rb trying to receive.CCCCCCCC

$ rb --help

rb version 0.12.20
Usage: rb [options] [filename.if.xmodem]
Receive files with ZMODEM/YMODEM/XMODEM protocol
    (X) = option applies to XMODEM only
    (Y) = option applies to YMODEM only
    (Z) = option applies to ZMODEM only
  -+, --append                append to existing files
  -a, --ascii                 ASCII transfer (change CR/LF to LF)
  -b, --binary                binary transfer
  -B, --bufsize N             buffer N bytes (N==auto: buffer whole file)
  -c, --with-crc              Use 16 bit CRC (X)
  -C, --allow-remote-commands allow execution of remote commands (Z)
  -D, --null                  write all received data to /dev/null
      --delay-startup N       sleep N seconds before doing anything
  -e, --escape                Escape control characters (Z)
  -E, --rename                rename any files already existing
      --errors N              generate CRC error every N bytes (debugging)
  -h, --help                  Help, print this usage message
  -m, --min-bps N             stop transmission if BPS below N
  -M, --min-bps-time N          for at least N seconds (default: 120)
  -O, --disable-timeouts      disable timeout code, wait forever for data
      --o-sync                open output file(s) in synchronous write mode
  -p, --protect               protect existing files
  -q, --quiet                 quiet, no progress reports
  -r, --resume                try to resume interrupted file transfer (Z)
  -R, --restricted            restricted, more secure mode
  -s, --stop-at {HH:MM|+N}    stop transmission at HH:MM or in N seconds
  -S, --timesync              request remote time (twice: set local time)
      --syslog[=off]          turn syslog on or off, if possible
  -t, --timeout N             set timeout to N tenths of a second
  -u, --keep-uppercase        keep upper case filenames
  -U, --unrestrict            disable restricted mode (if allowed to)
  -v, --verbose               be verbose, provide debugging information
  -w, --windowsize N          Window is N bytes (Z)
  -X  --xmodem                use XMODEM protocol
  -y, --overwrite             Yes, clobber existing file if any
      --ymodem                use YMODEM protocol
  -Z, --zmodem                use ZMODEM protocol

short options use the same arguments as the long ones