diff --git a/CMakeLists.txt b/CMakeLists.txt
index eed7691..f03691d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -34,9 +34,10 @@ add_custom_target(fxsdk ALL DEPENDS "${BIN}/fxsdk.sh")
 
 # fxlink
 configure_file(fxlink/config.h.in "${BIN}/include/fxlink/config.h")
-add_executable(fxlink fxlink/usb.c fxlink/filter.c fxlink/main.c
-  fxlink/properties.c fxlink/ud2.c fxlink/util.c)
-target_link_libraries(fxlink PkgConfig::libusb) # PkgConfig::libudev
+add_executable(fxlink fxlink/usb.c fxlink/filter.c fxlink/interactive.c
+  fxlink/main.c fxlink/png.c fxlink/properties.c fxlink/ud2.c fxlink/util.c
+  fxlink/protocol.c)
+target_link_libraries(fxlink PkgConfig::libpng PkgConfig::libusb) # PkgConfig::libudev
 target_include_directories(fxlink PRIVATE "${BIN}/include/fxlink")
 if(NOT FXLINK_DISABLE_UDISKS2)
   target_link_libraries(fxlink PkgConfig::udisks2)
diff --git a/fxlink/fxlink.h b/fxlink/fxlink.h
index e08d727..1d00369 100644
--- a/fxlink/fxlink.h
+++ b/fxlink/fxlink.h
@@ -18,4 +18,7 @@ int main_blocks(filter_t *filter, delay_t *delay);
 /* Main function for -s */
 int main_send(filter_t *filter, delay_t *delay, char **files);
 
+/* Main function for -i */
+int main_interactive(filter_t *filter,delay_t *delay,libusb_context *context);
+
 #endif /* FXLINK_FXLINK_H */
diff --git a/fxlink/interactive.c b/fxlink/interactive.c
new file mode 100644
index 0000000..9a17ac3
--- /dev/null
+++ b/fxlink/interactive.c
@@ -0,0 +1,232 @@
+#include "config.h"
+#include "fxlink.h"
+#include "util.h"
+#include "properties.h"
+#include "filter.h"
+#include "protocol.h"
+#include "usb.h"
+#include "png.h"
+
+#include <libusb.h>
+#include <string.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <time.h>
+#include <png.h>
+
+static char *output_file(char const *path,char const *type,char const *suffix)
+{
+	char *filename = NULL;
+	int counter = 1;
+
+	time_t time_raw;
+	struct tm time_bd;
+	time(&time_raw);
+	localtime_r(&time_raw, &time_bd);
+
+	while(1) {
+		asprintf(&filename, "%s/fxlink-%.16s-%04d.%02d.%02d-%02dh%02d-%d.%s",
+			path, type, time_bd.tm_year + 1900, time_bd.tm_mon,
+			time_bd.tm_mday, time_bd.tm_hour, time_bd.tm_min, counter, suffix);
+		if(!filename) continue;
+
+		/* Try to find a name for a file that doesn't exist */
+		if(access(filename, F_OK) == -1) break;
+
+		free(filename);
+		counter++;
+	}
+
+	return filename;
+}
+
+static bool message_new(message_t *msg, usb_fxlink_header_t const *h)
+{
+	int version_major = (h->version >> 8) & 0xff;
+	int version_minor = (h->version) & 0xff;
+
+	fprintf(stderr, "New message (v%d.%d): application '%.16s', type '%.16s', "
+		"size %d bytes\n", version_major, version_minor, h->application,
+		h->type, h->size);
+
+	msg->output = malloc(h->size);
+	if(!msg->output) {
+		err("cannot allocate memory for message of %d bytes", h->size);
+		return false;
+	}
+
+	msg->header = *h;
+	msg->size_read = 0;
+	msg->valid = true;
+	return true;
+}
+
+static void message_finish(message_t *msg)
+{
+	char const *path = ".";
+
+	if(!strncmp(msg->header.application, "fxlink", 16)) {
+		if(!strncmp(msg->header.type, "image", 16)) {
