+u_int16_t gemtag_calc_crc(unsigned char *data,u_int16_t len) {
+
+ u_int16_t crc_polynom;
+ u_int16_t crc_preset;
+ u_int16_t crc;
+ int i,j;
+
+ crc_polynom=0x8408;
+ crc_preset=0xffff;
+ crc=0xffff;
+
+ for(i=0;i<len;i++) {
+ crc^=data[i];
+ for(j=0;j<8;j++) {
+ if(crc&0x0001)
+ crc=(crc>>1)^crc_polynom;
+ else
+ crc=(crc>>1);
+ }
+ }
+ return crc;
+}
+
+int gemtag_transcieve(struct gemtag_handle *gh,unsigned char cmd,
+ unsigned char *tx,unsigned int tx_len,
+ unsigned char *rx,unsigned int *rx_len) {
+
+ unsigned char txbuf[256];
+ unsigned char rxbuf[256];
+ struct gemtag_cmd_hdr *txhdr;
+ struct gemtag_cmd_hdr *rxhdr;
+ u_int16_t *crcptr;
+ int ret,size;
+
+ txhdr=(struct gemtag_cmd_hdr *)txbuf;
+ rxhdr=(struct gemtag_cmd_hdr *)rxbuf;
+
+ txhdr->start=0xa5;
+ txhdr->seq=++(gh->seq);
+ txhdr->cmd=cmd;
+ txhdr->len=htons(tx_len);
+ size=sizeof(struct gemtag_cmd_hdr);
+ memcpy(txbuf+size,tx,tx_len);
+
+ /* crc check */
+ gh->capabilities=GEMTAG_CAP_CRC;
+ if(gh->capabilities&GEMTAG_CAP_CRC) {
+ size+=tx_len;
+ crcptr=(u_int16_t *)(txbuf+size);
+ *crcptr=gemtag_calc_crc(txbuf,size);
+ size+=2;
+ }
+
+ /* usb write */
+ hexdump(txbuf,size);
+ ret=usb_bulk_write(gh->handle,0x02,txbuf,size,0);
+ if(ret<0) {
+ perror("usb bulk write");
+ return ret;
+ }
+ printf("write of %d bytes successfull\n",ret);
+
+ /* usb read */
+ ret=usb_bulk_read(gh->handle,0x81,rxbuf,sizeof(rxbuf),0);
+ size=ret;
+ if(ret<0) {
+ perror("usb bulk read");
+ return ret;
+ }
+ printf("received %d bytes\n",ret);
+
+ if(rxhdr->seq!=txhdr->seq)
+ puts("transmitted/recieved header are not equal");
+
+ /* crc check */
+
+
+ *rx_len=ntohs(rxhdr->len);
+ memcpy(rx,rxbuf+sizeof(struct gemtag_cmd_hdr),
+ ret-sizeof(struct gemtag_cmd_hdr)+2);
+ hexdump(rxbuf,ret+2);
+
+ return 0;
+}
+