Multiple ECOM100 Cards

[sgsims]

Is there any value in placing more than 1 ECOM-100 card in a 205 rack in order to increase the # of read/write requests that happen every scan?

My understanding is that each Ecom card can only handle one transaction at a time and the communication time slice during each scan can handle a max of 128 bytes.  Am I correct assuming that if I have multiple ECOM100 cards and the combined length of their messages is less than 128 bytes that those could all happen on one scan?

I have two barcode scanners on a conveyor constantly scanning boxes and I need to send that barcode to a host and receive information back.  Wondering if I should provide an ECOM100 card for each scanner and possibly a third to handle the host communication.

Scott

[franji1]

First question, do the barcode scanners speak a protocol that the ECOM100's understand?  The only possible one would be Modbus/TCP.  The other "application layer" protocols are ADC specific, i.e. DirectNet and K-Sequence.

[sgsims]

Yep the Cognex Cameras speak Modbus/TCP

[franji1]

Good - so then are you worried about latency?  Do you have multiple barcode readers that you need to respond to very quickly (e.g. under 100ms?).  Actually. probably under 10ms?

[sgsims]

Yes,  I am worried about latentcy.  That was sort of what prompted my inquiry.  Should I be using some other sort of communication protocol?

[franji1]

You could buy one ECOM100 per barcode reader on a private connection with nothing else connected to mimimize latency, but what is acceptable latency - 1 second? 100ms? 10ms? 1ms, 100 us?

[sgsims]

So I would say something like 50 Milliseconds would work

[franji1]

I would try just 1, use a switch that ONLY has those devices on it.  Your PLC scan time will be some of the bottle neck.  The faster it is, the more packets you can get through.

Note that 260 CPUs allow more than 1 packet to be processed per ladder logic scan, so you may be okay.  The CPUs control the backplane, so the ECOM100 module must "wait its turn", but it can get more than 1 turn per scan - the 260 firmware determines that.

[sgsims]

That's interesting...Yes I get the scan time bottleneck that makes sense.  When you say "packet" do you mean read / write command from the PLC to the Ecom?  I thought those commands needed to be interlocked.  That is why I was thinking more than one card would help.

[franji1]

I misunderstood the application.  When I read "barcode scanner", I thought that the barcode scanner was the Modbus MASTER and the ECOM100/PLC was the Modbus SLAVE.  Are you saying the PLC is the MASTER and the barcode scanners are the SLAVE, hence, you must poll the scanners from the PLC ladder logic as fast as possible?

If that's the case, YES, you would be best with 1 ECOM card PER scanner to minimize latency because the backplane ONLY supports 1 OUTGOING transaction at a time (e.g. RX/WX/NETRX/NETWX/ECRX/ECWX instruction on that slot).  So it would take a MINIMUM of 4 ladder scans to poll 2 Modbus/TCP devices, just 1 scan for the outgoing and 1 scan for the incoming.  And make sure YOU ARE DOING NO OTHER OUTGOING TRANSACTIONS.  No ECEMAIL, no other peer-to-peer comm to other ECOM100s, etc.

[sgsims]

Ah yes..now were getting somewhere:)  Sorry I should have been clearer.  That's what you get for dealing with amateurs:)

So as long as the 128 byte limit is not reached during the communication "Time Slice" then the bus can handle one transation per ECOm card...right?

So,  yes  that is why I was thinking and ECOM for each camera and also an ecom for the communication I need to do with the host after I receive the code from the camera. So that's three ecoms.

I also am doing some remote I/O with the Terminator I/O so I guess I need a fourth com device which is the ERM.

Sound like I am on the right track?

[franji1]

Yes - 2 ECOM100s and an ERM.  Host Engineering thanks you for your business!   

Are you going to need a CTRIO module too?  That would give you the Host Engineering Hat Trick!

[sgsims]

Funny you mentioned the CTRIO I actually have an encoder on the job so I guess I will get the "Hat Trick"

Cheers!  and thanks for the help.

[franji1]

I recommend using the ECOM100 and ECRX IBoxes for doing your polling "as fast as possible".  You would have two ECOM100 IBoxes at the top of ladder logic, one for each ECOM100 module.

Then to run the ECRX's as FAST as possible, do
STR ST1  // always ON
ECRX // first ECOM100
ECRX // second ECOM100

and the "interlocking" and "requeing" auto-magically happens via these "ladder logic macro" IBoxes.

[sgsims]

Awesome...Thanks..Guess I have some testing to do:)