Echelon LonTal Stack - Manual (Page 46)

34 Designing the Serial I/O Hardware Interface
higher voltage drop (typically, 0.7 V), that is, the Schottky diode ensures that the
voltage drop is low enough to ensure a logic-low signal.
Host-driven reset of the Echelon Smart Transceiver or Neuron Chip should only
be an emergency means to recover from some serious error. In addition, the host
application or serial driver should always log the reason or cause for the reset,
along with timestamp information. An unrecoverable error that requires a reset
of the Echelon Smart Transceiver or Neuron Chip is generally evidence of a
malfunction in the host driver, the Echelon Smart Transceiver or Neuron Chip,
or the physical link layer, and should be investigated.
Selecting the Link-Layer Bit Rate
The serial link bit rate for the pre-built Layer 2 MIP images is fixed at 115, 200
bps. If you build a custom Layer 2 MIP image, you can specify a lower bit rate if
required for your hardware. The minimum bit rate for the serial link between
the Echelon Smart Transceiver or Neuron Chip and the host processor is most
directly determined by the expected number of packets per second, the type of
packets, and the size of the packets. Another factor that can significantly
influence the required bit rate is support for explicit addressing, an optional
feature that the LonTalk Stack application can enable and disable.
Recommendations: The following recommendations apply to general-use
L ON W ORKS devices:
• Echelon Smart Transceiver or Neuron Chip external clock frequency
o
10 MHz or higher for TP/FT-10 devices (for Series 5000 devices,
specify a minimum 5 MHz system clock rate)
o
5 MHz or higher for power-line devices
• Bit rate
o
38 400 bps or higher for TP/FT-10 devices
o
9600 bps or higher for power-line devices
To generate a more precise estimate for the minimum bit rate for the serial
interface, use the following formula:
( )
exp
* * 5
PPS BPT P EA P MinBitRate
Interface size type
+ + + =
where:
• The constant 5 represents general communications overhead
•
type
P is the packet-type overhead, and has one of the following values:
o
3 for network-variable messages
o
1 for application messages
• EA is the explicit-addressing overhead, and has one of the following
values:
o
0 for no explicit-addressing support
o
11 for explicit-addressing support enabled
•
size
P is the packet size of the payload, and has one of the following values: