By default the S7-300 has a limit of 4 static connections, of which 2 are reserved for a PG (programming device) and an OP (operator terminal). So this leaves only 2 connections for Citect. If you have another communication module (a CP) in the PLC rack, another static connection is used for this. This limitation can lead to a situation where you can not connect a redundant Citect I/O server to the PLC, because the S7-300 has run out of static connections it can use.

However you can also use the reserved connections for a PG and an OP for a Citect connection. Normally in COML S7 software you define a 03 03 address for the TSAP address of the PLC. The 01 03 address (normally reserved for a PG) and 02 03 address (normally reserved for a OP) can also be used for a Citect connection. Of course this connection is then no longer available for its designed purpose (PG or OP).

In newer firmware version of the S7300 you have also another method to increase the amount of static connections that are available.

Standard an S7300 allow 4 static and 8 dynamic connections. In the newer firmware versions (>2AF01) you can decrease the amount of dynamic connections, and this adds to the amount of static connections. So from 4/8 you could go to 5 static & 7 dynamic or 6 static & 6 dynamic connections. You set this in the Siemens CPU configuration software, in the so called hardware configurator.