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P300H P300 Series Modem Installation and Operating Handbook Page 149
`Delayed Character Mode` is generally not a show stopper, but can cause issues with packet based
communications.
At Paradise we have only been able to make the ESC channel enter this `delayed character mode` by
setting up a pair of modems in intentionally mis-matched configurations. However, now we have
investigated and understood the mechanism, we feel it is feasible that under repeated changes of
configuration the ESC channel might enter this `delayed character mode` (although we can’t get it into this
mode ourselves by this method). To avoid this possibility we have modified the modems embedded
software from V3.56 or later to ensure that during normal reconfiguration the ESC channel should not get
into this mode.
While we are confident that in general the ESC channel is not likely to get into this `delayed character
mode` it is still possible to make it enter this mode if the modems are operated with intentionally mis-match
configurations. For those that would like to understand why this happens the following text describes the
operation in more detail. The following text is NOT mandatory reading !
ESC channel `delayed character mode` (very technical)
The Async ESC is carried in the satellite overhead by multiplexing it into the synchronous data of the
satellite framing which is added to the main data. Each Async character is converted into a byte, buffered
through a small FIFO, and then sent synchronously into the bits available in the frame for the ESC channel,
using a leading 0 to indicate that the next 8 bits of overhead available to the ESC carry the 8 bits of a
character. These 8 bits might be at one bit per satellite frame, or if a high proportion of the overhead is
assigned to the ESC (such as in Closed Net Plus ESC mode) they could be successive bits (eg the whole
8 bits of satellite TS48).
This works fine, and normally if you mismatch the configuration of a pair of modems then the modems will
not talk to each other as they calculate different overhead percentages to accommodate the Async ESC
channel, the symbol rates do not match and hence they will not communicate.
However once the overhead reaches the 0.39% minimum, changes to the ESC configuration do not change
the overhead percentage (it is already at minimum), and the modems will communicate with mismatched
ESC settings. For example if one modem is set to provide a ESC and a backward alarm facility and the
other ESC only, then the modem without the backward alarm facility will route the parts of the overhead
which carry the backward alarm from the distant end to the ESC channel decoding circuitry (because that
is what it has been configured to do !). This can result in two problems:
1) When the configurations are mis-matched, there will be spurious characters output from the ESC
channel, or the ESC channel will suffer significant character errors (as the backward alarm status
is output as ESC data).
2) More significantly, the extra data routed to the ESC channel decoding circuitry from the overhead
(in our example one bit per frame which carried the backward alarm from the distant modem) is
NOT limited to 2400Baud by the sending modems physical Async ESC port, and may exceed the
rate at which the UART in the receive modem can output these spurious ESC characters.
Specifically, the small FIFO between the framing / overhead circuitry and the UART can get
massively overrun, and after such an overrun this may lead to the ESC operating in `delayed
character mode`. When in this mode, only a total modem reset will restore normal ESC channel
operation.
In summary, it is possible by mis matching the configuration of a pair of modems to set them so that they
continuously output spurious or errored characters from the ESC channel, or force the ESC channel into
the `delayed character mode` where a major (power down) type reset is required to restore normal ESC
operation. As the modems at each end of the link are simply operating as set, and don’t know that the
configurations are mis matched, it is not possible to automatically prevent or recover from such a
configuration.