diff --git a/avr/recv.c b/avr/recv.c
index 8447356..da9fc95 100644
--- a/avr/recv.c
+++ b/avr/recv.c
@@ -7,9 +7,9 @@
 #include <tabby/avr/link.h>
 #include <tabby/avr.h>
 
-static volatile tabby_printer_packet header;
-
-static volatile uint16_t i, b;
+static volatile uint16_t offset_header = 0,
+                         offset_body   = 0,
+                         size_body     = 0;
 
 /*
  * SPI byte receipt interrupt vector
@@ -20,13 +20,11 @@ ISR(SPI_STC_vect) {
 
     uart_putchar(in, NULL);
 
-    switch (i) {
+    switch (offset_header) {
         case 0: {
             if (in == TABBY_PRINTER_SYNC_1) {
-                header.preamble[0] = in;
-                i++;
-
-                b = 0;
+                offset_header++;
+                offset_body = 0;
             }
 
             break;
@@ -34,61 +32,54 @@ ISR(SPI_STC_vect) {
 
         case 1: {
             if (in == TABBY_PRINTER_SYNC_2) {
-                header.preamble[1] = in;
-                i++;
+                offset_header++;
             } else {
-                i = 0;
+                offset_header = 0;
             }
 
             break;
         }
 
-        case 2: {
-            header.type = in;
-            i++;
-
-            break;
-        }
-
+        case 2:
         case 3: {
-            header.compression = in;
-            i++;
+            offset_header++;
 
             break;
         }
 
         case 4: {
-            header.size = in;
-            i++;
+            size_body = in;
+
+            offset_header++;
 
             break;
         }
 
         case 5: {
-            header.size |= in << 8;
+            size_body |= in << 8;
 
-            if (header.size > TABBY_PRINTER_PACKET_MAX_SIZE) {
-                i = 0;
+            if (size_body > TABBY_PRINTER_PACKET_MAX_SIZE) {
+                offset_header = 0;
             } else {
-                i++;
+                offset_header++;
             }
 
             break;
         }
 
         default: {
-            if (b <= header.size) {
-                b++;
-            } else if (b == header.size + 1) {
-                b++;
+            if (offset_body <= size_body) {
+                offset_body++;
+            } else if (offset_body == size_body + 1) {
+                offset_body++;
 
                 out = TABBY_PRINTER_DEVICE_ID;
-            } else if (b == header.size + 2) {
-                b++;
+            } else if (offset_body == size_body + 2) {
+                offset_body++;
 
                 out = TABBY_PRINTER_OK;
-            } else if (b == header.size + 3) {
-                i = 0;
+            } else if (offset_body == size_body + 3) {
+                offset_header = 0;
             }
         }
     }
diff --git a/avr/send.c b/avr/send.c
index b3d1c17..aef40d9 100644
--- a/avr/send.c
+++ b/avr/send.c
@@ -10,12 +10,6 @@
 #include <tabby/avr.h>
 
 int main() {
-    uint16_t i            = 0,
-             b            = 0,
-             sheet_offset = 0;
-
-    uint8_t c;
-
     tabby_printer_packet header = {
         .size = 0
     };
@@ -29,22 +23,28 @@ int main() {
 
     uint8_t sheet[TABBY_PRINTER_SHEET_SIZE];
 
+    uint16_t offset_header = 0,
+             offset_body   = 0,
+             offset_sheet  = 0;
+
+    uint16_t sum_calc   = 0,
+             sum_footer = 0;
+
     uart_init();
 
     tabby_avr_link_init_master();
 
-    sei();
-
     while (1) {
-        c = uart_getchar(NULL);
+        uint8_t c = uart_getchar(NULL);
 
-        switch (i) {
+        switch (offset_header) {
             case 0: {
                 if (c == TABBY_PRINTER_SYNC_1) {
                     header.data[0] = c;
-                    i++;
+                    offset_header++;
 
-                    b = 0;
+                    offset_body = 0;
+                    sum_calc    = 0;
                 }
 
                 break;
@@ -53,9 +53,9 @@ int main() {
             case 1: {
                 if (c == TABBY_PRINTER_SYNC_2) {
                     header.data[1] = c;
-                    i++;
+                    offset_header++;
                 } else {
-                    i = 0;
+                    offset_header = 0;
                 }
 
                 break;
@@ -65,38 +65,45 @@ int main() {
                 switch (c) {
                     case TABBY_PRINTER_PACKET_INIT:
                     case TABBY_PRINTER_PACKET_CANCEL: {
-                        sheet_offset = 0;
+                        response.status = TABBY_PRINTER_OK;
+                        offset_sheet    = 0;
                     }
 
                     case TABBY_PRINTER_PACKET_JOB:
                     case TABBY_PRINTER_PACKET_DATA:
                     case TABBY_PRINTER_PACKET_INQUIRY: {
                         header.type = c;
-                        i++;
+                        offset_header++;
 
                         break;
                     }
 
                     default: {
-                        i = 0;
+                        offset_header = 0;
 
                         break;
                     }
                 }
 
+                sum_calc += c;
+
                 break;
             }
 
             case 3: {
                 header.compression = c;
-                i++;
+                offset_header++;
+
+                sum_calc += c;
 
                 break;
             }
 
             case 4: {
                 header.size = c;
-                i++;
+                offset_header++;
+
+                sum_calc += c;
 
                 break;
             }
@@ -106,43 +113,55 @@ int main() {
 
                 if (tabby_avr_printer_packet_toolarge(header.type,
                                                       header.size)) {
-                               i = 0;
-                               b = 0;
-                    sheet_offset = 0;
+                    offset_header = 0;
+
+                    response.status |= TABBY_PRINTER_FULL;
+
+                    goto respond;
                 } else {
-                    i++;
+                    offset_header++;
                 }
 
+                sum_calc += c;
+
                 break;
             }
 
             default: {
-                if (b < header.size) {
+                if (offset_body < header.size) {
                     if (header.type == TABBY_PRINTER_PACKET_JOB) {
-                        job.data[b] = c;
+                        job.data[offset_body] = c;
                     } else if (header.type == TABBY_PRINTER_PACKET_DATA) {
-                        sheet[sheet_offset++] = c;
+                        sheet[offset_sheet++] = c;
                     }
 
-                    b++;
-                } else if (b == header.size) {
-                    b++;
-                } else if (b == header.size + 1) {
-                    i = 0;
-                    b = 0;
+                    offset_body++;
 
-                    if (header.type == TABBY_PRINTER_PACKET_JOB) {
+                    sum_calc += c;
+                } else if (offset_body == header.size) {
+                    sum_footer = c;
+
+                    offset_body++;
+                } else if (offset_body == header.size + 1) {
+                    sum_footer |= c << 8;
+
+                    offset_header = 0;
+
+                    if (sum_footer != sum_calc) {
+                        response.status |= TABBY_PRINTER_SUM;
+                    } else if (header.type == TABBY_PRINTER_PACKET_JOB) {
                         tabby_avr_printer_send_sheet(sheet,
-                                                     sheet_offset,
+                                                     offset_sheet,
                                                      &response);
 
                         tabby_avr_printer_job_start(&job, &response);
 
-                        sheet_offset = 0;
+                        offset_sheet = 0;
                     } else if (header.type == TABBY_PRINTER_PACKET_INQUIRY) {
                         tabby_avr_printer_send_inquiry(&response);
                     }
 
+respond:
                     uart_putchar(response.device, NULL);
                     uart_putchar(response.status, NULL);
                 }