+			usb_fxlink_image_t *img = (void *)msg->output;
+			char *filename = output_file(path, msg->header.type, "png");
+
+			uint8_t **row_pointers = fxlink_protocol_decode_image(msg);
+			fxlink_png_save(row_pointers, img->width, img->height, filename);
+
+			printf("Saved image (%dx%d, format=%d) to '%s'\n",
+				img->width, img->height, img->pixel_format, filename);
+			free(row_pointers);
+			free(filename);
+			return;
+		}
+
+		if(!strncmp(msg->header.type, "text", 16)) {
+			printf("------------------\n");
+			fwrite(msg->output, 1, msg->header.size, stdout);
+			if(msg->output[msg->header.size - 1] != '\n') printf("\n");
+			printf("------------------\n");
+			return;
+		}
+	}
+
+	/* Default to saving to a blob */
+	char *filename = output_file(path, "blob", "bin");
+	FILE *fp = fopen(filename, "wb");
+	if(!fp) {
+		err("could not save to '%s': %m", filename);
+		return;
+	}
+
+	fwrite(msg->output, 1, msg->header.size, fp);
+	fclose(fp);
+	fprintf(stderr, "Saved as blob to '%s'\n", filename);
+	free(filename);
+}
+
+static void message_output(message_t *msg, void *buffer, int size)
+{
+	int data_left = msg->header.size - msg->size_read;
+
+	printf("Got %d bytes of message data!\n", size);
+
+	if(size > data_left) {
+		err("Too much data in message, dropping %d bytes", size - data_left);
+		size = data_left;
+	}
+
+	memcpy(msg->output + msg->size_read, buffer, size);
+
+	msg->size_read += size;
+	if(msg->size_read >= msg->header.size) {
+		fprintf(stderr, "Successfully read %d bytes\n", msg->size_read);
+		message_finish(msg);
+		msg->valid = false;
+	}
+}
+
+int main_interactive(filter_t *filter, delay_t *delay, libusb_context *context)
+{
+	libusb_device *dev = NULL;
+	libusb_device_handle *dh = NULL;
+
+	/* Wait for a device to be connected */
+	filter_clean_libusb(filter);
+	int rc = usb_unique_wait(filter, delay, context, &dev);
+
+	if(rc == FILTER_NONE) {
+		printf("No device found.\n");
+		return 1;
+	}
+	else if(rc == FILTER_MULTIPLE) {
+		printf("Multiple devices found, ambiguous!\n");
+		return 1;
+	}
+
+	if((rc = libusb_open(dev, &dh))) {
+		rc = libusb_err(rc, "cannot open device %s", usb_id(dev));
+		goto end;
+	}
+
+	/* Don't detach kernel drivers to avoid breaking the Mass Storage
+	   communications if fxlink is ever started while the native LINK
+	   application is running! */
+	libusb_set_auto_detach_kernel_driver(dh, false);
+
+	if((rc = libusb_claim_interface(dh, 0))) {
+		rc = libusb_err(rc, "cannot claim interface on %s", usb_id(dev));
+		goto end;
+	}
+
+	printf("Connected to %s, starting test.\n", usb_id(dev));
+
+	/* This buffer is used to receive messages; if the header is not complete
+	   it is left in the buffer, hence the extra room */
+	__attribute__((aligned(4)))
+	static uint8_t buffer[2048 + sizeof(usb_fxlink_header_t)] = { 0 };
+	/* Amount of data in the buffer */
+	int buffer_size = 0;
+
+	/* Current message */
+	message_t msg = { 0 };
+
+	while(1)
+	{
+		int transferred = -1;
+		rc = libusb_bulk_transfer(dh, 0x81, buffer + buffer_size, 2048,
+			&transferred, 2000);
+
