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".

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 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
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)

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