Hi everyone,

I was hoping someone would have an insight into the problem I'm currently having.
I'm using a Philips 89c51RD+IA in a serial datalogger design with the schematic shown at this link:

http://homepages.inspire.net.nz/~ricketts...ematic.gif (44.6K)

The circuit is built and the ISP is working fine, as is the LCD display.
The problem I'm having is with the button inputs through the 74HCT540N buffer. No matter what inputs are given to the buffer, the outputs (to Port 2) stay at 5v. VCC and GND to the buffer are fine, both OE's are being grounded fine, yet still the outputs stay at 5v when the inputs are made 5v (the 540 is inverting).

I've desoldered the 540 and checked it's operation on my EPROM programmer/chip tester - it seems to be fine.

When I pull the P2 lines low using a 5k resistor to ground the 89c51 picks this up fine.

Any ideas are greatly appreciated. This is my first 8051 based hardware design, so I hope it's just something small I've overlooked....

Many thanks,

George Ricketts