The method to judge max232 chip : good or bad

The serial port of the one-chip computer is TTL level, in the actual application, need to change TTL level into RS232 level. Many RS232 interface chip, of course, the most famous max232 max; chip with a lot of compatible with this chip, such as LINEAR LT1081, LT1181, HARRIS ICL232, etc., are very famous and commonly used chips, LINEAR string Row interface conversion chip in the early circuit design can often see, but recently it seems rare. ICL232 cheaper than the MAX232.

Look at the schematic block diagram of MAX232, need to notice 2:00:

1) Requires an external capacitor of 0.1u, or 1u of gallbladder or electrolytic capacitor. There is a 232 chip that does not need an external capacitor. However, it is more difficult to make a circuit that achieves the capacitor effect in the chip, so it is more expensive. Therefore, Generally use an external capacitor; the reason for the need for capacitance, because the RS232 level is working at between -9V ~ +9 V, the capacitor needs to convert 5V voltage Rs232 level required +10 V and -10 V; circuit Called charge pump, very image;

2) The logic of Rs232 is exactly the opposite of TTL. On the block diagram, the logic between input and output is reversed; for TTL levels, it is high when no 232 signal is asserted; for RS232, the voltage at TX is -8 to 9V, At 0V, if there is a signal, it is a series of signals that are interleaved from + 9V to -9V. Using a scope, you can see the signal changes.

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

1) to determine whether the chip is normal, see the MAX232 block diagram, using a multimeter to measure 2 and 6 feet, as long as the voltage of 2 feet between +8 V ~ +9 V, 6 feet -8V ~ -9V, basically you can determine This chip is good;

2) When Rs232 does not send a signal, look at TX terminal voltage as the high level of Rs232, namely -8V- -9V, when the signal is sent, the data is changing, this shows this Rs232 port is good; This The method is also suitable for measuring local serial ports.

The early start of the RS232 interface chip is MC1488 (send), MC1489. MC1488 because it only receives, it is a single 5V power supply work; MC1488 you need positive and negative 12V power supply; but in the IPC card, positive and negative power supply is not a problem, so in moxa C168 and other serial cards, still use MC1488, MC1489 Cheap low cost ah.

This chip, the majority of motorola.

RS232 drive capability is relatively strong, at least 10 meters or more, more courageous, used in 20 to 30 meters or even farther; this time, it is best to use the shielding effect will be better.

Relevant MAX232 series products

Learn More About：Texas Instruments

Author：Brittany Antonia （The author of article owns the copyright.）