Hi , I'm new to this forum and I need a little help with a circuit I'm trying to build on a breadboard. It's a RS232 serial level converter using the MAX232chip from Texas Instruments. I have the datasheet for MAX232 and it has a schematic in it showing it's Basic construction. The problem that I'm having is getting my PC serial port to "SEE the MAX232 circuit I built.
I'm suppling the circuit with 5v from a USB port, on my PC.
I'm trying to build a ALDL interface ,( Assembly Line Data Link ) using the
MAX232 . The ALDL uses TTL protocol to communicate. I'm trying to build this ALDL interface to Scan my automobile through my Laptop.
I have tried the LOOPBACK TEST, using Hyper Terminal, but no success.
I know that the serial port works, I can operate a Satellite Box and my telescope from the serial port.
I can't figure it out, I've built the circuit from scratch twice but no change.
Here is the data sheet link.
http://datasheets.maxim-ic.com/en/ds/MAX220-MAX249.pdf

Any help would be great

Sincerely Aliman

 

I assume you have used the following circuit from the datasheet.

First let's make sure the hardware is connected up correctly.

1. Is the two voltages at pin#2 and pin#6 correct?

2. Is the serial port on the PC a 9-pin one? If not, then the pin# shown in picture is not correct. For 25 pin Serial port, swap connection Pin#2 & pin#3 and connect ground to pin#7 instead.

3. Very important. After you have connected the cable to the MAX232 chip, but before connection to the PC serial port, you would need to measure the continuity between the MAX232 pin and the socket pin on the RS232 DB9 plug(look carefully at the DB9 plug, you will see very tiny pin number labelling to tell you where the pin is).

e.g.

pin#14 of MAX232 and pin#2 of RS232 DB9 plug
Pin#13......................pin#3..........................
Pin#15......................pin#5..........................

3. apply the loopback jumper as shown

4. fires up HyperTerm and test it.

 

I haven't used Hyperterminal but if there are settings for hardware handshaking they should be disabled.
I use Realterm which is free and has loads of handy features.

 

First let me say Thankyou for your help.

I have built 2 versions of this circuit, one using electrolytic capacitors and the other using ceramic capacitors. The circiut is so simple , yet troublesum.

There is +5.12v at pin 16 on both circuits. The circuit using electrolytic caps has +2.95v at pin 2 to ground, and -2.95v at pin 6 to ground.
The ceramic cap circuit has +4.24v at pin 2 to ground and +0.22v at pin 6 to ground. Wierd??

To your second question my PC has a 9 pin serial port.

To your third question the continuity between MAX232 pins and DB9 is OK !

Here is a copy of the circuit I'm trying to use the MAX232 converter in.

Aliman

 

There is +5.12v at pin 16 on both circuits. The circuit using electrolytic caps has +2.95v at pin 2 to ground, and -2.95v at pin 6 to ground.
The ceramic cap circuit has +4.24v at pin 2 to ground and +0.22v at pin 6 to ground. Wierd??

Not good. With a proper chip the circuit is very reliable and works 100% each times. I don't remember I had met any troubles with the MAX232 and I must have built over twenty of them in the past decade.

For some older MAX232 chips, you have to use capacitors of value 10uF ~ 2.2uF. Only modern ones can work with capacitors as low as 0.1uF.

What is the value of capacitor you have used?

My other advice is to build the circuit with just the MAX232 chip and the five capacitors, until the voltages measure on pin#2 and and pin#6 are correct.

Only then you can adds other circuits.

 

The MAX232's that I'm using, I just recently received them as samples from Texas In. , and Intercil. They are MAX232EIN ,MAX232IN and the HIN232CPZ from Intercil.
I have a pic of the circuit on a proto board, this one is using the HIN232.

Aliman

 

The HIN232CPZ can work with capacitor value of 0.1uF or higher.

Are you sure the capacitor values are correct?

What is the markings on the ceramic capacitor? They should be "104".

 

OK I think I have it !
I replaced those ceramics with electrolytic caps. C1 - C4 are 50v 1uf and C5 is 50v 10uf.
Now the voltages at PIN 2 is + 9.2v and PIN 6 is -8.69v , thats much better, no ?
Aliman

 

Yes, that's more like it.

Keep on the good work.

 

Thanks so much Chung, I now have the circuit working properly !!

One question, when I'm testing the circuit with RX and TX (Pin11 & 12)connected ,I'm using a OBD1 ALDL Interface Tester and the circuit Tests OK!
When I disconnect Pins 11 & 12, the circuit Test FAILS, is this normal ??

 

When I disconnect Pins 11 & 12, the circuit Test FAILS, is this normal ??

I don't know if it is normal.

When you disconnect the loopback jumper, you would need the real world signals to and from the device for the communication to function properly. i.e. the device which you intent to connect to Pin#11 & Pin#12.

Now, you must take into account of the serial communication protocol like baudrate, parity, start/stop bits etc. to ensure they are the same on both the laptop and the device.

Getting any one of the above protocol settings wrong you will get no response or garbage as a result.

 

The ALDL Baud rate for OBD1 is 8192, but I know that the MAX232's closest Baud rate is 9600. Do you think that it would still work ?

 

The ALDL Baud rate for OBD1 is 8192,

This is an odd baud rate not commonly used in the PC world. Just make sure that it is correct.

but I know that the MAX232's closest Baud rate is 9600. Do you think that it would still work ?

The MAX232 is an analog voltage conversion device and does not have a baud rate. It will work with signal of any baud rate. However, it has a maximum limit of how fast it can handle the signals which then translates to a limit of some hundreds of Kbps.

Whether you can have serial communicate at 8192bps depends only on the setup of the uart of your laptop.

 

This is an odd baud rate not commonly used in the PC world. Just make sure that it is correct.



The MAX232 is an analog voltage conversion device and does not have a baud rate. It will work with signal of any baud rate. However, it has a maximum limit of how fast it can handle the signals which then translates to a limit of some hundreds of Kbps.

Whether you can have serial communicate at 8192bps depends only on the setup of the uart of your laptop.

Can you tell me how to set up the uart on my laptop. One problem, my laptop does not have a serial port, I am using a Prolific Serial to USB converter using the PL-2303 chip. When I connect the interface I built to the serial converter on my Toshiba 300D Satellite ,it connects to the laptop. The software I'm using indicates that MAX232 Interface is Detected.

Aliman

 

I don't ever own a USB-Serial converter so I'll pass.

Anyone?

 

Thats cool, Thanks for all your help, you helped me get that circuit working properly.

Aliman