Click for homepage
Hybrid
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.

Equipment with built-in hybrid on this website
Siemens T-56 Fernschreib-Endsatz (teleprinter terminating unit) with signalling
TekaDe FAG-200 teleprinter connection device
Introduction
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:

  • 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.
Simple teleprinter circuit
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.


Digital current hybrid
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


Polar relays
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.

Trls-64a polar relay with seven coils

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

  1. 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.

Type code T-rls-64a
T-rls-46a polar relay (B) - left angle view
Top view of polar relay (B)
Coil(s) connection wires
A
×
A
1 / 4
Type code T-rls-64a
A
2 / 4
T-rls-46a polar relay (B) - left angle view
A
3 / 4
Top view of polar relay (B)
A
4 / 4
Coil(s) connection wires

Hybrid circuit
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).

Dominant current path when the system is in rest.

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.


Transmitting a character
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.

Current path when transmitting character bits.

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.


Receiving a character
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.

Current path when receiving a negative data bit from 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.


Transitional state
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?

Current flow during the transition of switch y1. The current is high enough to keep Y1 (y1) in its present position.

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.


Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
© Crypto Museum. Created: Tuesday 10 September 2024. Last changed: Thursday, 12 September 2024 - 10:37 CET.
Click for homepage