The memory is divided into three parts: program, word and discrete registers. The program (ladder logic) and word registers are usually stored in FLASH memory, while the discrete registers are stored in RAM memory.

PLC - Memory Map

Eugene Kowch | P.I.D. Consultants Inc.

Programmable Logic Controller
 - Memory Map -

By Eugene Kowch, P.I.D. Consultants Inc.

The memory is divided into three parts: program, word and discrete registers. The program (ladder logic) and word registers are usually stored in FLASH memory, while the discrete registers are stored in RAM memory.


All programmable logic controllers (PLCs) have a memory map. Depending on the make or model of the micro PLC, the memory could range from 2K to 16K words. The memory is divided into three parts: program, word and discrete registers. The program (ladder logic) and word registers are usually stored in FLASH memory, while the discrete registers are stored in RAM memory.

To demonstrate a typical memory map, let us assume that the micro PLC we are working with has a total of 16K words of memory. The first 8K words are reserved for word data, with register addresses ranging from 0 to 17777 (octal). These are read/write registers which the ladder logic retrieves or stores internal data. The next 8K words are set aside for the ladder logic, with register addresses ranging from 20000 to 37777 (octal). These registers are not accessible to the programmer. The program is usually written in ladder logic with a PC, compiled and stored in flash memory.

The remaining 700 words are where the discrete data is kept, with register addresses ranging from 40000 to 41277 (octal). These are also read/write registers which the ladder logic keeps track of the I/O (input/output) of the PLC.

The discrete memory area is for inputs, outputs and control relays. However, you can also access the bit data type from a discrete register. Each discrete register contains 16 consecutive discrete locations. The above diagram shows one of the discrete registers from each of the I/O types.


Comments (0)

This post does not have any comments. Be the first to leave a comment below.


Post A Comment

You must be logged in before you can post a comment. Login now.

Featured Product

Introducing the revolutionary Comelit Visto Smart Doorbell

Introducing the revolutionary Comelit Visto Smart Doorbell

The Visto Doorbell is a new Smart Doorbell from Comelit which combines a modern Italian design with all of the basic features you are looking for in a smart video doorbell. In fact, it is so smart that it can use the 2 wires from your existing doorbell and pass both the power and the data signals. This solves the common problem of having poor wifi coverage all the way at your front door. The Visto also has built in night vision, video recording on a SD card and in the cloud, motion detection, and multiple mounting options including an angle mount. The second thing we are introducing is the Visto Dealer referral program. Because we only sell through the Professional Distribution channel, we want to give our dealers who buy in that channel extra incentives to sell our product. For any security dealers or installers who go on our website www.vistodoorbell.com, and sign up for our program, we will offer the following: - Free referrals to customers in your area - Cashback program - 10% discount during the 1st month of the launch - Free Visto t-shirts and gear - A chance to win a trip for 2 to Venice, Italy Its that easy. Let us refer customer to you and get paid in the process. All while selling a wonderful Italian designed smart doorbell which is innovative and easy to install. So all dealers should protect the pro-channel and install products like the Visto which are made and sold only at your friendly local security and low voltage distributors.