I'm still learning and need help

What type of memory do todays PLC's use most offten?

Let's see...I knew it once...oh, what is it?--I can't remember. Oh yeah...no that's not it. I forgot. I had it memorized once. What's wrong with my memory?

Student, by chance?

(digital)

Usually DRAM.
That's why most PLCs require a battery to retain their programs.

SRAM works in a simlar way, but is generally more expensive.

BTW, my answer is 100% correct, but it might not be what your teacher is looking for. My advice to you is to read the book/take notes to find out what exactly he/she wants to hear.

get ready for some flaming action.
Not sure how specific you need to be but I would have to say Solid-State Memory.

LJBMatt said:

get ready for some flaming action.
Not sure how specific you need to be but I would have to say Solid-State Memory.

Click to expand...

You mean, I can throw away my IBM Toroidal core stacks? or are they considered solid state?

I remember core memory. We tried to migrate from core to bubble memory. Bubble memory did not work for programmable controllers. Stayed with good old core memory. What kind of memory did HAL use? "Dave, I'm losing my memory..."

PLCs most often use binary memory.

Decimal data is converted into binary by adding powers of the number 2. For example, the number 5 becomes 101 in binary (1*2^0 + 0*2^1 + 1*2^2, from right to left). It is easy to store binary numbers in a transistor circuit with few errors because the transistor(s) will be in saturation (on=1) or cutoff (off=0).

Data is most often handled by the PLC in bits, bytes, and words. Bits represent a single binary digit (0 or 1). A byte is 8 bits, representing a larger (0 to 255, or -127 to 128) number. A word is a group of bytes: typically two, but it varies from PLC to PLC.

There is also a group of bits called a nibble, which represents a half-byte (four bits, or fifty cents). A nibble can be represented in hexidecimal format. Hexidecimal is base 16, so the numbers 0-15 are represented by a single place holder. This is accomplished by substituting the letters A-F for the numbers 10-15.

Hexidecimal should NOT be confused with BCD! BCD places an artifical limit on each nibble, limiting them to the values 0-9. Each position (power of ten) of a decimal number is then represented by a single, distinct nibble. When the byte or word is displayed as a hexidecimal or BCD number, the value of the data will be immediately visible as a "real-world" value.

I hope this helps!

AK

Core & Bubble memory. Now thoes are OLD terms. Ive never seen a core memory matrix but I understand one had to weave small iron donuts in a wire matrix that would hang on a vertical grid. I've tried bubble memory but it was sloooowww, unreliable, and quickly overcome by EEProm, and UVprom etc..


elecman38,

My guess is the instructor is refering to the user aspects of PLC memory, such as program memory, ROM, and non-volitale RAM, and/or Programmable Read Only Memory.

Most (not all) newer PLCs have three types of memory.

ROM (read only memory) where the PLC instruction set, default settings and such are stored.

EEprom (Electricaly eraseable programable read only memory) is where the user program is stored. Can be replaced with UVprom, sometimes called Flash Rom etc..

RAM (Random Access Memory) is where the user program is executed. Wipes clean on power cycle. Operating system loads program from EEprom into RAM where it is executed.

My suggestion is to download and read the user manual for the PLC that you are using in your class.

I must be old

I have programmed commercial PLCS that had magnetic core memory (Square D Class 8881) and bubble memory (Sy/Max Model 700). Heck I've even done UV PROM and something called EAROM (electrically alterable ROM).

As I recall, toroid cores were pretty solid, specially when you bounced them off the back of someone's head.

In my house, we have a lot of bottled water, some soft, some hard. The hard water is in the freezer, it is also solid state water. Soft water is in the refrigerator, or on the counter.

Sorry, a slow posting night.....casey

casey, you must really be bored.