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The detailed method of judging the quality of RS232 chip

The detailed method of judging the quality of RS232 chip 


RS232 interface standard


Serial communication interface standards have been used and developed, and there are already several. But they are all improved and formed on the basis of the RS-232 standard. Therefore, we will discuss mainly RS-232C. The RS-232C standard is a communication protocol published in 1969 by the American EIA (Electronic Industries Association) and BELL and other companies. It is suitable for communication with data transmission rate in the range of 0 to 20000b/s. This standard specifies issues related to serial communication interfaces, such as signal line functions and electrical characteristics. Because the communication equipment manufacturers all produce the communication equipment compatible with the RS-232C standard, as a standard, it has been widely used in the communication interface of the microcomputer.

Before discussing the content of the RS-232C interface standard, two points should be made:

First of all, the RS-232-C standard was originally formulated by connecting data terminal equipment DTE (Data Terminal Equipment) and data communication equipment DCE (Data Communicate Equipment) for telecommunication. Therefore, the formulation of this standard does not consider the application requirements of computer systems. But now it is widely borrowed for the near-end connection standard between a computer (more precisely, a computer interface) and a terminal or peripheral. Obviously, some provisions of this standard are inconsistent with computer systems, or even contradictory. Armed with this background, it's not hard to understand where the RS-232C standard is incompatible with computers.

Send and receive

Secondly, the "send" and "receive" mentioned in the RS-232C standard are defined from the standpoint of DTE, not from the standpoint of DCE. Since in a computer system, information is often transmitted between the CPU and I/O devices, both of which are DTE, so both parties can send and receive.

The detailed method of judging the quality of RS232 chip is introduced

The serial port of the single-chip microcomputer is TTL level. In practical application, the TTL level needs to be converted into RS232 level. There are many RS232 interface chips, the most famous is of course max232 from maxim company; there are many chips that are fully compatible with this chip, such as LINEAR's LT1081, LT1181, HARRIS' ICL232, etc., all of which are very famous and commonly used chips, LINEAR's serial Row interface conversion chips are often seen in early circuit designs, but they seem to be rare these days. The ICL232 is cheaper than the MAX232.

Looking at the schematic block diagram of MAX232, we need to pay attention to 2 points:

1) An external capacitor of 0.1u is required, or a 1u bile electrolytic capacitor or electrolytic capacitor. There is a 232 chip that does not require an external capacitor, but because it is difficult to make a circuit in the chip to achieve the capacitor effect, it is also more expensive; so Generally, external capacitors are used; the reason why capacitors are needed is because the RS232 level works between about -9V and +9V, and a capacitor is required to convert the 5V voltage into the +10V and -10V required for the Rs232 level; the circuit is called charge pump, very figurative;

2) The logic of Rs232 and TTL are just the opposite. On the block diagram, the logic between input and output is reversed; for TTL level, when no 232 signal is sent, it is high level; for RS232, at this time, the TX terminal is -8~9V voltage, relatively For 0V; if there is a signal, it is a series of signals interlaced from +9V~-9V, and the change of the signal can be seen by using an oscilloscope.

Using the above characteristics, we can test the quality of the RS232 interface circuit.

1) To judge whether the chip is normal, refer to the block diagram of MAX232, use a multimeter to measure pins 2 and 6, as long as the voltage of pin 2 is between +8V~+9V and the voltage of pin 6 is between -8V~-9V, it can basically be concluded that this chip is It's good;

2) When Rs232 does not send a signal, see that the voltage of the TX terminal is the high level of Rs232, that is, between -8V~-9V. When the signal is sent, the data is changing, which means that the port of this Rs232 is good; this The method is also suitable for measuring the local serial port.

The early RS232 interface chips are MC1488 (send) and MC1489. Because MC1489 only has reception, it works with single 5V power supply; MC1488 needs positive and negative 12V power supply; but in the board card of industrial computer, positive and negative power supply is not a problem, so in multi-serial card such as moxaC168, MC1488, MC1489 are still used, Cheap and low cost. Most of this chip is motorola.

The driving ability of RS232 is relatively strong, at least more than 10 meters. It is more daring, and it is used at 20 to 30 meters or even further. At this time, it is better to use a shielded wire for better effect.

Since the RS232 interface circuit is the only path for the PC and the user to develop the system, it is the key to the realization of online debugging and download functions. In the development process of the user system, the problem of online communication failure often occurs. There are many reasons for this phenomenon, but the most likely one is the abnormal operation of the interface circuit. To judge whether the RS232 interface circuit works normally, you can check whether the RS232 communication is normal one by one according to the following steps.

① Whether the V+ and V- pin voltages are high enough (more than +8V and -8V respectively). If the voltage is lower, the ADM202 chip may be damaged.

②Is there a ±12V pulse signal on the R1IN pin? If it exists, it means that the communication signal sent by the PC is normal.

③ Whether there is a +5V signal on the R1OUT pin. If it exists, it means that the ADM202 is working normally when receiving the PC signal.

④ Check if there is a +5V signal on the T1IN pin. If it exists, it means that ADμC812 responds to the communication of the PC; otherwise, there is a problem with ADμC812 communication, indicating that ADμC812 is not working properly.

⑤ Whether there is a ±12V pulse signal on the T1OUT pin. If it exists, but ADμC812 cannot enter the online debugging state, it means that there is a problem with the serial interface cable between DB9 and the PC.

1. Using an oscilloscope to observe the waveform of TXD of MAX232 is a regular 0, 1 waveform, but the amplitude is very large; while RXD is not a regular 0, 1 change, the rising edge has a cut angle, and the voltage on the hypotenuse fluctuates slightly. But the TTL output is a regular 0, 1 change waveform! !

2. Use a multimeter to measure the voltage (VCC=5V):

The second pin has a change of 8.7V-9.3V,

The 6th pin has -8.4--(-9.0V) change;

1-3 feet are 4.9V-4.9V;

4-5 feet are 0.02V-3.9V

How to judge the quality of RS232 chip? Detailed method introduction

The hardware circuit of serial communication is shown in the figure above. We can use the following methods to determine whether there is a hardware problem in the serial port. Connect the 9th pin of the MAX232 to ground, and measure whether the output between the 3rd and 5th pins of the serial port is about 10V DC. Voltage? Connect the 9th pin of the MAX232 to 5V, and measure whether the DC voltage of about -10V is output between the 3rd pin and the 5th pin of the serial port? In this way, you can judge whether the MAX232 is in good condition and whether there is poor contact at the crimping point of the serial cable.

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