User`s guide

Page 27

Controlinc 1746-C (Version 5.21) Network Master Users Guide (2004-11-18)

As per affecting network performance,

unless the command is to write data to the actuators, the

PLC does not have to limit the rate in which these commands are sent to the 1746-C because the

1746-C only sends back the table information it already has stored in memory. Conceptually, these

do delay the overall scan time. However, the amount of delay is very minor from a global perspective.

(

Obviously, the PLC must still ensure all appropriate M0/M1 TRANSFER handshaking is performed)

As per writes that send data to the actuators, the rate should be limited. Please refer to the section

that discusses writing data to the actuators on the network.

Note 1: All tables can be read by the PLC. To ensure you only read from a read/write table, access it

with the command word set to “0” (zero).

Note 2: An invalid table ID sent to the 1746-C will default to a read request for table [0] data.

Note 3: If the 1746-C times out waiting for the previous M1 file to be read by the PLC, the previous M1

file will be over-written.

Note 4: On write commands from the PLC, reading the immediate feedback is optional for the PLC. In

other words, if the PLC has no need to inspect this table, then it can read it back but just throw

it out. However, servicing the M1 transfer request is important to overall system throughput.

For instance, the PLC program might be architected such that it treats an output pipe

(“command”) as just that … a one-way pipe that has no feedback. In this case, the PLC would

use other mechanisms for feedback (ex: other tables) and does not require the redundant

feedback on write commands.

Note 5: When a request for an M1 transfer is made by the 1746-C, the 1746-C program waits (in the

respective request function) until either the transfer completes or until a timeout is triggered

(currently set at 2 seconds).

This extended timeout time is to assist a PLC program that is having other (hopefully

temporary) difficulties and might take a while to return to normal behavior.

Therefore, to not slow down 1746-C network scanning activities, the PLC-RLL needs to service

these requests as soon as possible … even if it is to only throw away the table read back.

Servicing the request for an M1 transfer made by the 1746-C is critical for overall timing

efficiency.

Note 6: One main point to remember on M0/M1 transfer timing is that once the 1746-C responds to an

M0 transfer, the 1746-C will not return to process (setup for) another M0 file from the PLC until

it:

• responds with an immediate feedback M1 file transfer request containing the table being

addressed.

• finishes writing the entire table of values to the valves (if a write command).

• completes the set of scans in the next network scan time slice