Operation
RFI-9256 Radio Modem User Manual
Page 24 of 109
DATA PATH
Internally, the RFI-9256 stores a set of payload frames that are waiting to be transmitted, and a set of
payload frames that have been received but not yet processed. Combining this with the serial port interface
described in section 5, an overall picture of the data path in the RFI-9256 radio can be obtained. This is
shown for a master unit in Figure 8.
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 8: 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.