+		if(rc == LIBUSB_ERROR_NO_DEVICE) {
+			printf("Disconnected, leaving.\n");
+			break;
+		}
+		else if(rc && rc != LIBUSB_ERROR_TIMEOUT) {
+			rc = libusb_err(rc, "bulk transfer failed on %s", usb_id(dev));
+			continue;
+		}
+		if(transferred <= 0) continue;
+
+		buffer_size += transferred;
+
+		/* If there is an unfinished message, continue working on it */
+		if(msg.valid) {
+			message_output(&msg, buffer, buffer_size);
+			buffer_size = 0;
+		}
+
+		/* If the header is not yet fully transmitted, wait */
+		usb_fxlink_header_t *h = (void *)buffer;
+		if(buffer_size < (int)sizeof *h) continue;
+
+		/* Handle a new message */
+		if(h->version == 0x00000100) {
+			int data_size = buffer_size - sizeof *h;
+
+			if(!message_new(&msg, h))
+				printf("dropping %d bytes\n", data_size);
+			else
+				message_output(&msg, buffer + sizeof *h, data_size);
+
+			buffer_size = 0;
+			continue;
+		}
+		else {
+			err("invalid header, dropping %d bytes", transferred);
+			buffer_size = 0;
+		}
+	}
+
+	/* Save last unfinished message */
+	if(buffer_size > 0) {
+		printf("%d bytes not collected dropped\n", buffer_size);
+	}
+	rc = 0;
+
+end:
+	if(dh) {
+		libusb_release_interface(dh, 0);
+		libusb_close(dh);
+	}
+	if(dev) libusb_unref_device(dev);
+	return rc;
+}
diff --git a/fxlink/main.c b/fxlink/main.c
index c2e8adc..b5a5f27 100644
--- a/fxlink/main.c
+++ b/fxlink/main.c
@@ -12,8 +12,6 @@
 #include <stdio.h>
 #include <errno.h>
 
-/* Main functions for each mdoe */
-int main_list(filter_t *filter, delay_t *delay, libusb_context *context);
 int main_test(libusb_device *device, libusb_context *context);
 
 static const char *help_string =
@@ -31,7 +29,7 @@ static const char *help_string =
 "  -l, --list          List detected calculators on the USB ports (libusb)\n"
 "  -b, --blocks        List detected Mass Storage filesystems (udisks2)\n"
 "  -s, --send          Send a file to a Mass Storage calculator (udisks2)\n"
-"  --test              Communication tests by Lephe (libusb) [WIP!]\n"
+"  -i, --interactive   Interactive messaging with a gint add-in (libusb)\n"
 "\n"
 "General options:\n"
 "  -w DELAY            Wait up to this many seconds for a calculator to\n"
@@ -67,18 +65,18 @@ int main(int argc, char **argv)
 	// Command-line argument parsing
 	//---
 
-	enum { TEST=1, LIBUSB_LOG };
+	enum { LIBUSB_LOG=1 };
 	const struct option longs[] = {
-		{ "help",       no_argument,       NULL,       'h' },
-		{ "list",       no_argument,       NULL,       'l' },
-		{ "blocks",     no_argument,       NULL,       'b' },
-		{ "send",       no_argument,       NULL,       's' },
-		{ "test",       no_argument,       NULL,      TEST },
-		{ "libusb-log", required_argument, NULL, LIBUSB_LOG },
+		{ "help",        no_argument,       NULL,       'h' },
+		{ "list",        no_argument,       NULL,       'l' },
+		{ "blocks",      no_argument,       NULL,       'b' },
+		{ "send",        no_argument,       NULL,       's' },
+		{ "interactive", no_argument,       NULL,       'i' },
+		{ "libusb-log",  required_argument, NULL, LIBUSB_LOG },
 	};
 
 	while(option >= 0 && option != '?')
