Datatek DT-4180 - Manual (Page 94)

9 4 8 0 , 4 1 8 0 , 4 2 8 0 , 4 2 8 4 4 0 0 0 X A U s e r M a n u a l
04/09/09
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1 4 S Y N C H R O N O U S P R O T O C O L S W I T H
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The ports on a xxxx support the option of recovering clocks from the data lines for
synchronous protocols. When this occurs, the encoding for these protocols must be
either NRZI or a Biphase frequency modulation. The NRZI option is reliable for bit
stuffed protocols such as SDLC, but is subject to failure in a BiSync due to the potential
for insufficient bit cell transitions. This is not a limitation on the xxxx hardware, but rather
a fact of the encoding itself. The NRZI encoding on SDLC data streams is by far, the
most common variant of these protocols. It is this NRZI encoding which is used on the
isochronous LTS connections.
In order to use a xxxx port with recovered clocks, it should be programmed with the
protocol, the line encoding, the operation on the data, the baud rate, and the recovered
clock. Consider the following command:
port 1 prot=sdlc baud=1200 enc=nrzi dxe=dce clk=rcvd fill=flag
This command instructs the xxxx that the port is SDLC NRZI with recovered clocks. The
baud rate is required in order to properly recover the clocks from the data, and must
match the peer. The dxe=dce instructs the xxxx that it should control CTS from the peer
DTEs RTS, and not wait to send data. The clk=rcvd instructs the xxxx that the clocks
are to be recovered from the data rather than on a separate EIA lead. Please note that
this configuration would use the asynchronous DCE adapter (i.e. the AG adapter). The
fill=flag option is the default for an SDLC port and specification of the option is not
required unless the port had been previously configured otherwise.
Suppose that the port is to be connected to a modem device operating in 2-wire (half
duplex) mode, and the port is to be the DTE in that configuration. The xxxx port would
need to assert RTS and wait for CTS before sending data to avoid corruption on the half
duplex interface. The following command would issue that configuration.
Port 1 prot=sdlc baud=1200 enc=nrzi dxe=dte clk=rcvd fill=mark
Note that the only difference is the dxe=dte option. This instructs the xxxx to assert RTS
when there is data to send, and then wait for CTS to be asserted by the DCE before
actually sending the data. The fill=mark idles the line between frames in the mark state
as opposed to flags. Some modem devices do not handle flag idled lines well. If that is
the case, then this option should be used. It doesn’t hurt to mark fill on DTE ports.
Please note that this configuration would use the asynchronous DTE adapter (i.e. the
AH adapter).