windings on the coil.
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The circuit diagram below shows a simplified circuit diagram of
a teleprinter connection device (TCD), in a
4-wire double-current configuration.
The diagram can roughly be divided in four parts: (1)
teleprinter circuit, (2) compensation circuit, (3)
line circuit and (4) power supply (PSU).
There are two polar relays (Y1, Y2), one of which (Y1) has two
individual windings (1,2 and 3,4).
First, assume that the line is in rest and that nothing is sent or
received. In that case, the relay contacts are as shown above.
The left part – the teleprinter circuit – is pretty basic.
The teleprinter has its normally closed transmit contact (TC)
and its receive magnet (RM) connected in series.
The PSU circuit at the bottom supplies the necessary
+60V and -60V line voltages.
The current in the teleprinter circuit flows as follow:
From +60V to the contact y2, then through one of the coils (2,1) of
polar relay Y1, through resistor Rt and then to the teleprinter
transmit contact (TC), the teleprinter receive magnet (RM)
and finally to the -60V supply. The total voltage in this circuit is
120V (2 × 60V). The resistance of the coil of Y1 (RY1) is estimated at
500Ω. The current in the teleprinter circuit ( It ) is 40mA,
which is calculated as follows:
Another current flows through the compensation circuit ( Ic ).
From the +60V supply, the current goes through contact y2,
through winding (3,4) of polar relay Y1, then through resistor Rc
and finally to GND (0V). It is obvious that the current in the
compensation circuit is half that of the teleprinter circuit, since:
As It is dominant — it is twice as large as Ic —
the left branch of polar relay Y1 (2,1) 'wins'. As a result, its
contact (y1) will remain in the leftmost position (+60V).
Move the mouse over the circuit diagram above to see the dominant
current as a blue line.
Note that the rightmost section (line circuit) only contains
transmit contact (y1) and receive relay coil (Y2). Note the dots
aside the contacts and aside the coils of the polar relays. Current through
the coil in the marked direction, forces the corresponding contact
to the marked position.
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Transmitting something towards the exchange is pretty straight forward.
As soon as we press a key on the teleprinter, the teleprinter's transmit
contact (TC) will open and close in the rythm of the binary pattern of
the selected character. If TC opens, it causes the current in the left
winding of Y1 (2,1) to momentarily drop to 0 mA. As there is still a 20 mA
current in the right winding of Y1 (3,4), this causes relay contact (y1)
to switch to the rightmost contact (-60V). This effectively transmits a single
bit to the line. If TC closes, 40 mA will flow again through winding 2,1
of Y1, as a result of which it 'wins' again, causing contact y1 to return
to its leftmost positon.
The diagram above shows the dominant current path when the system is at rest.
Move the mouse over the diagram to see what happens when the transmit contact
(TC) of the teleprinter opens.
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Y2 relay is a single-coil polar relay. In rest, a positive current flows
from the exchange (a1) through Rrx, then through Y2 (1,2) and back to the
line (b1). When the line current is reversed, the current through Y2 will
also be reversed, and contact y2 will switch to the leftmost position.
As soon as a character is received, for each negative bit the current
through the winding of relay Y2 is reversed and, hence, y2 is toggled to the
leftmost position. In that state, the current in the teleprinter circuit
is interrupted, causing the receive magnet (RM) to disengage. At the
same time, y2 reverses the current (-20 mA) through the compensation circuit
by feeding -60V to winding 3,4 of Y1. The latter has the same effect as a
positive current through winding 2,1. As a result, contact y1
will remain in its present position, and no echo is sent to the line.
The circuit diagram above shows the dominant current path when the system
is at rest. Move the mouse over the diagram to see what happens when receiving
a negative bit.
Check out the flow in the compensation circuit in this situation:
From the GND of the power supply (0V),
it flows trough resistor Rc, then through winding 4,3 of Y1, through
contact y1 (now in the left position) and finally to the -60V of the power
supply. This completes the proof of a functional hybrid.
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In practice, relays are not ideal components. Coil windings have nasty
side effects each time the current suddenly changes. And worse: there is
a brief moment when switch y2 is currentless. This is the case during
the transition from one position to the other. What happens in this state?
One would expect contact y1 to be dropped momentarily (causing a bit to
be transmitted) but luckly this is
not the case. In the short moment when the wiper of y2 is in between
contacts, the current flows from GND (0V) through Rc, then through
winding 4,3 of Y1 and again through winding 2,1 of Y1, through Rt
and the teleprinter (RM and TC) to the -60V supply. The resulting current
is just 10 mA, but this is enough to keep Y1 — and its contact y1 —
in its present position. Move the mouse over the diagram above, to see
what happens during the transition of y2.
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© Crypto Museum. Created: Tuesday 10 September 2024. Last changed: Thursday, 12 September 2024 - 10:37 CET.
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