Dear Per, Erik,

Thank you for your advices.

The monitor program that resides in the EPROM in my 80C535 board can be accessed interactively via Hyperterminal. A command consists of a single keystroke like "?" for displaying Help, "h" for displaying 128 bytes of certain memory location, etc. My 80C535 can first respond to 3 to 4 keystrokes command, and then seems to hang or locked up. The system, if has been powered off for several hours, can boot up consistently, but again only for 3 to 4 keystrokes.

I use 7805 regulator, install properly bypass 0.1µF capacitor for each IC's Vcc to ground, and 22µF bypass capacitor for all. I also provide wide enough ground track and avoid ground loop.

A senior member of another forum said that the 80C535 has a watchdog timer that 8031 lacks, so I have to add to the originally written code for 80c31, a proper initialization code for the timer. I had consulted the datasheet and found that the Watchdog Timer can be cleared by setting two bits in two different special function register (IEN0.6 and IEN1.6) by two consecutive instructions. Or maybe I can use the timer in the contrary to just to overcome my lock-up problem.

Components supplier shop here usually doesn't understand functionalities of ICs they're selling, so we have to specifically mention the part number. I have contacted a shop in another bigger town for any of MAX810, MAX1232, MAX707 and MAX813, but they only have the MAX1232.

Yes right, all I need is the simple 3-pin MAX810 but I find only the complex MAX1232. I have read the datasheet, and it has a strobe input ST, which needs to be connected to an I/O of the microprocessor. The program then has to be modified because the microprocessor must toggle the ST input within a set period to verify proper software execution. I think that this programming is too advance for me so I still endeavor to look for the 3-pin chip.

Moreover, in fact, I just build the circuit and also obtain the Monitor program (firmware for EPROM) from the author's site. It can be downloaded for free. The author has stated that the Monitor program has been made very compact, and he uses many tricks to make it as compact as possible to let it fit in the 8K EPROM space. So if I have to insert 2 instructions to disable the internal Watchdog timer of the 80C535 for instance, maybe I have to learn the whole code to let me shorten the messages in there, just to let me to add instructions.

I appreciate any suggestion or comment, thank you.