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doc: Minor typo fix/rewording in drtio.rst

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David Nadlinger 2019-01-12 00:13:24 +00:00
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doc/manual/drtio.rst
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@ -98,14 +98,14 @@ Real-time and auxiliary packets
DRTIO is a packet-based protocol that uses two types of packets:
* real-time packets, which are transmitted at high priority at a high bandwidth and are used for the bulk of RTIO commands and data. In the ARTIQ DRTIO implementation, real-time packets are processed entirely in gateware.
* auxiliary packets, which are lower-bandwidth and are used for ancilliary tasks such as housekeeping and monitoring/injection. Auxiliary packets are low-priority and their transmission has no impact on the timing of real-time packets (however, transmission of real-time packets slows down the transmission of auxiliary packets). In the ARTIQ DRTIO implementation, the contents of the auxiliary packets are read and written directly by the firmware, with the gateware simply handling the transmission of the raw data.
* auxiliary packets, which are lower-bandwidth and are used for ancillary tasks such as housekeeping and monitoring/injection. Auxiliary packets are low-priority and their transmission has no impact on the timing of real-time packets (however, transmission of real-time packets slows down the transmission of auxiliary packets). In the ARTIQ DRTIO implementation, the contents of the auxiliary packets are read and written directly by the firmware, with the gateware simply handling the transmission of the raw data.
Link layer
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The lower layer of the DRTIO protocol stack is the link layer, which is responsible for delimiting real-time and auxiliary packets, and assisting with the establishment of a fixed-latency high speed serial transceiver link.
DRTIO uses the IBM (Widmer and Franaszek) 8b/10b encoding. The two types of 8b/10b codes are used: D characters, that always transmit real-time packet data, and K characters, that are used for idling and transmitting auxiliary packet data.
DRTIO uses the IBM (Widmer and Franaszek) 8b/10b encoding. D characters (the encoded 8b symbols) always transmit real-time packet data, whereas K characters are used for idling and transmitting auxiliary packet data.
At every logic clock cycle, the high-speed transceiver hardware transmits some amount N of 8b/10b characters (typically, N is 2 or 4) and receives the same amount. With DRTIO, those characters must be all of the D type or all of the K type; mixing D and K characters in the same logic clock cycle is not allowed.
@ -119,7 +119,7 @@ The series of K selection words is then used to form auxiliary packets and the i
Both real-time traffic and K selection words are scrambled in order to make the generated electromagnetic interference practically independent from the DRTIO traffic. A multiplicative scrambler is used and its state is shared between the real-time traffic and K selection words, so that real-time data can be descrambled immediately after the scrambler has been synchronized from the K characters. Another positive effect of the scrambling is that commas always appear regularly in the absence of any traffic (and in practice also appear regularly on a busy link). This makes a receiver always able to synchronize itself to an idling transmitter, which removes the need for relatively complex link initialization states.
Due to the use of K characters both as delimiters for real-time packets and as information carrier for auxiliary packets, auxiliary traffic is guaranteed a minimum bandwith simply by having a maximum size limit on real-time packets.
Due to the use of K characters both as delimiters for real-time packets and as information carrier for auxiliary packets, auxiliary traffic is guaranteed a minimum bandwidth simply by having a maximum size limit on real-time packets.
Clocking
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