Chapter Three
Operation
Page
19
When the WI-MOD-9-D unit detects data in the input buffer, it initiates a radio message. The
radio message will end when the number of transmitted bytes reaches the maximum message
length (configurable by the user), or if the input buffer becomes empty.
If the configured serial data rate is the same or more than the radio data rate, then data is
transmitted as soon as it enters the input buffer - data “streams” from the input buffer to the
radio port. If the serial rate is less than the radio rate, then the transmission will be delayed for a
period to allow sufficient data to build up in the input buffer to avoid the radio emptying the
input buffer before a complete serial message has been input. The WI-MOD-9-D will calculate
the amount of delay depending on the difference between the serial and radio rates.
The radio transmission will stop when the input buffer is empty or when the radio has transmitted
the maximum number of bytes (user configurable - maximum 530 bytes). If there is still data in
the input buffer, the WI-MOD-9-D will start another radio transmission.
If error checking is configured, then a 16 bit CRC error-check is added to the end of the
transmitted data packet. The receiving module will receive the full data packet and check the
CRC before outputting the data.
The maximum size of the data packet is configurable by the user (maximum is 530 bytes). If less
data than the maximum size is input to the WI-MOD-9-D, then the WI-MOD-9-D will transmit
the actual data input. If more data is input than the maximum size, then the WI-MOD-9-D will
transmit multiple packets until all of the data is transmitted.
Because of radio start-up delays, the effective radio data rate will be lower than the transmitted
data rate. If you are sending large blocks of data, and the serial rate is equal or more than the
radio rate, we recommend that you use CTS/RTS flow control to prevent the input buffer from
overflowing. If you use the maximum data rate of 115,200 bits/sec, the best effective overall
rate is approx 65,700 bits/sec using a serial speed of 76,800 bits/sec. If you use a serial speed of
115,200, the effective overall rate will be lower. If you are required to use a serial speed of
115,200 bits/sec, then we suggest that you configure the maximum packet size to 60 bytes - this
will give faster performance than the default setting of 530 bytes.
3.2.1 Character Type
The WI-MOD-9-D may be configured by the user to recognize the following types of characters
- 7 or 8 data bits, even or odd or no parity, 1 or 2 stop bits.
Most applications will require the character type to be the same at each WI-MOD-9-D modem in
the system. Nevertheless, the character type may be configured to be different at different WI-
MOD-9-D modems. Data is transmitted by radio as an eight-bit byte without stop or start bits. If
the input data is 7 data bits, then the byte transmitted by radio comprises the 7 bits plus a zero
bit. Input characters with 8 bits are transmitted as just the 8 data bits, with no parity. Because
the data is transmitted without parity, the user may configure CRC error checking to be added to
each transmitted data packet. Data is output at the destination module based on the character
type configured at that module - that is, the start/stop bits and parity is added to the radio data.