diff --git a/src/diag_io.c b/src/diag_io.c
new file mode 100644
index 0000000..36f92dc
--- /dev/null
+++ b/src/diag_io.c
@@ -0,0 +1,115 @@
+
+#include <stdint.h>
+#include <stdio.h>
+#include <errno.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "protocol.h"
+#include "diag_io.h"
+#include "diagchar_hdlc.h"
+
+/* transmit a msgb containing a DIAG message over the given fd */
+int diag_transmit_msgb(struct diag_instance *di, struct msgb *msg)
+{
+	int out_len, rc;
+	uint8_t packet[DIAG_MAX_HDLC_BUF_SIZE];
+	struct diag_send_desc_type send;
+	struct diag_hdlc_dest_type enc = { NULL, NULL, 0 };
+
+	send.state = DIAG_STATE_START;
+	send.pkt = msgb_data(msg);
+	send.last = msgb_data(msg) + msgb_length(msg) - 1;
+	send.terminate = 1;
+
+	enc.dest = packet;
+	enc.dest_last = packet + sizeof(packet) - 1;
+
+	diag_hdlc_encode(&send, &enc);
+
+	out_len = (enc.dest - (void *)packet);
+
+	rc = write(di->fd, packet, out_len);
+	if (rc != out_len) {
+		printf("Short write on packet.\n");
+		return -1;
+	}
+
+	msgb_free(msg);
+
+	return 0;
+}
+
+/* transmit a message from a buffer (nto msgb) as DIAG over the given fd */
+int diag_transmit_buf(struct diag_instance *di, const uint8_t *data, size_t data_len)
+{
+	struct msgb *msg = msgb_alloc(DIAG_MAX_REQ_SIZE, "DIAG Tx");
+
+	memcpy(msg->tail, data, data_len);
+	msgb_put(msg, data_len);
+
+	return diag_transmit_msgb(di, msg);
+}
+
+int diag_read(struct diag_instance *di)
+{
+	uint8_t buf[DIAG_MAX_HDLC_BUF_SIZE];
+	struct diag_hdlc_decode_type hdlc_decode;
+	struct msgb *msg;
+	int rc;
+
+	/* read raw data into buffer */
+	rc = read(di->fd, buf, sizeof(buf));
+	if (rc <= 0 ) {
+		fprintf(stderr, "Short read!\n");
+		return -EIO;
+	}
+
+	if (!di->rx.msg) {
+		di->rx.msg = msgb_alloc(DIAG_MAX_REQ_SIZE, "DIAG Rx");
+		di->rx.msg->l2h = di->rx.msg->tail;
+	}
+	msg = di->rx.msg;
+
+	hdlc_decode.dest_ptr = msg->tail;
+	hdlc_decode.dest_size = msgb_tailroom(msg);
+	hdlc_decode.src_ptr = buf;
+	hdlc_decode.src_size = rc;
+	hdlc_decode.src_idx = 0;
+	hdlc_decode.dest_idx = 0;
+
+	rc = diag_hdlc_decode(&hdlc_decode);
+
+	if (msgb_length(msg) + hdlc_decode.dest_idx > DIAG_MAX_REQ_SIZE) {
+		fprintf(stderr, "Dropping packet. pkt_size: %d, max: %d\n",
+			msgb_length(msg) + hdlc_decode.dest_idx,
+			DIAG_MAX_REQ_SIZE);
+		return -EIO;
+	}
+
+	msgb_put(msg, hdlc_decode.dest_idx);
+
+	if (rc == HDLC_COMPLETE) {
+		di->rx.msg = NULL;
+		rc = crc_check(msgb_data(msg), msgb_length(msg));
+		if (rc) {
+			fprintf(stderr, "Bad CRC, dropping packet\n");
+			msgb_free(msg);
+			return -EINVAL;
+		}
+		msgb_get(msg, HDLC_FOOTER_LEN);
+
+		if (msgb_length(msg) < 1) {
+			fprintf(stderr, "Message too short, len: %u\n", msgb_length(msg));
+			msgb_free(msg);
+			return -EINVAL;
+		}
+
+		if (di->rx.rcvmsg)
+			(di->rx.rcvmsg)(di, msg);
+		else
+			msgb_free(msg);
+	}
+
+	return 0;
+};