I've only used Modbus RTU on the 7800, not Controlbus, but some Controlbus info is explained on the spec sheet for the model QS7850A General Purpose Interface ControlBus Module
in part:
The protocol definition and device information is available to any company that wants to create a system to communicate
with the Q7700 Network Interface Unit or any device
connected to it. This information is contained on a single IBM format 1.44 Mbyte floppy disk, part number 206610A.
Except for a read me file, the contents of this disk are compressed in a zip file.
The disk contains:
- The protocol specification
- Device and point description files for the control devices, calls to a library of access routines (both the program and the access routines are included in C language source form)
- Instructions and a ROM image file to convert any QSxxx
card into a ?Testcard? device that implements all of the data
- A PC program to test and demonstrate the GPI Card and
its protocols. (An RS-232 to RS-485 adapter will be needed
to connect the PC RS-232 port to the GPI Card RS-485
port.)
- A PC program to demonstrate the GPI Card binary to
ASCII conversion of reply messages.