Variable Timer Preset

[mikeb 1]

Hi, I am new to Allen Bradley. I have an ML1100. I need to set up a TON timer, but the preset needs to be adjusted from the LCD & Keys. It looks to me like the preset value in a TON cannot be specified as a data memory, it has to be a constant. Am I correct? What is the best way to achieve this? I was thinking of using a data register and increment it with a clock pulse, then compare with the value from the LCD, but even that doesn't seem simple. If it was a Mitsi, I would use a D register for the timer value...easy! Thanks in advance.

[Ken Moore 23]

Use a MOV instruction to move a value from an integer to the preset. MOV N7:0 T4:0.PRE

[mikeb 1]

Thanks for that. Its the sort of thing thats obvious when you know.

[Ken Moore 23]

Glad to help

[TConnolly 13]

No, you're not really correct. It is data memory, its not anything else. First off, a timer is not a device. It is a computer instruction that operates on a section of memory. The AB supports strong data types, and a T file is a section of memory where the words are divided into three word groups for use by a timer instruction. The timer preset is one of the words in the timer data structure located in memory, just like an integer (in fact it is an integer). The preset in any time structure is at the memory address TX:X.1. Using .PRE is a shortcut supported by the programming software to enforce strong data typing. You can pre program that word to be a particular value when you enter the timer instruction. You could also leave it at 0 and have that word in memory written to programatically. You can also program most HMI devices such as a Panel View, or the Redlion G3 or the C-More, and many others, to write directly to any desired word address of not only N files but also any word of a T file, C file, R file, etc. So depending on what LCD/keypad you are using you can probably write directly to the preset word of your timer memory. However if your HMI does not support range limiting then better practice is to write to an N word and include logic to make sure that a valid preset has been entered. Edited 17 Mar 2009 by Alaric

[BobLfoot 302]

One caveat to Ken's suggestion be sure and limit check N7:0 somewhere. For example. GEQ N7:0 0 MOV N7:0 T4:0.PRE. I can guarantee that if you move a negative number into T4:0.pre the cpu will fatal crash. Actually used this fact onetime as a troubleshooting aide in a high speed sorter to halt program execution when a set of conditions was true.

[Ken Moore 23]

Yep, should have included the range checking. A negative preset will always fault a cpu, I use this to check my fault shutdown logic.

[mikeb 1]

Thanks for all the replies. I understand it all much better now. I'm using the built in keypad and LCD on an ML1100, to allow the timer value to be adjusted. After more searching I found a whitepaper on the Rockwell website that described what I want to do. As I said i'm finding my way with Allen Bradley at the moment. I've used Mitsubishi for so long now, that it just takes a bit of time to get to grips with a different way. Once again thanks to you all, I should get to test it out later in the week.

[BobLfoot 302]

Slightly off topic Mike, but the Mitsi's power up with all real world outputs off until the program turns them on - correct? If so BE WARNED, AB uses an output table to record the last state of the output and this is the state it powers up to until the program changes it. This was a big key learning for me when I shifted from Toshiba and Omron PLCs to ABs.