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How many 485 serial ports can be installed?

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  • MVP 2023

You didn't say which UniStream model, so I'm looking at the 10" unit with the separate CPU.

You get one RS485 port on the CPU.

According to the specification guide for the USC-P-B10 CPU-for-Panel -


The UAC-02RSC has one RS232 port and one RS485 port.  I fired up UniLogic and started dropping them on the back of the panel.  I was able to drop four modules.

So the answer is five RS485 ports.

Joe T.

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Thanks for your help, I posted my problem in the visilogic forum where I have 9 modbus rs485 slaves and with only one port I was able to convincingly connect up to 3 slaves. each slave has a timeout of 100 millisec to go to network failure. the data exchange is to force 5 coils and read 11 coils each. (in less than 100 millisec) so when I connect the 4th slave, it goes to failure for more than 100 millisec. (manufacturer's fixed value) softstart PSTX ABB.
with your answer I can think that each port works with 2-3 slaves. but there is another problem with unistream that in the configuration sheet, you can not decrease the speed of penny popping at less than 100 millisec.
I have added my code developed with visiligic. I'm still not happy with this work, I think I lack knowledge to have a fast 485 network (with 9 slaves)
I forget to say that slaves have only 19200 bps.

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  • MVP 2023
On 6/23/2018 at 5:52 AM, gongue2005 said:

I forget to say that slaves have only 19200 bps.

I try to find information about PSTX ABB   and see  parameter list with communication settings.

12.09 - settings  for communication speed,

Also find some note in


: If there is no message passed between the PSTX softstarter and the Modbus RTU master for more than
100ms, the PSTX softstarter will trip on fieldbus communication failure protection (P1E00) and with the default
configuration, the motor will be stopped. If the fieldbus communication system is setup in such a way that
commands/requests are not continuously passed between the PLC and softstarter, this protection function should
be disabled. The parameter 19.4 (Fieldbus failure op) can then be set to "Off".



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hi all, I managed to stabilize the 9 PSTX by installing 3 RS485 ports on a V1040 connecting 3 soft start on each port. They worked very well until I change the screen. the last 3 slaves of each network are going to fail.
it seems that the processor stops the ports momentarily when it loads the new screen. What do you think of this?

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  • MVP 2023

Something with ABB modbus is wrong ....
Since this is only software starters, their management requires a maximum of 9 outputs (on-off control)

and 9 (alarm) or 18 inputs (alarm + normal operation-end of speed rump).

You may need to drop modbus from the list and use the unitronics I/O expansion modules.

By the way,  maybe you have wiring issue 

I did not know how to check the ports momentarily stops.

Maybe transmission is not hardware buffered and only software data flow control used (interrupted by screen change).

Or screen change command is active for to long time.


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