Open collector output
The open collector output (OC) (in German “open collector ”, also “unconnected collector”) is an output of an integrated circuit with a bipolar transistor with a free collector output. Mostly it is used to allow the connection to a higher voltage level or the logical links AND (English. AND ) and OR (eng. OR ) as so-called Wired-AND and Wired-OR- connections in bus systems like the I²C - Allow bus.
Since field effect transistors are increasingly being used in integrated circuits which have a so-called drain connection instead of a collector connection, this output is also referred to as an open drain output .
Explanation
The inner workings of an analog or digital IC can generally be divided into input connections, switching logic and output connection (output connections). In the vast majority of cases, a voltage level between VO L and VO H is set at the output . Any intermediate values are also allowed for analog ICs; for digital ICs, the internal switching logic forces a “Low, 0” (VO L ) or “High, 1” (VO H ) to be present at the output . It is important to understand that with the output circuit shown (which can deviate from the principle shown due to emitter followers , short-circuit fuses and other variations) the output voltage level is always between VO L and VO H , and cannot be lower or higher. Depending on the circuit, VO L can be equal to V- and VO H equal to V +, so that, to put it simply, the output voltage VO is between V- and V +. If both limits are reached, the term rail-to-rail technology is used because in the circuit diagram V- and V + look like the rails of a railroad track.
It now happens that one has to work in an electronic circuit with a common ground and different V + levels. A typical example is the mixing of analog signal processing with any V + level and processing of the signals on microcontrollers , where V + must be fixed at +5 V. Or outputs from digital processing must be brought to voltage levels greater or less than +5 V. These are indicated by the different V + lines at the top in the adjacent picture. While passing on analog voltage levels from left to right can still be unproblematic (+3.635 V is always +3.635 V), it looks much more critical with the digital information "0" and "1" because in the middle "1 “Z. B. is +4.8 V, which in the right part, with a V + voltage of z. B. 20 V is interpreted as "0", since +4.8 V is well below 0.5 · 20 V = +10 V.
The link between the components of one level to the other are open collector outputs, where, as shown, the collector of a transistor is led to the outside of an IC connection without any further internal circuitry (open = open). It therefore behaves like an open-close switch to the outside world (although it is not defined which logical state open and closed are assigned to). In the example shown, resistors that are connected to the V + level of the "receiving component" are switched to ground by the open collector transistor when it is switched on. In other words, the input of the receiving component sees a binary "0". If the open collector transistor is not activated, i. i.e., it is not conductive, the level is pulled to V + of the input component (provided this resistance is small compared to the input resistance). The input therefore sees a binary "1". This is why these resistors are also called pull-up resistors . These are not implemented in most standard ICs, so they have to be provided externally, but there are microcontrollers in which individual ports can be configured as inputs with an integrated pull-up resistor.
Open collector connections can also have properties that the rest of the transistors in an IC lack. A classic example are driver transistors for numeric displays, which z. B. can be supplied with up to 30 V, although the ICs as members of the 74 series of digital standard ICs are only approved for V + = 5 V. The "discharge" connection of the famous NE555 timer IC is an open collector connection with a very high permissible current load.
Digital ICs and using the OC
The open collector output is one of five possible output types (also totem pole , tri-state , complementary output and open emitter output ) for digital integrated circuits . The collector resistance of the output stage is omitted, so that several outputs can be combined to form a bus.
Tasks can certainly arise in which many gates have to be linked to one another on the output side. If z. B. 25 gate outputs are to be combined by an OR gate, one would have to lead 25 lines to an OR gate with 25 inputs. This is not only very complex, such gates are also not available on the market. This problem could be solved by cascaded OR gates, but would then also get different signal propagation times .
It is better to use gates with an open collector output here. These have an npn transistor at the output, wherein the emitter to ground (engl. Ground), and the collector output is led unswitched to the output. Such outputs can now easily be switched in parallel and connected to V + with a common collector resistor.
With positive logic, the output voltage U a is only in the HIGH state if all connected gate outputs are also high, ie all output stages are blocked. On the other hand, it can be seen that the output voltage goes into the LOW state if only one output is in the LOW state. This results in an AND link of the outputs for positive logic.
With this open-collector circuit technology, an OR link can also be implemented by connecting the negated outputs of the gates with their open collectors and then negating them. According to de Morgan:
A corresponding circuit shows:
It can be seen that the AND link of the negated gate outputs with subsequent negation provides an OR link. WIRED-AND and WIRED-OR structures are used e.g. B. used in the implementation of "programmable logic arrays" (PLA).
Symbols according to the IEC standard
The diamond describes the high-resistance output and the line the low-resistance connection from the level to the output. Their arrangement above or below shows which state is at which level. If the diamond is now above the horizontal line, the H level is high-resistance (no voltage is applied) or the L level (ground) is directly connected to the output.
| symbol | description |
|---|---|
 |
General sign for an "open exit", e.g. B. the open collector output |
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Open output H-type with low-resistance H-level and high-resistance L-level Wired-OR links are possible with these outputs . An external "pull-down" resistor must also be connected to the wired OR connection. |
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Open L-type output with high-impedance H level and low-impedance L level Wired AND operations are possible with these outputs . An external "pull-up" resistor must also be connected to the wired AND link. |
literature
- Gerhard H. Schildt: Introduction to technical computer science . Springer, Vienna; New York 2003, ISBN 3-211-83853-8 .