User Manual

Operation
RFI-9256 Radio Modem User Manual Page 18 of 96
4kB output buffer
4kB input buffer outgoing frame buffers
incoming frame buffers
Frame 1
f
1
Frame 2
f
2
Frame Time
20ms
M M S S
Figure 4: Overall data path in the RFI-9256 radio
Due to the framing structure over the air, and the data path shown above, the RFI-9256 cannot be regarded
as a direct wire replacement. It will induce additional latency into the communications link, as well as
potentially causing changes in the timing between bytes (inter-character delay).
LATENCY
The RFI-9256 will introduce latency into the system. This latency is caused by the following factors:
 Serialisation delays. Serialisation delays are caused by the time taken for the incoming RS-232
bit stream to be converted back into bytes. The serialisation delay for each serial port can be
calculated in milliseconds using Equation 1.
baud
bits
t
serial
1000=
Equation 1: Latency induced by serialisation delay for a single serial port in ms
Where bits is the number of bits in a byte (including start, stop and parity bits), and baud is the baud
rate of the serial port.
Thus, for 9,600 baud, 8N1 the serialisation delay is around 1ms per serial port.
 Framing delays. Framing delays will occur depending on where data arrives relative to the start of
a frame. Consider the situation in Figure 5.