It's a matter of simple math to calculate whether you can poll 32 devices for a 16bit variable in 2 seconds.

Firstly, what is the baud rate? Let's say 19200 baud - 1920 chars/second or 520uS/char

Next look at the packet sizes for the request/response

request - 8 bytes + 4char space for Modbus RTU (actually 3.5)
response - 7 bytes + 4 char space

= 23 bytes * 520uS/byte = about 12mS per request/response transaction assuming the slave responds immediately.

12mS * 32 = 384mS. Well within your 2 second requirement. The slaves don't need to respond immediately - we have around 50mS per poll to spare.

If you were to use Modbus ASCII, the amount of chars at least doubles but we don't have the 4 char space requirement.

request: 17 chars (confirmed)
response: 15 chars (guessed)
= 16mS per transaction.


Can it be done? The numbers tell the story.

The baud rate could be lowered to 9600.
But it is always good to have some headroom - you can poll each slave for a number of 16 bit registers and still have time to spare.

The benefits of using Modbus:
open standard
supported by many software/hardware vendors
well known
easy migration to Modbus TCP/IP - use a module like the Advantech ADAM 4572 which allows you to use standard networking - fibre,WiFI etc

Downsides:
none really....




*disclaimer* This was a quick calculation and the data presented may not be accurate - do your research to confirm.