I picked up several ADP Parts Locator Interface Units at a local surplus electronics store awhile back. They are nice little boxes with a small alphanumeric keyboard, 24 character LCD display and a telephone interface. The "firmware" for the box is in a socketed EPROM. I pulled the EPROM and burned a new one to display several messages on the LCD in a forever loop. I have not traced out the whole circuit yet, but the problem I am seeing is the loop runs OK for 30 minutes and then stops leaving the last message being displayed on the LCD. I have not made any headway figuring out what is causing the system to stop. I assume it is some type of watchdog that times out after 30 minutes, but none of the chips on the board seem to serve that function. I at first thought that it was getting an interrupt and jumping to a nonexistant interrupt routine, but I then read that interrupts are disabled on power up until you enable them. I don't.

Here is a list of the IC on the board and I welcome any insight.

Thanks
Carlos Qualls

1 - 80C31BH
1 - CD74HCT32E
2 - MM74HCT541N
1 - 74ACT138P
1 - 74ACT573P
1 - TP5088N
1 - 24 Character LCD display
1 - keyboard
1 - socketed EPROM
1 - RAM MS62256L-10PC
1 - DS1210 battery backup for the RAM chip
Plus other nonIC parts for the telephone interface.