Block Recovery Protocol (pdf) - Güralp Systems Ltd

Specification
3 Other considerations
3.1 Terminal passthrough
BRP receivers may request terminal passthrough by transmitting a lone ^S character
(ASCII code 19), or by sending a specially-formulated ACK packet (preferred
method; see section 2.4). If the transmitter is not currently transmitting, a single
^S should be sent; otherwise, the receiver should wait for its next ACK packet
before requesting BRP mode. This avoids incompatible behaviour between the
CD24 and the DM24 1 , and avoids a long wait if no packets are currently being
transmitted.
There is no clear indicator of when passthrough mode has been activated. The
CMG-DCM receiver code currently assumes passthrough mode is activated when
it has requested passthrough and it receives bytes that do not belong to a block
transmission (table 2.2).
The terminal passthrough should time out after some seconds if there is no activity
on the link. This should be enforced by the BRP transmitter. The receiver may
end the link at any time by sending a ‘GO’ command (this is actually transmitterspecific,
but all current known transmitters implement GO, although there may be
other commands for other situations). The transmitter should send the sequence
0x1B 0x11 to signify it has exited passthrough mode.
3.2 Real-time status
Due to a design flaw, the CMG-DCM mk2x hardware does not contain a realtime
clock module. The CMG-EAM hardware does, but the clock must be synchronised
somehow. The DCM must attempt to retrieve its time from the connected
digitisers. This process is not easy — only status packets can be used, because
1 The CMG-CD24 always honours ^S characters, whereas the CMG-DM24 may ignore them
if it is busy transmitting a packet.
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