Differences between TTL level and RS232 level

Differences between TTL level and RS232 level

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When I encountered a problem about the level selection, I forgot the definition of the RS232 level. At that time, I was unable to respond. After I came back, I found out the difference between the two, it may take more than two years for you to forget some common sense that you were familiar with before. This is not a good phenomenon. :-), or copy the differences between the three common levels here. it is used to deepen the effect of memory ..

What are TTL level, CMOS level, and RS232 level? What are their differences? Generally, the CMOS level has a higher noise margin than the TTL level.

(1) TTL level standard

Output L: <0.8 V; H:> 2.4 v.

Input L: <1.2 V; H:> 2.0 V

The output low level of the TTL device must be less than 0.8 V, and the output high level must be greater than 2.4 v. If the input value is lower than 1.2v, it is regarded as 0. If the input value is higher than 2.0, it is regarded as 1. Therefore, the low-level noise tolerance of TTL-level input is only (0.8-0)/2 = 0.4 V, and the high-level noise tolerance is (5-2.4)/2 = 1.3 V.

(2) CMOS level standard

Output L: <0.1 * VCC; H:> 0.9 * VCC.

Input L: <0.3 * VCC; H:> 0.7 * VCC.

Because the CMOS power supply uses 12 V, the input below 3.6v is low, the noise margin is 1.8 V, higher than 3.5v is high, and the noise margin is 1.8 V. Higher noise tolerance than TTL.

(3) RS232 Standard

The level of logic 1 is-3 ~ -15 V. The logic 0 level is + 3 ~ + 15 V,Note that the definition of the level is reversed once.

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What is the difference between TTL and CMOS?

1. The upper and lower limits of the level are defined differently. CMOS has a greater anti-noise area. For a 5 V power supply, TTL is generally 1.7v and 3.5v, while CMOS is generally 2.2 V and 2.9v, which is not accurate and is for reference only.

2. The current driving capability is different. TTL generally provides 25 mA of the driving capability, while CMOS is generally around 10 mA.

3. The required current input size is also different. Generally, TTL needs to be around 2.5 mA, and CMOS almost does not require current input.

4. Many devices are compatible with TTL and CMOS. datasheet will describe. If you do not consider the speed and performance, General devices can be switched. However, you need to note that sometimes the load effect may cause the circuit to work abnormally, because some TTL Circuits need the input impedance of the next level as the load for normal operation.

1. logical levels of TTL and CMOS Circuits

Voh: output voltage of logical level 1

Vol: output voltage of logic level 0

VIH: input voltage of logical level 1

VIH: input voltage of logic level 0

TTL circuit critical value:

Vohmin = 2.4 V volmax = 0.4 V

Vihmin = 2.0 V vilmax = 0.8 V

CMOS circuit critical value (power supply voltage + 5 V)

Vohmin = 4.99 v volmax = 0.01 v

Vihmin = 3.5 V vilmax = 1.5 V

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2. logical level conversion between TTL and CMOS

The CMOS level can drive the TTL level.

The TTL level cannot drive the CMOS level, and the pulling resistance must be added.

3. Features of common logical chips

74LS series: TTL input: TTL output: TTL

74hc series: CMOS input: CMOS output: CMOS

74hct series: CMOS input: TTL output: CMOS

Cd4000 series: CMOS input: CMOS output: CMOS

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Figure: RS232 and TTL/CMOS level inversion.

Differences between TTL level and RS232 level