All I want to do is just use the Modbus for basic I/O functions (operator panel, lube control,coolant control, etc.) and allow my motion control card to do the servo and encoder work.
Dave -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Peter C. Wallace Sent: Friday, October 26, 2007 9:55 AM To: Enhanced Machine Controller (EMC) Subject: Re: [Emc-users] Starting a new project Snip_________________________________________________________________ > Wellll - not quite. > Unless they use the same clock source, there will be skew. Every once > in a while, you'll get two packets in a single servo period, or you'll > get none. That doesn't even take into consideration the jitter you > get on the PC end, which will cause the same problem even if you do > have a common clock source. You either need a request/response > system, or you need the remote unit to update frequently enough that > the data isn't too stale no matter when you read it. For example, if > you have a 1ms period, and 25 us old data is "close enough", then send > packets at 4 or > 5 KHz, and the data can never be more than 25 or 20 us old. Of > course, you then need some way of marking a set of data as complete > (so HAL can read from it) and as "in use" (so the net driver doesn't > write to it) - all of that non-blocking ... I've been thinking about a related problem with our RS-422 serial interfaced Amps and controllers, and the general problem of using the hardware for more precise timing, but leaving EMC in control of timing. With a FPGA, its possible to make a hardware timer thats phase locked to EMCs servo thread, basically just a DDS that the thread reads at 'time 0' This read has two functions, 1. The value read is the current timer mismatch with EMC thread, which tells you whether you are 'locked' 2. A hardware side effect of the read is to tweak the DDS to (slowly) correct the mismatch. Once the DDS is locked, the DDS accumulater bits can be compared with a list of timer values in FPGA memory (> compare), allowing hardware actions to be taken and any desired "phase angle" during the servo thread period. This way for example, our outgoing serial packets can have jitter in the nS range, or a PPMC card connected to a FPGA created EPP port could have read-ahead done just before the data is required... Peter Wallace Mesa Electronics (\__/) (='.'=) This is Bunny. Copy and paste bunny into your (")_(") signature to help him gain world domination. ------------------------------------------------------------------------ - This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
