 | Bit 3 |   1 = DDCD (Delta Data Carrier Detect = DCD changed)   |
   | Bit 2 |   1 = TERI (Ring Indicator changed)                    |
   | Bit 1 |   1 = DDSR (Delta DSR = DSR changed)                   |
   | Bit 0 |   1 = DCTS (Delta CTS = CTS changed)                   |
   +-------+--------------------------------------------------------+
   Bits 0-3 are reset when read.
   Bit 4 is the <A HREF="RR0C.HTM">$DE0C</A>'s RTS during loopback test.
   Bit 5 is the <A HREF="RR0C.HTM">$DE0C</A>'s DTR during 