MAX232 R1out and R2out voltage

[sidz]

Hi,
i assembled a MAX 232 circuit to convert the input from Serial port of computer.

After completing the wiring and stuffs when turned on the circuit to check if it is working (not connected to computer still)
just power on and checked the voltage level at the PIN-12 (R1out) & PIN-9 (R2out) of max232 both constantly shows 5.25 volts.

Does the above voltage expected on these pins when the circuit is just powered on and not even connected to the computer serial port?

cheers,
sid

[waltr]

Did you install the four 1.0uF caps as per Figure 5 of the data sheet?
I assume you are applying +5 to pin 16 and ground to pin 15.

If you did this then you should measure +10V on pin 2 and -10V on pin 6.

Pin 12 & 9 will measure +5V when pin 13 & 8 are 0 to -12V. This is expected.
Look at the diagram in figure 5. The little circles on the amplifier symbols means that the output logic level is inverted.

If you have the correct voltages on pins 15 & 16 then measure pin 12 and connect pin 13 to pin 2. Pin 12 should go to 0V.

[sidz]

Thanks waltr. i get the exact voltage levels as you mentioned.

i have a second question:
When i connect this(max232) to the serial port and write data in the serial port i see a voltage drop on pin 12 when it is measured against GND.

the voltage drop varies based on the no of bits written on the serial port.
like if i write a datastream containing 8 bytes i note a drop of 0.6 to 0.8 volts.
i wanted to ask does this little variation in voltage enough for a PIC like 16F628A to understand.

The serial port settings are:
Baud Rate:9600
DATABITS: 8
STOPBITS: 1
PARITY_NONE

[waltr]

If you are measuring pin 12 of the MAX232 with a multimeter then you will see a different voltage depending on the number of bits that are low or high. A multimeter averages the voltage measured and will not be able to measure a bit at 9600 Baud as one bit is only 104uSec long. An O'scope is the proper instrument to measure the voltage levels of a serial stream.

What was the hex value of the bytes you sent? If sending from a PC terminal program like Hyperterm, look up the ASCII table to see what the hex value of different characters are, example: 'A' = 0x41, 'U' = 0x55, DEL key = 0x7F.

As an experiment you could send many bytes of a value with the most number of bits high. This would produce the lowest average voltage at pin 12. Or clip the MAX232's input (pin 13) to pin 6 (-10V) and measure pin 12.

To 'stream' the data fast create a file, save it then have the terminal program send it.
Additional experiments could be done with different number of bits high and low.

So, it looks like you have the MAX232 working properly.
Have you gotten the PIC's UART to work? An easy test is to how the PIC send back what even it receives.

[sidz]

hey thanks waltr.
I dont have access to any O'scope so will try out directly after assembling the PIC.

will keep you posted.

cheers,
-sid

[waltr]

You could 'loop-back' the MAX232 output to the input (pin 12 to pin 11 with pins 13 & 14 going the your PC Com port) and any character typed should be echoed on screen.