Status & Memory Bits

[ad2002 0]

Hi, Can any body tell me the difference between status & memory bits used in the mitsubishi plc range. Why would you typically use one over the other? Thanks Andy

[navillusi 2]

Not sure exactly what you mean between "memory and status" bits, M - Internal relays - auxilliary relays used by the cpu and are not latched (cleared at power failure or cpu reset) L - Latch relays - auxilliary relays used by the cpu and are latched (not cleared at power failure) F - Annunciators - Internal relays convenient for equipment fault / failure detection V - Edge relays (q cpu) - similar to a one-shot B - Link relays - CPU devices used to refresh LB relays in a melsecnet network S - Step relays - relays for step instructions in sequential function chart

[Guest Guest]

Im not sure either what exactly you mean by status and memory bits - so I will guess too. Maybe you mean retentive and none retentive internal relays. A none retentive M relay that was on before power off - will be off when power is resumed. A retentive M relay that was on before power off will still be on when power is resumed. A typical use of this would be to remember which way something was moving. If a motor or solonoids move something back and forth and you need to know which way it was moving after power failure, you would typically use a retentive relay for this.

[JimRowell 1]

Mitsubishi uses the term "Memory bit" to describe a memory location that is 1 bit wide, is directly addressable and is not I/O. In other words, an internal relay. A "diagnostic" bit is a memory bit that indicates or controls the state of the system. In other words, they have pre-defined functions. Example, there are bits for indicating whether the plc is running or stopped, whether an error has occurred, operation flags for zero, borrow, carry, etc. Some of the diagnostic bits are referred to by Mitsubishi as "Status" bits. I wouldn't be surprised if a lot of people use the terms interchangably. Anyway, its not important what you call them. A status or diagnostic bit has a pre-defined function. Other memory bits do not and are available for whatever use you wish. Jim

[Guest Krassu]

I have also a question like as2002 had. What are differences between S and M registers? Both are latched when adress starts from 500.

[Crossbow 295]

S bits are step indicators in STL (stepladder) which is Mitsubishi's flowchart language. The S bit for your inital flowchart step should be set on first scan of the PLC, and then the flowchart will update the bits as it transitions from step to step. They really have no other use than STL. M bits are where the PLC stores internal bits and you write marker bits in your PLC program. The M bits over 8000 have special functions, like first scan, always on, clock pulses, error bits, etc. FX Programming Manual II on Mitsubishi's website has a complete detailed list of all of the memory areas in the FX series (FX1S, FX1N, FX2N, FX2NC) which you might find of use. I think it's part number is JY992D84801 Just go into Documents And Downloads link in upper right, pick Manuals, Pick Industrial Automation, then PLC, then PLC-F and it will be in the list.