RE: Modbus-TCP "authorization"

[Abdalla Ahmad via Tech-talk <tech-talk@aps.anl.gov>]

You can try writing a record for each authorization step with passive scan (or no SCAN) and then each record process the next with FLNK, with this you can only process the first record and the rest are processed sequentially. We do have a similar situation where a Modbus device requires setting a password for administrator access, we made a record that just writes a value to a certain address and we have admin access to the device.

 

Best Regards,

Abdalla Al-Dalleh

Control Engineer

SESAME

 

From: Tech-talk <tech-talk-bounces@aps.anl.gov> On Behalf Of ??????? ??????? via Tech-talk
Sent: Tuesday, November 18, 2025 6:33 AM
To: tech-talk@aps.anl.gov
Subject: Modbus-TCP "authorization"

 

Hi!

I have a Modbus-TCP device which requires a so called "authorization" to
be performed BEFORE any actual I/O can take place; without such an
authorization all register writes and even reads result in exception 2
("illegal data address").

The "authorization" consists of several register reads and writes, in
specific order:

   1. Write INT32_LE:0x89ABCDEF to holding registers 0x2100,0x2101
   2. Write INT32_LE:0x01234567 to holding registers 0x2102,0x2103
   3. Write 0 to holding register 0x2104
   4. Read discrete input 0x2200

Is there some mechanism to perform this sequence immediately upon connect?
Looking at "modbus" module documentation, I see nothing relevant.

Thanks in advance!

Vitalii