Comparison of DCB multiplexer interface and the EIA/TIA 561 interface
DCB uses the RJ45 connector for its SR series 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-synchronous interface ... employing serial binary data interchange".
|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|
|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 hardware flow control were printers, directly attached to comm equipment or through a terminal aux port.
Now that we have a proliferation of modems attached to PC's, RTS/CTS has become more common. At this time, DCB has chosen to stay with pin 3 as hardware flow control (if so configured by the user) and RTS is used as a path to the remote DCD signal.
|Pos #||SR Terminal Port||Net Mgmt Port||SR Composite Port|
|1||DSR (out)||-||RxClk (in)|
|2||DCD (out)||-||TxClk (in)|
|3||Busy (in)||DCD (in)||DCD (in)|
|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)|
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