-	switch((option = getopt_long(argc, argv, "hlbsf:w::", longs, NULL)))
+	switch((option = getopt_long(argc, argv, "hlbsif:w::", longs, NULL)))
 	{
 	case 'h':
 		fprintf(stderr, help_string, argv[0]);
@@ -86,7 +84,7 @@ int main(int argc, char **argv)
 	case 'l':
 	case 'b':
 	case 's':
-	case TEST:
+	case 'i':
 		mode = option;
 		break;
 	case LIBUSB_LOG:
@@ -141,7 +139,7 @@ int main(int argc, char **argv)
 	libusb_context *context = NULL;
 
 	/* Initialize libusb for corresponding modes */
-	if(mode == 'l' || mode == TEST) {
+	if(mode == 'l' || mode == 'i') {
 		if((rc = libusb_init(&context)))
 			return libusb_err(rc, "error initializing libusb");
 		libusb_set_option(context, LIBUSB_OPTION_LOG_LEVEL, loglevel);
@@ -168,18 +166,8 @@ int main(int argc, char **argv)
 		rc = err("this fxlink was built without UDisks2; -s is disabled");
 		#endif
 	}
-	else if(mode == TEST) {
-		libusb_device *dev = NULL;
-		int rc = usb_unique_wait(filter, &delay, context, &dev);
-
-		if(rc == FILTER_NONE)
-			printf("No device found.\n");
-		else if(rc == FILTER_MULTIPLE)
-			printf("Multiple devices found, ambiguous!\n");
-		else if(rc == FILTER_UNIQUE) {
-			rc = main_test(dev, context);
-			libusb_unref_device(dev);
-		}
+	else if(mode == 'i') {
+		rc = main_interactive(filter, &delay, context);
 	}
 
 	if(context) libusb_exit(context);
@@ -240,48 +228,9 @@ int main_list(filter_t *filter, delay_t *delay, libusb_context *context)
 }
 
 //---
-// WIP tests
+// libudev tests; work but not useful yet
 //---
 
-int main_test(libusb_device *dev, libusb_context *context)
-{
-	libusb_device_handle *dh;
-	int rc;
-	(void)context;
-
-	if((rc = libusb_open(dev, &dh)))
-		return libusb_err(rc, "cannot open device %s", usb_id(dev));
-
-	/* When possible detach any existing driver */
-	libusb_set_auto_detach_kernel_driver(dh, true);
-
-	if((rc = libusb_claim_interface(dh, 0))) {
-		libusb_close(dh);
-		return libusb_err(rc, "cannot claim interface on %s", usb_id(dev));
-	}
-
-	uint8_t buffer[2048];
-	int transferred = -1;
-
-	while(1)
-	{
-		rc = libusb_bulk_transfer(dh, 0x81, buffer, 2048, &transferred, 500);
-
-		if((rc == 0 || rc == LIBUSB_ERROR_TIMEOUT) && transferred > 0)
-			fwrite(buffer, 1, transferred, stdout);
-		if(rc)
-			rc=libusb_err(rc,"cannot perform bulk transfer on %s",usb_id(dev));
-
-		fprintf(stderr, "Transferred: %d\n", transferred);
-		if(rc || transferred == 0) break;
-	}
-
-	libusb_release_interface(dh, 0);
-	libusb_close(dh);
-	return rc;
-}
-
-/* libudev tests, work but not useful yet */
 #if 0
 #include <libudev.h>
 int main_udev_test(libusb_device *dev)
diff --git a/fxlink/png.c b/fxlink/png.c
new file mode 100644
index 0000000..89f9453
--- /dev/null
+++ b/fxlink/png.c
@@ -0,0 +1,42 @@
+#include "png.h"
+#include "util.h"
+#include <stdio.h>
+
+/* fxlink_png_save(): Save a bitmap into a PNG file */
+int fxlink_png_save(png_byte **row_pointers, int width, int height,
+	char const *path)
+{
+	png_struct *png = png_create_write_struct(PNG_LIBPNG_VER_STRING,
+		NULL, NULL, NULL);
+	if(!png)
+		return err("failed to write PNG: png_create_write_struct");
+
+	png_infop info = png_create_info_struct(png);
+	if(!info)
+		return err("failed to write PNG: png_create_info_struct");
+
+	FILE *fp = fopen(path, "wb");
+	if(!fp) {
+		png_destroy_write_struct(&png, &info);
+		return err("failed to open '%s' to write PNG: %m", path);
+	}
+
+	if(setjmp(png_jmpbuf(png))) {
+		fclose(fp);
+		png_destroy_write_struct(&png, &info);
