There are a number of addressing schemes used by PLC manufacturers. Let’s take a quick look at how memory locations (including hardware I/O) are accessed with RSLogix 500. Along the way, let’s define some terms.INSTRUCTION – RSLogix’s command language is comprised of “instructions”. An XIC (it looks like a normally open contact – – ) is an instruction. A timer is an instruction. A few of the most common instructions are described below.BIT – an address within the PLC. It can be an input, output or internal coil, among others.In RSLogix, there are a couple of ways to show the address of a bit.
Put the CPU in stop mode and turn it off. Take out the SD card and turn the CPU back on without the SD card installed. From the display go to Settings Reset Factory Defaults, and select the factory defaults with no SD card installed. While the CPU is resetting, put the SD card in a computer and look at it with Windows Explorer.
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The default is:type:word/bitFor example, an address that references an output of a MicroLogix 1100 is O:0/0. That is:O:0/5 means that it is a physical output.O: 0/5 means that it uses Slot 0 (in the case of the 1100, this output is onboard)O:0/ 5 means that it is the fifth output on the PLC.By the way, don’t get the capital “O” confused with a zero.RUNG – A section of the PLC ladder program that terminates in an output function of some type.
Just like in an electrical ladder diagram, a rung has some type of output that is turned on or turned off by the preceding entities in the rung. The first rung in a ladder program is always 0000.HARDWIRED INPUT – a physical connection to the PLC from an input device (switch or sensor, etc.).Allen-Bradley uses the capital letter “I” to designate a hardwired input.
An address that describes an input on an SLC 500 is I:4/0.Similar to the output structure,I:4/0 means that it is a physical input.I: 4/0 means that it uses Slot 4 (the 5th slot in the rack).I:4/ 0 means that it is the first input on the card.Don’t get the capital “I’s” confused with ones.HARDWIRED OUTPUT – a physical connection from the PLC to an output device (relay or pilot light, etc.) As was said above, an address that references an output of an SLC 500 is O:5/0.INTERNAL COILThis is a programmable bit used to simulate a relay within the PLC. The internal coil has no connection to the outside world. It does not connect to an output card. Internal coils are used to store information. The “contacts” of this “relay” can then be used multiple times in other parts of the program.In RSLogix, the “B3” (binary) file is commonly used for all the internal coils. There are many other words in other files that have bits you can use as internal coils, but we are going to stick with the B3 file for our application.B3:0/0 means that it references an internal Binary fileB3: 0/0 means that it uses the first word in the tableB3:0/ 0 means that it is the first bit in the word.Note that, unlike the Output and Input files, you have to use the file number in the address.
In this case, the default file number is 3.TIMERA timer is a programmable instruction that lets you turn on or turn off bits after a preset time.The two primary types of timers are TON for “timer on delay” and TOF for “timer off delay”.Timers in A-B SLC and MicroLogix processors use file 4 for their timers.T4:0 means that it references an internal Timer fileT4: 0 means that it uses the first timer in the tableThe address T4:0 simply refers to the timer. Each timer has bits that turn on after the timing function is complete. You can address this bit by simply putting a “/DN” after the timer address.