Comparison of DCB multiplexer interface and the EIA/TIA 561 interface
DCB uses the RJ45 connector for its SR multiplexer products. This decision was made long before there was a standard for this connector (EIA/TIA 561). By chance or by using superior insight, It looks like we did it right.
EIA/TIA 561, Approved March 20, 1990
The standard EIA/TIA 561 specifies a "simple 8 position non-synchronousinterface ... employing serial binary data interchange".
| Pin# | DCB | EIA/TIA | Comments | |
| 1 | DSR | RING | DCB applies +12 volts to DSR. It is not under processor control | |
| 2 | DCD | DCD | Data Carrier Detect | |
| 3 | BUSY | DTR | Printers and many TTY devices use DTR for flow control | |
| 4 | SG | SG | Signal Ground | |
| 5 | RD | RD | Received Data | |
| 6 | TD | TD | Transmit Data | |
| 7 | CTS | CTS | Clear to Send | |
| 8 | RTS | RTS | Is mapped to remote end DCD, as in multi-drop modems and dial up half duplex modems. This is used as the "terminal on/wakeup" to a host computer. On a Unix system, RTS on at the terminal end maps to DCD at the host, which enables a "login" prompt. |
DCB designed and implemented the interface before the 561 spec was released.The fact that the interfaces are nearly identical is by chance. (or is it?)
DCB uses pin 3 for busy (DTR), since in 1989/1990, most devices using hardwareflow control were printers, directly attached to comm equipment or through aterminal aux port.
Now that we have a proliferation of modems attached to PC's, RTS/CTS hasbecome more common. At this time, DCB has chosen to stay with pin 3 ashardware flow control (iif so configured by the user) and RTS is used as a pathto the remote DCD signal.
| Pos # | SR Terminal Port | Net Mgmt Port | SR Composite Port |
| DTE/DCE >> | DCE | DTE | DTE |
| 1 | DSR (out) | - | RxClk (in) |
| 2 | DCD (out) | - | TxClk (in) |
| 3 | Busy (in) | DCD (in) | DCD (in) |
| 4 | SG | SG | SG |
| 5 | RD (out) | TD (out) | TD (out) |
| 6 | TD (in) | RD (in) | RD (in) |
| 7 | CTS (out) | RTS (out) | RTS (out) |
| 8 | RTS (in) | CTS (in) | CTS (in) |