+		return 1;
+	}
+
+	png_init_io(png, fp);
+	png_set_IHDR(png, info,
+		width, height, 8,
+		PNG_COLOR_TYPE_RGB, PNG_INTERLACE_NONE,
+		PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
+
+	png_write_info(png, info);
+	png_write_image(png, row_pointers);
+	png_write_end(png, NULL);
+	png_destroy_write_struct(&png, &info);
+	fclose(fp);
+	return 0;
+}
diff --git a/fxlink/png.h b/fxlink/png.h
new file mode 100644
index 0000000..acc76a7
--- /dev/null
+++ b/fxlink/png.h
@@ -0,0 +1,14 @@
+//---
+// fxlink:png - Tools to output PNG images with libpng
+//---
+
+#ifndef FXLINK_PNG_H
+#define FXLINK_PNG_H
+
+#include <png.h>
+
+/* fxlink_png_save(): Save a bitmap into a PNG file */
+int fxlink_png_save(png_byte **row_pointers, int width, int height,
+	char const *path);
+
+#endif /* FXLINK_PNG_H */
diff --git a/fxlink/protocol.c b/fxlink/protocol.c
new file mode 100644
index 0000000..d67b5d5
--- /dev/null
+++ b/fxlink/protocol.c
@@ -0,0 +1,141 @@
+#include "protocol.h"
+#include "util.h"
+
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <endian.h>
+
+//---
+// Image format
+//---
+
+static int img_bytes_per_row(int format, int width)
+{
+	if(format == USB_FXLINK_IMAGE_RGB565)
+		return 2 * width;
+	if(format == USB_FXLINK_IMAGE_MONO)
+		return (width + 7) >> 3;
+	if(format == USB_FXLINK_IMAGE_GRAY)
+		return 2 * ((width + 7) >> 3);
+
+	return 0;
+}
+
+static void decode_rgb565(void *pixels, int width, int height, int size,
+	uint8_t **row_pointers)
+{
+	int bpr = img_bytes_per_row(USB_FXLINK_IMAGE_RGB565, width);
+
+	for(int y = 0; y < height; y++) {
+		void *row = pixels + y * bpr;
+
+		for(int x = 0; x < width; x++) {
+			/* Don't read past the read buffer if the image is incomplete */
+			void *input = row + 2 * x;
+			uint16_t color = 0;
+			if(input - pixels + 2 <= size) color = *(uint16_t *)input;
+
+			color = be16toh(color);
+
+			row_pointers[y][3*x+0] = (color >> 11) << 3;
+			row_pointers[y][3*x+1] = ((color >> 5) & 0x3f) << 2;
+			row_pointers[y][3*x+2] = (color & 0x1f) << 3;
+		}
+	}
+}
+
+static void decode_mono(void *pixels, int width, int height, int size,
+	uint8_t **row_pointers)
+{
+	int bpr = img_bytes_per_row(USB_FXLINK_IMAGE_MONO, width);
+
+	for(int y = 0; y < height; y++) {
+		void *row = pixels + y * bpr;
+
+		for(int x = 0; x < width; x++) {
+			/* Don't read past the read buffer if the image is incomplete */
+			void *input = row + (x >> 3);
+			int byte = 0;
+			if(input - pixels + 1 <= size) byte = *(uint8_t *)input;
+			int color = (byte & (0x80 >> (x & 7))) ? 0 : 255;
+
+			row_pointers[y][3*x+0] = color;
+			row_pointers[y][3*x+1] = color;
+			row_pointers[y][3*x+2] = color;
+		}
+	}
+}
+
+static void decode_gray(void *pixels, int width, int height, int size,
+	uint8_t **row_pointers)
+{
+	int bpr = img_bytes_per_row(USB_FXLINK_IMAGE_MONO, width);
+
+	for(int k = 0; k < 2 * height; k++) {
+		void *row = pixels + k * bpr;
+		int y = k % height;
+
+		for(int x = 0; x < width; x++) {
+			/* Don't read past the read buffer if the image is incomplete */
+			void *input = row + (x >> 3);
+			int byte = 0;
+			if(input - pixels + 1 <= size) byte = *(uint8_t *)input;
+
+			int color = (byte & (0x80 >> (x & 7)));
+			/* Everything is inverted */
+			if(!color) color = (k >= height) ? 0xaa : 0x55;
+			else color = 0x00;
+
+			row_pointers[y][3*x+0] += color;
+			row_pointers[y][3*x+1] += color;
+			row_pointers[y][3*x+2] += color;
+		}
+	}
+}
+
