Block Recovery Protocol (pdf) - Güralp Systems Ltd

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Block Recovery Protocol — Transmission of GCF over Serial Start block: ‘G’ Length GCF block Sequence number ❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤ ❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤❤ Checksum Table 2.2: GCF block transmission. other point in the sequence 1 . If neither signal is received, the transmitter will generally store the unacknowledged blocks in some sort of memory until they can be transmitted later. 2.3 ACK packets The format of a valid ACK packet is shown in table 2.3. The ACK packet is tied to the last block received by the stream ID, which is transmitted in little endian format 2 . Note that this should be the stream ID of the last block received, regardless of whether that block was out of sequence, or corrupted (in which case the stream ID might be wrong; but this simply means that the ACK will be ignored by the transmitter). The ‘Ctrl’ bit is set to 0 if a control code is used (see table 2.4 below), or 1 if a control code is not used, in which case the ACK packet is further tied to the block 1 Unfortunately, some BRP transmitters have modes where, due to the implementation, rewinding does not work (e.g. CMG-DM24mk3 in adaptive mode). There is no way for the receiver to detect this. In such a mode, the receiver should NAK blocks that have corrupt checksums, but it should not pay attention to the sequence numbers. Otherwise, there will be suboptimal behaviour as the receiver continues to try to rewind but the transmitter does not perform this — although no data should be lost, transmission bandwidth will be wasted. This could be implemented as an option that the user must explicity activate in the receiver. 2 An earlier version of serial link protocol used only the first two bytes of the ACK/NAK packet. It was only deemed necessary to use the least siginificant byte of the stream ID, since that would capture the tap and component information. Once BRP was introduced, this packet was extended in a backward-compatible manner, leading to the strange layout of bytes. 6 SWA-RFC-BRPR — Issue B
Specification 15 14 13 12 11 10 ACK byte: 0x01 9 8 7 6 5 4 3 2 Stream ID LSB 1 0 Ctrl Sequence ID Stream ID MSB, little-endian Table 2.3: ACK packet layout. Control code Description Digitisers 0x00 Acknowledge, ignore sequence number All 0x13 (^S) Break in to command mode All 0x54 (‘T’) Initiate software trigger CD24 Table 2.4: Control codes that may be used in BRP ACK packets. by least significant 7 bits of the sequence number. For example, if the transmitter’s last block was sequence number 0x67, the receiver would ACK it with 0xE7 in byte 2 of its ACK packet. If the ACK of the previous packet (0xE6) was delayed, the transmitter would not match the sequence number and would discard the delayed ACK packet. 2.4 ACK commands In addition to simply acknowledging correctly received blocks, the ACK packet can be used to send some special commands. These are detailed in table 2.4. A special code of 0 will cause the digitiser to accept the ACK packet as long as the stream ID matches (i.e. the block sequence number is ignored 3 ). To request terminal passthrough mode on the digitiser, a control code of 0x13 (byte 2 as 0x13) is sent. The CD24 digitiser firmware also recognises a control code of 0x54 to initiate a software trigger. 2.5 NAK packets Finally, the format of a NAK packet is shown below. The NAK packet contains a field which tells the transmitter which block sequence number to rewind to. 3 This method of acknowledgement was widely used until later versions of the CD24 and DM24 firmware. However, the new method of sending the sequence byte with the top bit set is backwards compatible with the old firmware so it should be used by any new receiver software. September 27, 2008 7
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