PLC Connect- Modbus/TCP - Coils Addressing Requirements
Dear Patrick, good evening.
I was reading your comments on the question (https://forum-controlsystems.abb.com/3410141/PLC-Connect-Controller-Addressing-
for-Communicating-Modbus-Registers) then, I've got a doubt.
You state that "PLC Connect will always read a minimum of 16 consecutive coils or discrete inputs and it will always start on an address that is a multiple of 8." then, I understand that PLCC imposes rules on the Modbus-Slave communication map. Is there any documentation listing those rules?
I am trying to understand if Modbus-Slave is allowed to have gaps like in the example in the picture below. It has starting addresses multiple of 8, followed by 16 positions in each column and the coils to be read are marked with an X at the right, with few gaps between them.
Thank you in advance!
I was reading your comments on the question (https://forum-controlsystems.abb.com/3410141/PLC-Connect-Controller-Addressing-
for-Communicating-Modbus-Registers) then, I've got a doubt.
You state that "PLC Connect will always read a minimum of 16 consecutive coils or discrete inputs and it will always start on an address that is a multiple of 8." then, I understand that PLCC imposes rules on the Modbus-Slave communication map. Is there any documentation listing those rules?
I am trying to understand if Modbus-Slave is allowed to have gaps like in the example in the picture below. It has starting addresses multiple of 8, followed by 16 positions in each column and the coils to be read are marked with an X at the right, with few gaps between them.
Thank you in advance!
Asked 7 months ago
Voted best answer
1
Hi Marco,
It should be OK, but it's possible that you could get a telegram starting from coil 672 and not 680.
It should be OK, but it's possible that you could get a telegram starting from coil 672 and not 680.
Answered 7 months ago
by roger.a.nilsson Rank: 1540 on 3/26/2020 1:39:34 PM | Like (0) | Report
in document: 9ARD118131-016 800xA PLC Connect & SoftPoint Server
Version 6.0.3 Rollup 4 Release Notes and
Product issue number: 800xAPLC-OL-6003-005 (page 2)
ISSUE: Support provided for reading single coil in Modbus
TCP.
PLCC Modbus TCP sends request for 8 coils at
time. Incase coils are not configured in multiples of
8, Slave might reject the request as
ILLEGAL_ADDRESS_RESPONSE.
Addressing format to read single coil for Modbus
TCP: CSXXX (Example:CS101 to read coil status
of 101)
Correction: New addressing format introduced to read the
single coil in Modbus TCP.
In case coils are configured in multiples of 8
then the old addressing format can be
used(Ex:C101)
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