Siemens S7-300PLC from entry to the master of 50 classic quiz(three)

40: Can I use SM321-1CH20 instead of SM321-1CH80 without changing the hardware 

configuration?

The technical parameters of the SM321-1CH20 and SM321-1CH80 modules are the same. The only difference between

 the SM321-1CH80 can be applied to a wider range of environmental conditions. So you do not need to change the 

hardware configuration.

41: I / O for direct access, we must pay attention to what?

Note that in an S7-300 configuration, an incorrect value is read if there is direct I / O read access across the module

 (a few bytes at a time with this command). Hardware can view the specific address.

42: Does the SM321 module need to be connected to the DC 24V?

Not required, in the case of the SM 321 module with MLFB 6ES7 321-1BH02-0AA0 it is no longer necessary to connect the 

DC 24V.

43: How to plan the simulation module SM374 in STEP 7 hardware configuration? How to 

find this module in the hardware catalog?

The SM374 analog module can be used in three modes: as a 16-channel digital input module, as a 16-channel digital 

output module, as a hybrid digital input / output module with 8 inputs and 8 outputs.

If you use the SM 374 as a 16-channel input module, configure a 16-channel input module - Recommended: SM 321: 

6ES7321-1BH01-0AA0,

If you use the SM 374 as a 16-channel output module, configure a 16-channel output module - Recommended: SM 322:

 6ES7322-1BH01-0AA0,

If you use the SM 374 as a hybrid input / output module, configure a hybrid input / output module (8 inputs, 8 outputs) 

- SM 323: 6ES7323-1BH01-0AA0 recommended.

44: Is there any short-circuit of the sensor when measuring current, is the analogue input I

 + of module 6ES7 331-1KF0.-0AB0 destroyed?

When measuring current, a short-circuit of the sensor occurs and the analog input I + of module 6ES7 331-1KF0.-0AB0 is

 not destroyed. The module has built-in overcurrent protection. Each 50-ohm resistor in the module has a PTC element on 

the front to prevent the module's input channel from being damaged.

45: If the CPU is switched off, does the 2-wire measurement transmitter continue to supply

 power?

If the transmitter module is inserted in position "D" and the module is powered by an external voltage on pin 1 and pin 20, 

the 2-wire measurement transmitter continues to supply power. The power supply current remains constant even if the CPU

 is switched off.

46: Can I use the absolute error limits listed in the module's documentation when measuring 

temperature (in Fahrenheit) with an S7-300 analog input module?

The specified error limit can not be used directly. The basic error and operating error are described in terms of absolute 

temperature and Celsius. It must be multiplied by a factor of 1.8 to convert it to Fahrenheit temperature units.

Example: S7-300 AI 8 x RTD: The specified temperature input operating error is +/- 1.0 degrees Celsius. When measured

 in Fahrenheit, the maximum acceptable error is +/- 1.8 degrees Fahrenheit.

47: Why can not use the commercial digital multimeter to read the constant current for

 reading impedance on the analog input block?

Almost all S5 / S7 analog input devices still work in a complex manner, ie all channels are plugged into the only one AD

 converter in turn. This principle also applies to the constant current necessary for reading the impedance. Therefore, 

the current to be read through the resistor is only used for short-term readings. For a SM331-7KF02-0AB0 with a selected 

interface that suppresses "50Hz" and 8 parametric channels, this means that the current will flow once every 180ms, with 

a 20ms read impedance each time.

48: Why does the voltage output of the S7-300 analog output group exceed the tolerance?

 Terminal S + and S- for what purpose?

When using the analog output module SM 332, care must be taken to return the assignment of the inputs S + and S-.

 They serve the purpose of compensating for the performance impedance. When connecting the actuator's two contacts 

with separate S + and S- wires, the analog output adjusts the output voltage so that the actual voltage on the actuator is

 the desired voltage.

If you want to get compensation, then the actuator must be connected with 4 wires. This means that for the first channel,

 you need:

Note: Due to the open sensor terminals (S + and S-), the output voltage is adjusted to a maximum of 140 mV (for 10V). 

g For this assignment, the 0.5% voltage output can not be used with error limits.

49: How to connect a potentiometer to 6ES7 331-1KF0-0AB0?

The sampling end and the first end of the potentiometer are connected to M +, the end is connected to M-, and the S- and 

M- are connected together.

Note: The maximum available resistance is 6K. If the potentiometer supports direct output of a variable voltage, then the 

potentiometer should have a V + connected to the first terminal and M- connected to M-.

50: How to connect a PT100 temperature sensor to the analog input module SM331?

PT100 resistance varies with temperature. If there is a constant current flowing through the RTD, the drop in voltage 

across the RTD varies with temperature. Constant current is applied to contacts Ic + and Ic-. The analog module SM331

 measures the change in current at M + and M-. By measuring the voltage you can determine the temperature.

There are three types of PT100 to analog input group connections: the 4-wire connection gives the most accurate 

measurement.

1) The formula for 3-wire connection shows only the actual measurement of analog input module SM331 (MLFB No. 

6ES7 331-7Kxxx-0AB0) b ".

2) In the S7-300 series, there are several analog inputs that pass multiple determinations. They set the line resistance of

 the common return line and make mathematical compensation. The accuracy achieved is almost comparable to the 4-wire

 connection. An example of such a module is SM331 (MLFB No. 6ES7 331-7PF00-0AB0).