Page Appendix G Appxg
User Manual: Appendix G D3-350 CPU User Manual
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PLC Memory In This Appendix. . . . — DL350 PLC Memory 1G G--2 PLC Memory PLC Memeory Appendix G DL350 PLC Memory When designing a PLC application, it is important for the PLC user to understand the different types of memory in the PLC. Two types of memory are used by the DL350 CPU: RAM and EEPROM. RAM is Random Access Memory and EEPROM is Electrically erasable Programmable Read Only Memory. The PLC program is stored in EEPROM, and the PLC V--memory data is stored in RAM. There is also a small range of V--memory that can be copied to EEPROM which will be explained later. The V--memory in RAM can be configured as either retentive or non--retentive memory. Retentive memory is memory that is configured by the user to maintain values through a power cycle or a PROGRAM to RUN transition. Non--retentive memory is memory that is configured by the PLC user to clear data after a power cycle or a PROGRAM to RUN transition. The retentive ranges can be configured with the handheld programmer using AUX 57 or DirectSOFT (PLC Setup). The contents of RAM memory can be written to and read from for an infinite number of times, but RAM requires a power source to maintain the contents of memory.The contents of RAM are maintained by the internal power supply (5VDC) only while the PLC is powered by an external source, normally 120VAC. When power to the PLC is turned off, the contents of RAM are maintained by a “Super--Capacitor”. If the Super--Capacitor ever discharges, the contents of RAM will be lost. The data retention time of the super--Capacitor backed RAM is 3 weeks maximum, and 4 1/2 days minimum (at 60° F). An optional batery, D2--BAT--1, can be added to maintain RAM retentive memory if the DL350 is ever without external power (see page 3--6 for a detailed explanation). The contents of EEPROM memory can be read from for an infinite number of times but there is a limit to the number of times it can be written to (typical specification is 100,000 writes). EEPROM does not require a power source to maintain the memory contents. It will retain the contents of memory indefinately. PLC user V--memory is stored in both volatile RAM and non--volatile EEPROM memory. Data being stored in RAM uses V1400 -- V7377 and V10000 -- V17777. Data stored in EEPROM uses V7400 -- V7777 Data values that must be retained for long periods of time, when the PLC is powered off, should be stored in EEPROM based V--memory. Since EEPROM is limited to the number of times it can be written to, it is suggested that transitional logic, such as a one--shot, be used to write the data one time instead of on each CPU scan. Data values that are continually changing or which can be initialized with program logic should be stored in RAM based V--memory. DL350 User Manual, 2nd Edition PLC Memory Non--volatile V--memory in the DL350 G--3 There are two types of memory assigned for the non--volatile V--memory area. They are RAM and flash ROM (EEPROM). They are sharing the same V--memory addresses; however, you can only use the MOV instruction, D2--HPP and DirectSOFT to write data to the flash ROM. When you write data to the flash ROM, the same data is also written to RAM. If you use other instructions, you can only write data to RAM. When you read data from the nonvolatile V--memory area, the data is always read from RAM. Writing Data Reading Data RAM V4000--V4377 V4000--V4377 V4000--V4377 V4000--V4377 There is no way to read data from the Flash ROM directly. Other instructions (OUT, OUTD...) MOV MOV D2--HPP D2--HPP DirectSOFT DirectSOFT Flash RAM After a power cycle, the PLC always copies the data in the flash ROM to the RAM. If you use the instructions except for the MOV instruction to write data into the non--volatile V--memory area, you only update the data in RAM. After a power cycle, the PLC copies the previous data from the flash memory to the RAM, so you may think the data you changed has disappeared. To avoid trouble such as this, we recommend that you use the MOV instruction. LD K2222 OUT V4000 RAM Flash RAM V4000 = 1111 V4000 = 1111 Copy V4000 = 2222 V4000 = 1111 V4000 = 1111 V4000 = 1111 Not changed Cycle power This appears to be previous data returning. DL350 User Manual, 2nd Edition Appendix G Flash RAM PLC Memory RAM 1
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