Hello dear friends,

I have some novice experience with 8051 programming in Assembler.
Recently I have to repair several Vehicle ECU(Electronic Control Unit). The ECU employs two Siemens 80C535 microprocessors in the unit, so I wanted to test the 80C535.

Since it is difficult to find an 80C535 Tester, I decided to build a simple one myself. With many 8051 circuits are in the Internet, I was succeded in building an 80C31 Development Board. It can be connected to PC running Hyperterminal via RS232 port, and has a Monitor Program in the EPROM. I can download a hex format 80C31 program from the PC into the board's RAM and run it. It has also an 8 bit port to connect to the outside world.

Afterwards, I built a same board, but I substituted an 80C535 in place of the 80C31. I implemented some modification of course :
1. Set the Reset pin normally high, merely by connecting a 10µF from pin 10 (Reset) to Gnd. (while 80C31 needs this pin normally low).
2. Connected a 0.1 µF from pin 37 to Gnd.
3. Connected a 22 µF from pin 11 (V reference) to Gnd. Also connected a 100 Ohm resistor from this pin to +Vcc.

I connected PSEN, Read, Write, EA, PE and ALE appropriately, and set Port 0 and Port 2 for memory access just the way the 80C31 does.

However, when the board had been built and connected to the PC (running Hyperterminal), no Welcome Message nor any character appeared on the Hyperterminal. I have checked all the hardware and nothing was wrong.

Please advice whether I have to do some more setting, or anything. I appreciate every suggestion, thank you.

Jusman Sulaiman