Todd Dice (Customer) asked a question.
Port 3 SC102; Port_3_Ready_Flag on Click Plus C2-03CPU confusion
What causes the SC102 bit to stop working? And how does one re-start it when it stops?
I'm trying to write Modbus code to operate two GS10s and one Sure Servo 2 over port 3.
Programming to operate just one drive hasn't been an issue in the past, but geez, introduce more than one and I'm flummoxed. This is where I really messed up not using a P1K but alas...
Anyhow, this is also a good time to request you add a program in the Comms section of your example programs page of Click Modbus to multiple drives/servos/etc.
Thanks!
Good morning!
Click is interesting when dealing with multiple drives on serial comm ports compared to Productivity or BRX PLC's. Users must make sure to interlock serial communication instructions on Click PLC's whereas Productivity and BRX offer an option to "automatically" do this for users. Read further below for a synopsis of the Click example programs set up and behavior.
Example code for two GS10 drives can be found on a GS10 drive page as a downloadable Zip file which has Click, BRX, and Productivity examples.
Follow the link here to find the example program zip file on a GS10 drive page under the Downloads section: https://www.automationdirect.com/adc/shopping/catalog/drives_-a-_soft_starters/ac_variable_frequency_drives_(vfd)/micro_vfds/gs11n-22p0#
Follow the link here to find the example program zip file on a SureServo2 drive page under the Downloads section: https://www.automationdirect.com/adc/shopping/catalog/motion_control/sureservo2_(sv2a_series)_servo_systems/460v_servo_motors,_drives,_motor_cables/sv2a-4040#
Click PLC's: Why the Comm Instructions Are Interlocked — and Why It Matters
The core issue:
RS-485 is a shared, half-duplex bus. Only one device can transmit at a time. If two Send or Receive instructions attempt to execute simultaneously — or before the previous transaction has completed — the messages will collide on the wire, corrupting both, and the drives will receive garbage or nothing at all. On a VFD network this is not just a nuisance; a corrupted command word could cause a drive to fail to stop, fail to fault, or behave unpredictably.
How the interlock works:
The counter CTD1 (Comm Steps) acts as a token — only one rung's communication instruction has its enable condition true at any given moment. Each SD and RD instruction is conditioned on CTD1 equaling a specific value (0 through 5). Since the timer increments the counter only after the 500ms dwell time, each instruction gets its own exclusive time slot. No two comm instructions can be active simultaneously.
The 500ms timer spacing adds an additional buffer — it is not just sequencing but rate-limiting, giving each drive time to process the command and prepare a response before the next instruction fires. The documentation notes that increasing this value creates more time between instructions, which may be necessary on noisier networks or with slower device responses.
The practical consequence of skipping interlocks:
Without this sequencing, all SD and RD instructions would be conditioned only on
Always_ON and would attempt to execute every scan — likely hundreds of times per second — resulting in continuous bus collisions, permanent communication errors, and no drive control whatsoever.
In short, the interlock scheme is needed on Click PLC's to transform a free-for-all on the serial bus into an orderly, round-robin polling sequence — which is a better architecture for any multi-drop Modbus RTU network.