AB Read vs Write bandwidth

[JTCat]

I am working on a CLX that has to message tag values to and from a DH+ network that is somewhat congested. It is in the works to remove nodes from the DH+ network and put them on Ethernet with 5/05's.

Am I correct in assuming that 'write' instructions use less bandwidth than 'read' instructions?

It seems to me that a read instruction has to communicate both ways to retrieve data. First being to request the data from the other processor, then the data has to be sent back to the requesting processor.

Whereas a write instruction just blasts the information out there without regard to whether there is a processor listening for it or not.

I understand the whole troubleshooting read vs writes, but that's not an issue in this regard. I just want to know if using write operations is less bandwidth intensive than read operations.

Thanks,
JTC

 

[lfe]

Write operations will take a similar time than read operations since the other side has to reply to inform if the operation was successful or not.

 

[pturmel]

Whereas a write instruction just blasts the information out there without regard to whether there is a processor listening for it or not.

Simply not true. Writes have status replies. Reads are small requests with status + data replies. Writes have data in the request and a simple status reply. About the same bidirectional size.

 

[Peter Nachtwey]

MSG reads and writes require TCP connections and very send or receive requires an ack or nak. There isn't any significant difference but any difference would be due to the length of transfer.


BUT!....



The size of the transfer is not as significant as the number of transfers. If the data you want to read or write is close enough together then it is best to read or write whole blocks even if some of the data doesn't need to change or you don't want all the data that is read.


The old SLC 500s were very limited on the number of connections they could support. The Control Logix has a higher limit but I don't remember what it is. In any case, years ago there was a change to the Ethernet/IP spec where if the device ran out of connections, the connection with the oldest transmission was reused.