Question about spare ram margin for 1756 redundant system

[Fan Liu 1]

In my 1756-L61 standard redundancy system I only get 15% spare ram after my logic programming finished. The 15% include enabled redundancy option in rslogix5000. If disable the option there would be 25% ram left. In my program I use LAD,FB and ST language.And I have do my best to limit the ram use .For example most of bool variables are optimized by bool array alias tag, and using a lot of UDT structure. My question is what is the recommended spare ram for stable redundant system and what percent of spare ram is the system margin. Due to uncertain changes in commission site I think L61 is a bad choise for my project although the choice is not taken by myself. Thanks for any comment on this issue.

[BobLfoot 301]

Have you acquired a copy of the RA "Redundancy Bible" which details how to configure, program and commission RA redundancy systems with 1756 processors. Failure to follow the guidelines has caused more headaches than most anything else.

[Fan Liu 1]

I have read 1756-um523_-en-p manual which may be so-called "Redundancy Bible" but not found any detailed information about spare ram issue.

[Steve G 1]

Check out chapter five of the manual. It gives you the best way to set up data types and programming methods. I did not see an exact limit to the ram usage for a redundancy system but there is a lot of useful info in the chapter. I am currently working on a redundancy system and originally had a L61, but went up to a L62 due to changes in the I/O count and program length needed. I always like to have the ability to add things later and not have to worry about if it will cause problems later. The location I work at is always changing and it is better to have it and not need it than to need it and not have it. One last note and I say this to save you from my own dumbness, read the whole manual so you are not surprised during setup and commissioning time. Make sure you are reading the correct manual, i.e. standard vs. enhanced redundancy system. When I and ordered the plc parts for my system I must have been reading both standard and enhanced manuals because I ordered and L62 an EN2T and a CNBR. The EN2T is used in an enhanced redundancy system and a CNBR is used in a standard redundancy system, so in the end I had to send the CNBR back and order and CN2R card. Not a big mistake since I caught the error before I opened it, but it could have been and couple of thousand dollar mistake. Edited 2 Mar 2011 by Steve G

[Fan Liu 1]

Dear Steve, thanks a lot for your reply. I just worry about small spare ram will lead redundant system to be unstable or crash when switchover occurs. Also it maybe out of ram if there are lot of changes in the site. Processor replacement is impossible now because the project T&C will come soon. It is a taking over project and I got a passive position now.