Card identification. Returns a 66. This is the same value returned by the HP 98644 Serial Interface. It signifies that the following differences from the HP 98626 Interface are present:
1. The optional receiver/driver lines are not present.
Register 7 does nothing.
2. Configuration switches are not present. Defaults are
9600 baud, 8 bit, no parity on a PC.
3. The physical connector is a RS-232-C 9 or 25 pin connector.
