What selection should I use to get best performance over a serial connection? The primary factor is of coarse the baude rate but what are the optimum settings for parity, error checking, NAK, ENG & ACK?

Factors affecting speed of serial connection with RSLINX

Does selecting either BCC or CRC error checking make any difference to the speed of transmission when connecting a Compact logix L30 to a RSview32 SCADA.

Same query RE Stop bits, & Parity. i.e. what selection wil give the best speed. I know that the speed is primerly determined by the selected baud rate, but do these other factors have an effect on the speed?

Thanks for any comments.

Steve

 

Stick to the CRC - many AB/PLC cannot be changed, plus the CRC error detection is orders of magnitude 'safer'. The 1 extra byte sent (BCC is 1 byte, CRC is 2) won't affect performance enough to justify reducing safety. The calculation time is tiny compared to serial shift times (ie: baud rate).

Assuming low error rate, NONE of the ACK/NAK/ENQ settings will speed you up. AB PLC ACK/NAK independent of "understanding", so they ACK/NAK fairly fast without even needing to parse or process the PCCC message carried.

All AB/PLC can "buffer" atleast 4 outstanding requests, so no need to wait for a response before sending more requests (most AB/PLC buffer 10 or more). Using DF1 Full-Duplex is much faster than Half-Duplex (likely you knew that).

Make sure the "embedded response" is true; this can almost double your performance. This means RSLinx and PLC can "embed" the ACK/NAK within a message and don't need to (as the DF1 spec implies) only return ACK/NAK between messages.

Make sure you use good cable and good shielding to reduce "noise" that causes retries.

best regards
- LynnL, www.digi.com

 

Thanks for the advice. Where do I set "Embeded response" (For a compactlogix 1769L30)? I don't see any such thing in RSLINX.

Thanks Again
Steve

 

As I understand it RSlinx auto-detects it; it won't use unless the PLC does first. So look into the RS-232 config of the L30 and enable it there - if it is available. I'm not a Control/CompactLogix expert. But given the nature of CIP-via-DF1, I don't think it will help you any.

Now I see why your concern to maximize serial comms. RSLinx has little role in helping you for RSLogix up/download. It is RSLogix5000 that decides *HOW* to use the many services of RSLinx, and RSLogix is really hoping for Ethernet's brute-force behavior.

I have not looked at CompactLogix-via-DF1 for 2 years, but previously RSLogix5000 would open a single CIP connection for the download and run pretty much in a half-duplex paradigm. An 80K program created about 350 CIP packets, and these are actually fragmented into DF1 and include an application layer ACK (in DF1's view) per fragment. So it is a lot of back-n-forth on the serial line with lots of little delays adding up.

For faster up/downloads your only options will be to maximize your baud rate (I think CompactLogix support 38k) or invest in a newer CompactLogix CPU with direct Ethernet.

best regards
- LynnL, www.digi.com (experts in Ethernet-to-Serial)

 

I have an issue, and that is trying to tune RSLinx via Serial DF1 on a Microwave link, would you know of any hints on doing this? Certainly m,ost channels are working except I get two that seem to fail often.

 

That does sound like a time-out setting is just on the edge.

Not to sound like a parrot, but make sure you carefully search AB's online knowledge base. They do an above-average (excellent) job of documenting such specialty solutions - plus I know Ab PLC get used often with Radio modems so the issues should be well known.

If that doesn't answer you may need to contact Rockwell. I have heard there are lots of hidden configuration options for RSLinx, but they (correctly in my view) want you changing them only when someone knowledgable confirms it will help and not hurt you. It is amazing how many smart engineers think "trial-n-error" (sometimes near-random) adjustment of settings is "tuning".

best regards
- LynnL, www.digi.com

 

I have the similar problem funny enough on a Microwave link.

I am using westermo td23s at 1200baud to transport values to a front end PLC over 4wire multidrop analogue cards.

one front end PLC works fine going north bu the PLC going south come with the error DF1 port Buffer full. we are waiting to get an answer back from Rockwell but maybe you are having the the same problem and itr could be the buffer in the DF1 port not clearing. how can I clear the Buffer?

Rgds

Grant