diff --git a/c34/c34.txt b/c34/c34.txt
index bee927f..8544155 100644
--- a/c34/c34.txt
+++ b/c34/c34.txt
@@ -13,9 +13,8 @@ Datenpaket besteht aus 8 byte:
 header  2 byte  00FF
 pck_id  1 byte  N
 data    4 byte  val (big endian)
-chksum  2 byte  Summe ueber byte[0]=pck_id,byte[1]=data[0],...,byte[4]=data[3]
-                byte1: (sum_i byte[i]) & 0xFF
-                byte2: (-1 - sum_i byte[i]*(5-i)) & 0xFF = ~(sum_i byte[i]*(5-i)) & 0xFF
+chksum  2 byte  Fletcher16 (hier: byte2 1's-complement am Ende)
+
 
 Telemetrie pck_id N:
 0x14: date: 0x00027173 =   160115 -> 2015-01-16
@@ -29,7 +28,7 @@ GPS-lat/lon wie NMEA mit Faktor 1e4
 
 
 Checksum:
-2 byte
+2 byte, Summe ueber byte[0]=pck_id,byte[1]=data[0],...,byte[4]=data[3]
 byte1: (sum_i byte[i]) & 0xFF
 byte2: (-1 - sum_i byte[i]*(5-i)) & 0xFF = ~(sum_i byte[i]*(5-i)) & 0xFF
 
@@ -44,6 +43,10 @@ FrameID 0x15 (time):
  00FF 15 0001E605 01C2
  00FF 15 0001E606 02C1
 
+-> Fletcher16:
+5*byte[0]+4*byte[1]+3*byte[2]+2*byte[3]+1*byte[4]
+= byte[0] + (byte[0]+byte[1]) + (byte[0]+byte[1]+byte[2]) + (byte[0]+byte[1]+byte[2]+byte[3]) + (byte[0]+byte[1]+byte[2]+byte[3]+byte[4])
+
 
 PTU-Data: float32?
 
diff --git a/c34/c34dft.c b/c34/c34dft.c
index ba92a91..ea7d44f 100644
--- a/c34/c34dft.c
+++ b/c34/c34dft.c
@@ -306,6 +306,7 @@ void printGPX() {
         printf("\n");
 }
 
+// Chechsum Fletcher16
 unsigned check2(ui8_t *bytes, int len) {
     int sum1, sum2;
     int i;
@@ -321,6 +322,19 @@ unsigned check2(ui8_t *bytes, int len) {
 
     return sum2 | (sum1<<8);
 }
+/* // equivalent
+unsigned check16(ui8_t *bytes, int len) {
+    unsigned sum1, sum2;
+    int i;
+    sum1 = sum2 = 0;
+    for (i = 0; i < len; i++) {
+        sum1 = (sum1 + bytes[i]) % 0x100;
+        sum2 = (sum2 + sum1) % 0x100;
+    }
+    sum2 = (~sum2) & 0xFF;  // 1's complement
+    return sum2 | (sum1<<8);
+}
+*/
 
 double NMEAll(int ll) {  // NMEA GGA,GLL: ll/1e4=(D)DDMM.mmmm
     int deg = ll / 1000000;