ISP Download to DS89C450 using MAX3232 ?

[dxpwny]

I'm looking for advice on what I might do to get some RS232 modules I bought to work with my DS89C450 circuit. I bought a variety of RS232 modules off of Ebay, and the ones that use the MAX232CSE work fine. The ones that use the MAX3232ESE do not work completely.

All modules allow me to communicate with my DS89C450 circuit all the way up to 115200 baud. The MAX3232 based modules DO NOT allow me to program the parts via the in-system programming method however. I always download to the device at 38400 baud. I use a 29.7412 MHZ crystal that allows me 'perfect' timing at 38400 ... 115200 baud.

The two buffers I am using are NC7SZ125M5X single gate buffers. I use 0.01 capacitors next to each part, from VCC to GND. While trying to get this to work, I have been using a 8 inch long cable from my PC motherboard to the RS232 module. I use a 12 AC adapter into a 7805 regulator for circuit power.

I have tried using both a 74AC125 and 74HC125, and saw no change. I also tried changing the charge pump capacitors - using both 0.01/0.1 uF, and saw no change either, polarized and not polarized.

I'd appreciate any advice on how I might get these MAX3232ESE based modules working.

 

[Amar Dhore]

From your above circuit and datasheet for MAX3232, you receving on RX2 and transmitting on TX1. Are grounds of RS232 driver and controller tied together?

 

[dxpwny]

Yes, all the grounds a tied together.

Why is Rx2 / TX1 a problem ?

In email I received is says "... you receving on RX2 and transmitting on TX1. Well, that's is a problem." yet I do not see the phrase "Well, that's is a problem." here in the thread. That is a problem for me.

Did you edit your original post to remove that phrase ?

Thanks for the reply btw.

 

[Harald Kapp]

Why is Rx2 / TX1 a problem ?

I don't think this is a problem. The TX and RX parts of the MAX are completely interchangeable.
What measurement capabilities (instruments) are available to you? Can you verify the correct and matching waveforms on pins 8/9, 13/12 and 11/14 of the MAX with an oscilloscope? If not, you may use the soundcard of a PC for in this scenario.

 

[Amar Dhore]

Yes I realized that after I posted. Rx2 / TX1 should not be a problem.

1. Check your baud rate?
2. In put and output waveform with scope as Harald mentioned.
3. How long is the TTL cable?