Communication speed and response, amount of data points on a bus

A frequently asked question is: How many I/O modules can be connected to a bus? Physically,  up to 250 addresses with several thousands of I/Os would be possible. However, regarding the system response time the reasonable values for usual HVAC applications are:

  • Up to 300 physical datapoints (inputs and outputs) for a MiniPLC (IPLC201, IPLC301)
  • Up to 400 physical datapoints (inputs and outputs) for a MiniPLC (IPLC500, IPLC510)
  • Up to 500 physical datapoints (inputs and outputs) for a IPLC.1 (industrial PC with a touch screen).

An example of a I/O module set for a MiniPLC is

3x        MCIO (3x 29 I/O)
1x        M420 (16 DI)
2x        M210 (2x 8 DO).

The rule of thumb is that a MiniPLC301(201) can host four compact modules MCIO2 plus some extra binary I/O modules for the required I/O mix.

The response for one I/O module, using default communication speed of 9600 bps, is about 20 ms, so to refresh all I/Os the PLC needs about (3 + 1 + 2) * 20 ms = 120 ms plus 300 ms to process the function blocks application, thus the input – PLC – output response is lower than 500 ms.

The bus response can be accelerated by

  • setting higher communication speed, which, in general, is using non-default values together with higher risk of EMC problems. It is recommended to use higher communication speeds only if necessary, and at modules within the panel. The sensitivity to correct bus termination increases at the same time (the first and the last modules must have the BUS END switch set to ON).
  • separating the bus to 2 segments and their connecting to two ports of the process station (e.g. IPCT.1).

With standard HVAC systems using maximum amounts of datapoints per bus (see above) the system response is fast enough.

For monitoring systems (data collecting only) or integrated room controls (with no control loops over the bus), the datapoint amounts may be up to twice as high. However, to bring more than about 100 Modbus addresses on the bus is not advisable, because of difficullt diagnostics in case of short circuit or EMC problems.