Analog & Digital Signal Transmission and Data Acquisition Cards
Analog Transmission Signals
This method has two disadvantages:
- The maximum distance is one meter.
- The value of the transmitted signal is not less than 0.1 V.
If these conditions are not met, the voltage drop and interference significantly degrade the signal. This is solved by using a voltage-current converter in the range of 4 to 20 mA, i.e., there are 4 mA to 0V and the maximum voltage is 20 mA. Using the converter is able to increase the transmission distance up to 3 km and also facilitates the detection of faults (if the receiver measures 0 A, something goes wrong).
Digital Signal Transmission
Much can be done in parallel, as in series. The factor that determines the choice of method used is the distance of transmission.
Parallel Transmission
This method should not be used for distances up to 2 or 3 meters because the cost of the line would be too high (1 line per bit). It is done through a set of parallel lines called a bus. The standard bus has 16 lines: 8 for data transmission, 3 are to indicate that the message is ready to be transferred, and 5 to ensure the orderly flow of data.
Series Transmission
This method is used for large transmission distances and can use low-cost cable. Data from the various sensors can be selected by a multiplexer to transmit data in an orderly manner along the line. This method uses 3 lines for the transmission of data: one for transmission, another for reception, and another for ground. The signal data are between -3 and -15 V for the logic state “1” and between +3 and +15 V for the logic state “0”. These voltage margins are to prevent degradation of the signal due to interference. The transmission speed is 20,000 bits/s with a maximum capacity of 2500 pF. The maximum distance is 17m.
There are some methods that use a differential working mode; this signal is transmitted by two wires, and the receiver captures the difference between the two. In this way, we eliminate the interference.
The Modem
The modem is used to transmit digital data over long distances via public telephone lines. A modem modulates the digital signal in an analog waveform; this signal is on the telephone line, and another modem demodulates the analog signal into a digital one. There are three forms of variation that a modem can use:
- Frequency Modulation: a 2200 Hz represents a “1” and 1200 Hz a “0”. The speed is low.
- Amplitude Modulation: if the amplitude is high, it represents a “1”, and if it is low, a “0”.
- Phase-Shift Modulation: If the receiving modem detects a mismatch, it represents a “0”; at other times, it represents a “1”.
Data Acquisition Cards
A data acquisition card is an element capable of making decisions based on the state of the process at every moment with the help of a computer through a communication channel (keyboard for entering data and viewing screen or printer).
Features:
- Versatility, due to the computer itself and the card.
- Low price, since it does not have to be equipped with a power supply or external controllers.
Structure
In a data acquisition board, there are four major blocks that output data to the internal bus of the card, and then to the PC bus: Analog Inputs, Analog Outputs, Digital Inputs, and Digital Outputs.
Analog Inputs
The card comes equipped with multichannel analog connections which can be done in two ways:
- Common Mode: uses one line, so an error voltage appears due to lead resistance. This error voltage coupled to the potential noise signals distorts the measure.
- Differential Mode: this method uses two lines so that the card will show the potential difference between the two lines, thus eliminating interference and increasing accuracy.
If we handle multiple analog channels at once using a multiplexer (to reduce the cost of drivers because the same line transmits data from all sensors), then the multiplexer should put a variable gain amplifier to determine the sensitivity of the card. At the amplifier output will be the sample and hold circuit, which captures the signal and stabilizes it. After an A/D converter translates the digital signal so that the PC understands the program. The type of converter used is determined by the resolution and scanning speed.
- Resolution: is the digital word size of the output (number of bits that is encoded).
- Scan Speed: the maximum number of data per second that the converter can scan.
A trigger circuit is responsible for generating momentum that indicates when to start the process of acquiring a sample. There are three types:
- Shot by Software: is done when the program is running on the PC.
- Shot by Programmable Time Intervals: is made by a square wave generated by the PC whose frequency is set in the program.
- External Triggering: is generated by an external system to the card. Used when you want to synchronize the acquisition process with some external event.
Digital Inputs
The card has digital input channels (8 bits) which are assigned binary information (“0” or “1” with TTL voltage levels).
Analog Outputs
Cards have one or more output channels through their respective D/A converters (the resolution is important to keep in mind), that generate a continuous analog voltage proportional (between +10 and -10 V) to the numerical value of the digital data that has to be input. The measurement accuracy depends on the auto-calibration systems at the card’s disposal. The accuracy of the card, and therefore the measures, is related to the resolution of the converter, but it is not the same data.
Digital Outputs
The function of the digital outputs is to allow communication with other cards or controlling instruments and control systems of two states (such as a contactor). These outputs are programmed with words of 8 bits, so that the output connector has 8 pins with “1” or “0” (TTL compatible). The cards can be designed with the capacity to generate interrupts so that the PC is not constantly paying attention to the procurement process, and the card can perform other tasks. Direct Memory Access is a technique that allows the passage of data directly from the RAM card without CPU intervention.