# 0x90 Data Frame Weirdness

**URL:** <https://forums.digi.com/t/0x90-data-frame-weirdness/1682>\
**Category:** XBee - 802.15.4\
**Created:** [May 9, 2011, 1:25pm UTC](https://forums.digi.com/t/0x90-data-frame-weirdness/1682 "2011-05-09T13:25:07Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Neptune](https://avatars.discourse-cdn.com/v4/letter/n/b5a626/32.png) [@Neptune](https://forums.digi.com/u/Neptune)\
**Post date:** [May 9, 2011, 1:25pm UTC](https://forums.digi.com/t/0x90-data-frame-weirdness/1682/1 "2011-05-09T13:25:07Z")

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Hello,

I’m sending a 0x10 Transmit Request frame from my PC through a coordinator in API mode (with escape characters, AP=2), to an XBee configured as a router, also in API mode with AP=2, hooked up to an Arduino. I’m sending a frame that specifies a length of 15, with one byte of payload data thusly:

buf[0] = 0x7E; // Start byte  
buf[1] = 0x00; // High byte of message length  
buf[2] = 15; // Low byte of message length  
buf[3] = 0x10; // Frame type: transmit request  
buf[4] = 0x00; // Frame ID: set to 0 for no reply  
buf[5] = (XBSH & 0xFF000000) \>\> 24; // MSB of XBee target 64-bit address  
buf[6] = (XBSH & 0x00FF0000) \>\> 16;  
buf[7] = (XBSH & 0x0000FF00) \>\> 8;  
buf[8] = (XBSH & 0x000000FF);  
buf[9] = (XBSL & 0xFF000000) \>\> 24;  
buf[10] = (XBSL & 0x00FF0000) \>\> 16;  
buf[11] = (XBSL & 0x0000FF00) \>\> 8;  
buf[12] = (XBSL & 0x000000FF); // LSB of target 64-bit address  
buf[13] = 0xff; // MSB: XBee target 16-bit address  
buf[14] = 0xfe; // LSB: Xbee target 16-bit address  
buf[15] = 0x00; // Broadcast radius (set to 0 for max hops)  
buf[16] = 0x00; // Transmission options (none)  
buf[17] = 0x34; // Payload  
cksum = 0;  
for(i=3; i\<18; i++)  
cksum += buf[i];  
buf[18] = 0xFF - (cksum & 0xFF);  
write(fd, buf, 19);

The receiving Xbee gets a 0x90 Receive Packet, however the length is 13 and the bytes received are:

buf[0] = 0x7E correct, start delimiter  
buf[1] = 0x00 correct, msb of length  
buf[2] = 0x0D lsb of length = 13  
buf[3] = 0x90 correct, frame type  
buf[4] = 0x00 correct, frame ID  
buf[5] = 0x7D huh? Shouldn’t this be 0x13?  
buf[6] = 0x33 Shouldn’t this be 0xA2?  
buf[7] = 0xA2 Umm… everything is offset a byte now  
buf[8] = 0x00  
buf[9] = 0x40  
buf[10] = 0x6F  
buf[11] = 0x80  
buf[12] = 0xA3  
buf[13] = 0x00  
buf[14] = 0x00  
buf[15] = 0x01 Since everything is shifted, I lost my payload byte

Any idea what’s happening? This happens consistently over and over. The coordinator and router are in the same room. Both interfaced at 9600 baud. I can’t seem to discover any configuration options that would cause this behavior!

Thanks in advance for any suggestions…

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<div class="post-metadata">

**Author:** ![Neptune](https://avatars.discourse-cdn.com/v4/letter/n/b5a626/32.png) [@Neptune](https://forums.digi.com/u/Neptune)\
**Post date:** [May 9, 2011, 1:27pm UTC](https://forums.digi.com/t/0x90-data-frame-weirdness/1682/2 "2011-05-09T13:27:23Z")

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Also, I forgot to add that maybe a second afterwards, the 0x34 payload byte comes through with the checksum! It’s like the frame is fragmented.

I may be getting hit with a double-whammy.

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<div class="post-metadata">

**Author:** ![johnf](https://avatars.discourse-cdn.com/v4/letter/j/848f3c/32.png) [@johnf](https://forums.digi.com/u/johnf)\
**Post date:** [May 9, 2011, 10:24pm UTC](https://forums.digi.com/t/0x90-data-frame-weirdness/1682/3 "2011-05-09T22:24:44Z")

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buf[5] = 0x7D huh? Shouldn’t this be 0x13?  
buf[6] = 0x33 Shouldn’t this be 0xA2?

What you’re seeing is the result of using API=2.

7D is the escape character. That means the following character (0x33) has been XORed with 0x20. Together, 7D 33 is decoded as 0x13, which is what you expected.

My packet-check program (see the pinned posts) may perhaps help with packet analysis.

As a general rule, I’d suggest using API mode 2 only when you really need it - such as when you want end-to-end flow control. Otherwise API mode 1 works just fine, and is easier to work with.

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<div class="post-metadata">

**Author:** ![Neptune](https://avatars.discourse-cdn.com/v4/letter/n/b5a626/32.png) [@Neptune](https://forums.digi.com/u/Neptune)\
**Post date:** [May 9, 2011, 11:07pm UTC](https://forums.digi.com/t/0x90-data-frame-weirdness/1682/4 "2011-05-09T23:07:05Z")

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WHOA!!

Wow man, thank you so much - it would have taken me a long time to discover that (if ever!).

Kudos to you, and thanks again. You made my day.