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Transceiver module TRX433-10C2 / TRX868-10C2 (Vers. 1.0E) 8 Buffer overflow Be careful that the two buffers (transmittion- and receiving buffer) don’t overflow. That can be ensured by adhere the right timing or by using the handshake (see paragraph Example of data flow and Handshake with RTS/CTS). Overflow of the transmittion buffer The transmittion buffer has, additionally to its 61 bytes, a 2-byte overflow buffer. So when the transmittion buffer is full (61 bytes full) and further data are still transferred over RS232 then max. 2 bytes can be saved in the overflow buffer. Those will be transferred in the transmittion buffer as soon as there is space in there. When the overflow buffer is full as well then further over the RS232 received data will be rejected. Overflow of the receive buffer The receiving buffer has, additionally to its 61 bytes, a 2-byte overflow buffer. This buffer is only used for saving the RSSI-values in addition to the payload if the function “Output of the telegram-RSSI” respectively “output of the no signal condition-RSSI” is enabled and if the whole 61 bytes of the receiving buffer are full with the radio telegram. When data are received over radio even if the buffer is already full (61 bytes full), then the whole incoming radio telegram will be rejected if the CRC16 is on. If the CRC16 is off then the already received data will be saved and the rest of the incoming radio telegram will be rejected. Handshake with RTS/CTS The communication over the RS232-interface can be controlled with help of RTS/CTS to avoid overflowing of buffers. RTS and CTS are only used for the handshake when the handshake is activated. When the handshake is disabled then the RTS and CTS are ignored. Function RTS As soon as 61 bytes are saved in the transmittion buffer then the transceiver module stops the receiving of data over RS232 by setting the RTS-Pin on high. The user may not send further data over the RS232interface to the transceiver module until the RTS-pin changes to low again. The RS232-interface enables again (RTS-Pin reset to low) once ≤58 bytes are saved in the transmittion buffer. Function CTS With an activated handshake, the user can stop the output of data of the transceiver module by setting the CTS-Pin to high. The transceiver module doesn’t output any more data over the TxD-Pin until the CTS-Pin is set back to low by the user. Note If the CTS-Pin is used as handshake, don’t send any data to the transceiver module during the high-time of the CTS-Pin because these data would be interpreted as configuration data. The CTS-Pin has the double function handshake (CTS) and interface mode (MODE). Figure 2 shows the cycle of a data communication of 64 bytes with use of the handshake. There, module 1 is the transceiver and module 2 is the receiver. Additionally you can see how many bytes are saved in the transmittion buffer (TX-BUF) of the sending transceiver module and in the receiving buffer (RX-BUF) of the receiving transceiver module. module 1 module 2 bytes in TX-BUF ... 0........... 1........... ........60......... 61................................................................60.59.58.57.56. 56.55.55.54.53. 53.... ....3......................................................... 2...1...0..................................... ............... RS232, RTS-pin RS232, RxD-pin RF-transmission RS232, TxD-pin Byte 1 Byte 2 Byte 60 Byte 61 start RF-transmission after 2-3 bytes break 32 Bit Preamble +2ms Startup 24 Bit SYNC Byte 62 Length Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6 Byte 7 Byte 8 Byte 9 Byte 10 Byte 11 Byte 63 Byte 64 Byte 60 Byte 61 CRC16 32 Bit Preamble +2ms Startup Byte 1 Byte 2 24 Bit SYNC Byte 3 Length Byte 62 Byte 63 Byte 64 bytes in RX-BUF ... 0......................... ........................................................................................................................................................... .................. 61.........60.........59.........58.........57.........56. 59... 58.......... .........0.... Figure 2 data communication with handshake start RF-transmission after 2-3 bytes break Byte 4 Byte 5 CRC16 Byte 6 Byte 7 Byte 64
Transceiver module TRX433-10C2 / TRX868-10C2 (Vers. 1.0E) 9 CRC16 The CRC16 is a 16-Bit check sum which checks the payloads in the DataField for correctnes. If the higher application doesn’t check the data then you should definitely use the CRC16. CRC16 off The radio data are not checked for correctness. The RS232 receives a data byte from the DataField of the radio telegram and outputs it immediately. The higher application has to deal with the checking of the data. CRC16 on The payloads in the DataField are CRC16-checksum-verified and the calculated checksum is included on the end of the radio telegram. That means that the data of the receiving transceiver module will only be output over RS232 when CRC16 corresponds. By wrong CRC16, the whole radio telegram will be rejected. Fast send The configuration Fast send decides when the radio transmission of the over the RS232 received payloads starts and in which way the radio telegram length is transmitted. Fast send off The data which were received in one piece over the RS232 are buffered in the transmittion buffer. As soon as no further data is received over the RS232 for the time of 2-3 RS232-bytes then the radio transmission starts and the current stored payloads in the transmittion buffer are transmitted. The length of the radio telegram is therefore defined at the beginning of the radio transmission (in the LengthField, the number of bytes of payloads are entered). When further data are received over RS232 during the radio transmission, they will be buffered in the transmittion buffer and they’ll build a new radio telegram as soon as the radio transmission of the current radio telegram is complete. Fast send on The radio transmission starts once the first byte is received over RS232 and will not stop until no byte is left in the transmittion buffer. Therefore the length of a radio telegram is not known in the beginning of the radio transmission. That’s why no LengthField is added to the radio telegram. To label the radio telegram length, a bit is added to every payload byte in the DataField which shows the end of the payloads. Note • Although the radio transmission starts earlier with Fast send than without, one bit is added to every payload byte in the DataField for the identification of the radio telegram length. Thereby, a radio telegram with fast send on and more than 8 bytes payload gets longer than one without fast send. • Due to the fact that Fast send means more computing effort, Fast send can only be used up to a RF data rate of 19.2kbits/s (respectively 9.6kbits/s by 8MHz µP clock frequency). • The radio telegram length is limited to 61 bytes with Fast send when CRC16 is enabled because the receiver has to buffer the whole radio telegram until CRC16 is checked. The user has to make sure that the length of the radio telegram is no more than 61 bytes payload. • With disabled CRC16 and Fast send on, a radio telegram length with over 61 bytes payload is possible. • The pause with the period of 2-3 RS232-bytes for the start of the radio transmission without Fast send varies depending on the adjusted configuration. To make sure that the radio transmission really will start you should keep a minimum pause of three RS232-bytes.
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