Does anyone make a PLC that could be mounted on a PCB that is 1 inch square or smaller to control 5VDC devices? All I need is purely digital I/O and some internal timers.
Sounds more like a job for a single chip micro -- have you looked at the Microchip line of microcontrollers?
Also, the Atmel AVR is a nice single chip FLASH controller. It is competitive to the PIC, but I like their product line better. I've succesfully used a GNU (Open source) "C" Compiler ported to AVR. I haven't used it in quite a while though.
Check out this website: Ladder logic compiler for a PIC microcontroller
They call that a microprocessor and the language, assembler, greatly resembles IL. Seriously. You can also get uPs that you can program in higher level languages. I haven't seen a RLL chip, but it's certainly doable, just little demand. Of course, a $5 uP from the automation folks would be $325 and an oblate spheroid geometry so it would only fit their $100 sockets. See if you can't do what you need with a PIC, a Rabbit or an old Z80. Getting it down to 1 chip is kinda limiting but they are out there.
Try a PIC chip or a Basic Stamp. The Basic Stamp is a PIC with a basic interpreter ROMMed in and mounted on a very small daughter board. There are
other daughter board micro-controller solutions as well.
Cheap (about 20€), ladderdiagram programming (included)....
Such devices are known as microcontrollers. As you have to control 5 VDC devices, the same can be done with open collector or TTL outputs. A full
blown PLC might be overkill and very big. The smallest PLC I have seen measured 8x10 cm without the power supply. The specs were 5 vdc 8 nos. DO/DI.