Below, the three interface circuits are briefly described:

TTL

TTL is the English abbreviation of Transistor-Transistor Logic. As can be seen from its name, the original intention of this interface level standard is to be used between digital systems based on triode structure.

For digital circuits working under the TTL interface standard, the standard power supply for the internal active devices should be 5V. The output and input conditions are as follows:
For the output end, the voltage requirement for state 1 is greater than or equal to 2.4V, and the voltage requirement for state 0 is less than or equal to 0.5V;
For the input end, the judgment requirement for state 1 is greater than or equal to 2.0V, and the judgment requirement for state 0 is less than or equal to 0.8V.

That is to say, it needs to be greater than a certain threshold to represent level 1, and less than a certain threshold to represent level 0.
Comparing the voltage requirements of the output and input terminals, it can be seen that the voltage output requirements of the output terminal are more stringent than the dual-threshold judgment standard of the input terminal. This is mainly done to take into account noise interference and the transmission speed of electrical signals between output and input, thereby making the dual-threshold judgment standard more reliable.

RS232

RS232 is a serial physical interface standard formulated by the Electronic Industry Association (EIA). RS is the abbreviation of Recommended Standard, which means recommended standard in Chinese, and 232 is the identification number. The RS232 bus standard has a total of 25 signal lines. Here we only discuss its digital level interface determination standard.

The standard power supply of RS232 is ±12V or ±15V. The voltage requirement for state 1 is between -15V and -3V, and the voltage requirement for state 0 is between 3V and 15V.

RS485

RS485 is equivalent to an upgraded version of RS232. Similar to LVDS, RS485 also uses differential form to transmit information (but RS485 really transmits two voltage signals), so its anti-interference performance is better than RS232. Here, we are also only concerned about its digital level interface criteria.

In RS485 state 1, the voltage difference between the two lines is required to be between 2V and 6V; in state 0, the voltage difference between the two lines is required to be between -6V and -2V.

Ucreate Electronic Group often uses the above interfaces when manufacturing materials for customers. For example, power bank customers use TTL and RS485 circuit interfaces, and BeagleBone's development board using TI's AM3352 as the main chip uses a serial port. UART etc. These interfaces are mainly used to connect computers for program burning, key burning, and testing of various functions such as wired, wireless, Bluetooth, RTC, etc.

Can different standards be mixed together?

The interface level standards between various digital systems are introduced above. Usually when using them, it is strongly recommended that everyone choose the same standard for both sides of the digital system interface. However, sometimes due to some configuration conditions of the two parties, it may not be possible to find a unified level standard for communication. So at this time, is there no other way besides designing an interface conversion circuit board? No, in fact, some different interface level standards are compatible.

First of all, it is impossible for single-ended and differential to be compatible because they are different from the physical connection. But for the same type of interface, if the output of the A-level standard meets the input of the B-level standard, then it is said that the output of A can drive the input of B. If vice versa, then it is said that the two level standards A and B can drive Drive each other.

For example, the CMOS output can drive the TTL input, but not vice versa, because the TTL state 1 output is only greater than or equal to 2.4V, which cannot reach the greater than or equal to 3.5V required by CMOS to determine state 1; but LVTTL3V3 and LVCMOS3V3 can can drive each other because their outputs can meet each other's input determination requirements.

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