Specifications

Although you can request that one or both UARTs are disabled (via the Feature Bits), and you can
request that there is no Packet Serial mode UART (by setting the Default UART parameter to 255),
both of these user requests will be ignored unless there is also a valid SNAPpy script loaded into the
unit. If the parameter is set to a value outside the range of UARTs on your module (other than 255),
UART1 (UART0 on modules with only one UART) will be the default.
If there is no SNAPpy script loaded, a fail-safe mechanism kicks in and forces an active Packet Serial
port to be initialized on UART1 (or UART0, if so specified in this parameter), regardless of the other
configuration settings. This was done to help prevent users from “locking themselves out.”
If there is a SNAPpy script loaded, then the assumption is that the script will take care of any
configuration overrides needed, and the Feature Bits and the Default UART setting will be honored.
ID 13 – Buffering Timeout
This lets you tune the overall serial data timeout. This value is in milliseconds, and defaults to 5. This
value controls the maximum amount of time between an initial character being received over the serial
port, and a packet of buffered serial data being enqueued for processing. Regardless of the number of
characters buffered or the rate at which they are being buffered, each time this timeout passes any
buffered data will be queued.
Note that other factors can also trigger the queuing of the buffered serial data. In particular see the next
two NV Parameters.
The larger this value is the more buffering will take place. In TRANSPARENT MODE, every packet
has 12-15 bytes of overhead, so sending more serial characters per packet is more efficient. Also,
when using MULTICAST TRANSPARENT MODE, keeping the characters together (in the same
packet) improves overall reliability.
The tradeoff is that the larger this value is, the greater the maximum latency can be through the overall
system.
ID 14 – Buffering Threshold
This value indicates the total packet size threshold used when sending packets of data. The size
defaults to 75 bytes. If no timeout limit is reached first, this parameter will cause buffered data to be
enqueued when there is sufficient data to cause the packet, including header, to be at least this many
bytes long. At higher serial rates, this size can be overshot between SNAP checks of the packet size.
There is no guarantee that packets will necessarily be precisely this size.
Each packet of data sent includes a header, which comprises 12 bytes for multicast packets and 15
bytes for unicast packets. So the actual amount of serial data sent in each packet will be reduced by
either 12 or 15 fewer bytes, depending on whether the data is being sent by multicast or unicast.
Additionally, if the feature bit in NV Parameter 11 indicates that SNAP should be using its second
CRC to prevent data corruption, the data payload will be reduced by an additional two bytes. If you
want to send N bytes of data per packet, this parameter should be set to N + 12 for multicasting or N +
15 for unicasting, or N + 14 for multicasting with a secondary CRC or N + 17 for unicasting with a
secondary CRC.
Page 96 of 202 SNAP Reference Manual Document Number 600-0007K