Internet PLC Forum

My current solution. I added "delay 10" statements between each setlcd command. This increases the LCD screen transfer time from ~22ms to ~62ms, and of course ties up the PLC in that custom function. However, the LCD errors seem to have gone away, although tests continue to be run.

Lucas

For anyone stumbling onto this posting with similar problems, I've sourced my particular trouble to a faulty DMC20481 module. The trouble seems resolved with a replacement module, complete with better performance seen with the new module (line writes occur visibly slower on "faulty" module, with character phasing when new characters are written to screen).

Lucas

Hi all,

I'm wondering if there is a way to control the T100MD-2424+'s LCD module, other than using a series of SETLCD commands? Also, how big is the PLC's internal LCD message buffer? I realize that the design objective of the PLC's LCD module probably didn't include this low-level access, but I figure it is worth asking about.

I'm currently debugging an LCD error and would like to be able to control the exact messages sent from the PLC to an Optrex LCD module (DMC20481). The PLC's SETLCD commands are on a ladder with a 0.5 second trigger. My debugging code has 4 20-character lines being sent (all 1's, all 0's, all 1's, all 0's). An external switch is used in the triggered custom function to send an alternate message sequence similar to the previous, except the character "1" lines and character "0" lines are flipped. Measurements of the PLC LCD output lines demonstrate that the full 4-lines are transferred in ~22ms.

Every once and awhile, the LCD display seems to receive configuration messages that result in random effects (display reconfigures to 2-lines, display completely blanks, display shifts line text to the right several places, ect). This occurs most reliably during the transition between the two messages (when I throw that external switch), but errors are also noted during "steady-state" message, albeit far less frequently.

After initial configuration, no further explicit configuration messages are sent via the SETLCD commands, but monitoring the LCD control lines shows that the PLC does send configuration message as part of its SETLCD implementation (presumably to indicate which line each set of characters is to be written). The RS line is measured as triggering between each set of 20 characters, with two E edges seen within that window.

My current debugging narrative describes a buffer being flooded by the back-to-back SETLCD commands specifying 80 characters + line descriptor control bits. My next experiment involves inserting delays between the execution of each SETLCD command, which should alleviate any buffer flooding concerns. However, to find the root cause of exactly why this is occurring only sporadically, I would need to control the exact messages that are being sent. This would allow me to more precisely test which buffer is possibly being flooded (Optrex module, or PLC module).

Lucas

Hi all,

I have a set of questions regarding the performance factors involved in operating a T100MD-2424 PLC. I'll post them in the order I find logical, but I wholly admit the existence of unconfirmed assumptions that may make the questions moot, irrelevant, or misdirected. Please view these questions from a performance viewpoint, not a clarity-of-implementation or any such subjective viewpoint.

- What is the maximum scan frequency of the ladder scan? If I have a single ladder, with a single ladder element tied to an input channel, how fast can I pulse the input and have the PLC catch the pulse?

- What sort of performance hit is expected with an linearly increasing ladder rung count? Say each ladder rung is uniform, and of the simple sort described above.

- What is the relative performance hit of implementing functionality in custom function BASIC versus ladder logic-triggered RELAYS? What is the performance hit involved with using logical BASIC conditionals versus implementing multiple ladder rungs? (By BASIC I'm referring to if/else if/then/else conditional logic blocks)

- Is there a location where I can find the operating specifications of the T100MD-2424? I'm looking for a datasheet with operational characteristics, more so than an "installation" or "operation" guide.

Thanks in advance for any insights!

Lucas