Hello,
I know the Alert 500 system quite well, I've modified a few in the past. I currently work for a global company that has invested in technology and software to upgrade Alert 500 systems to Siemens S7. It is against the rules of this forum to advertise your own company, so I won't.
But if I may I would like to give some advice. As you know the Alert 500 has been out of production for a long time and availability of spare parts has become a serious problem. So your company should make a strategy to upgrade the Alert 500. This upgrade can be done by putting profibus adapters in the old I/O cabinets and connecting to a Siemens S7 (hence removing the PDP11). The old I/O can then be replaced with Siemens I/O little by little.
For your specific request of suggestions of how to replace the Sattgraph with a new SCADA, your options are:
1. find a SCADA package which can use a COMLI DDE I/O server
2. Pass data through a PLC that supports COMLI communications and another protocol that is supported by your favorite SCADA package.
One thing to be careful with is synchronization of data in COMLI. The maximum number of registers that can be transferred in one COMLI message is limited. To avoid problems, ensure that the registers that must be synchronised are sent in the same COMLI message.
Also, you say that the Alert 500 cannot be touched at the moment. This could cause you a lot of problems if the current COMLI setup requires modification as you might need to modify the Alert 500 program as well.
Kind Regards
Alastair