+------------+                  +-----------+
|     TXD out|>--------+  +----<|TXD out    |
|            |         |  |     |           |
| DTE        |     +-- |--+     |       DTE |
|            |     |   |        |           |
|     RXD  in|<----+   +------->|RXD in     |
+------------+                  +-----------+