Elpro Technologies 905U-D User Manual download pdf (Page 19)

Chapter Three Operation

Page 19 © January 2011

When the 905U-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 905U-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 905U-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 905U-D, then the 905U-D will transmit the

actual data input. If more data is input than the maximum size, then the 905U-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 905U-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 905U-D modem in the

system. Nevertheless, the character type may be configured to be different at different 905U-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.

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Elpro Technologies 905U-D User Manual Page 19