If TCL JTAG is used to issue a Shift-DR instruction in read-only mode and in addition if TDI is set to '1', invalid data is returned. This occurs only when using a Platform USB Cable.
Using Shift-DR in read-only mode and setting TDI to 1 will use fast read, and there is a problem with the way in which fast read is implemented. There are two alternatives to using read-only and setting TDI to 1.
1. If TDI does not matter when shifting out bits, then set TDI to 0.
2. Alternatively, just use read/write mode and make TDI a buffer filled with constant 0's or constant 1's.
This will be fixed in ChipScope 9.1.02i.