+uint8_t **fxlink_protocol_decode_image(message_t *msg)
+{
+	usb_fxlink_image_t *img = (void *)msg->output;
+	void *pixels = msg->output + sizeof *img;
+
+	/* Compute the amount of data for the specified image format and size */
+	int bytes_per_row = img_bytes_per_row(img->pixel_format, img->width);
+	int expected_size = img->height * bytes_per_row;
+
+	/* Check that the correct amount of data was sent */
+	int size = msg->size_read - sizeof *img;
+	if(size < expected_size)
+		printf("warning: got %d bytes but needed %d, image will be "
+			"incomplete\n", size, expected_size);
+	if(size > expected_size)
+		printf("warning: got %d bytes but needed %d for image, dropping extra"
+			"\n", size, expected_size);
+
+	/* Allocate row pointers */
+	uint8_t **row_pointers = malloc(img->height*sizeof *row_pointers);
+	if(!row_pointers) {
+		err("failed to write allocate memory to decode image");
+		return NULL;
+	}
+
+	for(size_t y = 0; y < img->height; y++) {
+		row_pointers[y] = calloc(img->width, 3);
+		if(!row_pointers[y]) {
+			err("failed to write allocate memory to decode image");
+			for(size_t i = 0 ; i < y; i++) free(row_pointers[i]);
+			free(row_pointers);
+			return NULL;
+		}
+	}
+
+	/* Decode image */
+	if(img->pixel_format == USB_FXLINK_IMAGE_RGB565)
+		decode_rgb565(pixels, img->width, img->height, size, row_pointers);
+	if(img->pixel_format == USB_FXLINK_IMAGE_MONO)
+		decode_mono(pixels, img->width, img->height, size, row_pointers);
+	if(img->pixel_format == USB_FXLINK_IMAGE_GRAY)
+		decode_gray(pixels, img->width, img->height, size, row_pointers);
+
+	return row_pointers;
+}
diff --git a/fxlink/protocol.h b/fxlink/protocol.h
new file mode 100644
index 0000000..9595e3e
--- /dev/null
+++ b/fxlink/protocol.h
@@ -0,0 +1,71 @@
+//---
+// fxlink:protocol - Custom fxlink protocol
+//---
+
+#ifndef FXLINK_PROTOCOL_H
+#define FXLINK_PROTOCOL_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+/* See the gint source for details on the protocol */
+typedef struct
+{
+	uint32_t version;
+	uint32_t size;
+	uint32_t transfer_size;
+
+	char application[16];
+	char type[16];
+
+} usb_fxlink_header_t;
+
+/* Subheader for the fxlink built-in "image" type */
+typedef struct
+{
+	uint32_t width;
+	uint32_t height;
+	int pixel_format;
+
+} usb_fxlink_image_t;
+
+/* Pixel formats */
+typedef enum
+{
+	/* Image is an array of *big-endian* uint16_t with RGB565 format */
+	USB_FXLINK_IMAGE_RGB565 = 0,
+	/* Image is an array of bits in mono format */
+	USB_FXLINK_IMAGE_MONO,
+	/* Image is two consecutive mono arrays, one for light, one for dark */
+	USB_FXLINK_IMAGE_GRAY,
+
+} usb_fxlink_image_format_t;
+
+//---
+// Utilities in this implementation
+//---
+
+/* Message currently being transferred */
+typedef struct
+{
+	usb_fxlink_header_t header;
+	/* Valid when we are reading a message */
+	bool valid;
+	/* Data already read in this message */
+	uint32_t size_read;
+	/* Data buffer */
+	char *output;
+
+} message_t;
+
+/* fxlink_protocol_decode_image(): Decode an image into RGB888 format
+
+   This function decodes the message into an RGB888 image and returns an array
+   of row pointers with the image data (free the array and each element of the
+   array after use).
+
+   If there are not enough bytes in the input, it pads with zeros, and if there
+   are extra bytes, they are dropped; in both cases a warning is printed. */
+uint8_t **fxlink_protocol_decode_image(message_t *msg);
+
+#endif /* FXLINK_PROTOCOL_H */