MAX232A with BS2OEM

mrsparkle

Hi,
I am trying to develop a test system based on the BS2 OEM, and I would like to design my own PCBA. I thought I would use a MAX232A to do the RS-232 translation, so I built the proto board shown in the included schematic and used the PBASIC2 DIP28 part from a functional BS2 OEM board, as well as its EEPROM. (Note that I used a crystal oscillator instead of the ceramic resonator that comes on the BS2 OEM board, because they seem to be obsolete and I can't find a direct replacement.) The clock and all the power supplies are working, but all I get from the Basic Stamp software is "No Basic Stamps found". Any ideas?

thanks,
Mr Sparkle

Dr_Acula

I'm more used to the propeller rather than the BS2 but just looking at the schematic two things - for a max232 the capacitors are usually 1uF rather than 0.1uF, and also I'm not sure where the DTR reset line goes - is "ATR" connected to anything?

Do you have a reference design this has been copied from?

For instance this is the max232 one http://circuits.datasheetdir.com/122/MAX232-circuits.jpg

The max3232 is the 3V version and this does use 0.1uF.

Also quick test - should be around +9V on pin 2 of the max232 and -9V on pin 6.

Also probably doesn't mean anything but there is a pin 10 and 11 marked on the DB9 connector but this part only has 9 pins.

If this is the correct chip http://www.me.psu.edu/rahn/me462/bs2_fig2.gif then should there be a transistor somewhere to invert the reset signal?

Chris Savage

Hello,

I see two issues in your schematic. The one that really stands out is that there is no longer a connection between the DTR line and the MCLR line. They should not be directly connected, but the DTR line is used to initiate a reset of the BASIC Stamp which is required for identification or programming. Without the proper connection this will never happen. The other, perhaps less obvious issue is the way the SIN/SOUT lines are connected as both an "echo" resistor and a diode are required for these lines to function correctly. This is detailed at the following link, but at line level, not RS-232: http://www.savagecircuits.com/bs2oem

A customer took this concept to the MAX232 and I posted his schematic here: http://forums.parallax.com/showthread.php/83019-BS2-OEM-quot-LITE-quot-Schematics?p=627767&viewfull=1#post627767

I hope this helps.

G McMurry

Here is a schematic from one of my old projects that used BS2 OEM. I don't remember how I came up with all of it. I must have seen it used somewhere. In particular, I don't know why I used the signal diode and the 4k7 resistor. I do know, that it worked fine... Maybe this will help you.

Greg

BTW - I used a 10p ribbon connector for my programming port. I don't typically put DB-X connectors directly on my PCBs. You would use a straight translation from 10 pin (2X5) to DB-9 with a ribbon cable.

Chris Savage

The 4.7K resistor provides echo, which is required by the BASIC Stamp Editor for programming and identification. The diode isolates the SOUT line from adversely affecting (blocking echo) the RX (PC side) line in certain circumstances. The SOUT line can only drive the signal low. I have described this connect many times over the years to those wanting to create an OEM implementation with fewer parts.

G McMurry

So, back in 2005 when I built this project, you probably described it to me! Thanks!

Greg

Chris Savage

G McMurry wrote: »

So, back in 2005 when I built this project, you probably described it to me! Thanks!

Greg

That is entirely likely and possible, sir. I also edited my explanation a bit more on the diode function.

mrsparkle

Thanks everyone,
Mr S