diff --git a/README.md b/README.md
index 7e382f2..f2e1ed3 100644
--- a/README.md
+++ b/README.md
@@ -22,7 +22,7 @@ See main.c for example of how to use the library.
 
 ## What is missing
 
-- Setting error type flags in the ESR when error is added to queue
+- <s>Setting error type flags in the ESR when error is added to queue</s>
 - There may be some bugs in the status registers (not tested much)
 - Character data argument ("string without quotes", enum-like)
 - Number units and metric suffixes (k,M,G,m,u,n)
diff --git a/main.c b/main.c
index 551f4eb..58e2890 100644
--- a/main.c
+++ b/main.c
@@ -23,11 +23,24 @@ int main()
 
 	send_cmd("APPLY:SINE 50, 1.0, 2.17\n"); // floats
 	send_cmd("DISP:TEXT \"Hello world\"\n"); // string
+
+	// test user error
+	send_cmd("*CLS\n"); // string
+	send_cmd("USERERROR\n"); // string
+	send_cmd("SYST:ERR:COUNT?\n");
+	send_cmd("SYST:ERR:NEXT?\n");
 }
 
 
 // ---- Test device impl ----
 
+
+const SCPI_error_desc scpi_user_errors[] = {
+	{10, "Custom error"},
+	{0} // terminator
+};
+
+
 void scpi_send_byte_impl(uint8_t b)
 {
 	putchar(b); // device sends a byte
@@ -78,6 +91,13 @@ void cmd_DATA_BLOB_data(const uint8_t *bytes)
 }
 
 
+void cmd_USRERR_cb(const SCPI_argval_t *args)
+{
+	printf("cb USRERR - raising user error 10.\n");
+	scpi_add_error(10, "Custom error message...");
+}
+
+
 // Command definition (mandatory commands are built-in)
 const SCPI_command_t scpi_commands[] = {
 	{
@@ -97,5 +117,9 @@ const SCPI_command_t scpi_commands[] = {
 		.blob_chunk = 4,
 		.blob_callback = cmd_DATA_BLOB_data
 	},
+	{
+		.levels = {"USeRERRor"},
+		.callback = cmd_USRERR_cb
+	},
 	{0} // end marker
 };
diff --git a/scpi.pro.user b/scpi.pro.user
index c49305f..f2d5683 100644
--- a/scpi.pro.user
+++ b/scpi.pro.user
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE QtCreatorProject>
-<!-- Written by QtCreator 3.5.1, 2015-12-05T18:39:54. -->
+<!-- Written by QtCreator 3.5.1, 2015-12-05T19:13:04. -->
 <qtcreator>
  <data>
   <variable>EnvironmentId</variable>
diff --git a/source/scpi_errors.c b/source/scpi_errors.c
index 980e98e..7ce33bb 100644
--- a/source/scpi_errors.c
+++ b/source/scpi_errors.c
@@ -21,7 +21,7 @@ static struct {
 } erq;
 
 
-void scpi_add_error(SCPI_error_t errno, const char *extra)
+void scpi_add_error(int16_t errno, const char *extra)
 {
 	bool added = true;
 	if (erq.count >= ERR_QUEUE_LEN) {
@@ -45,6 +45,18 @@ void scpi_add_error(SCPI_error_t errno, const char *extra)
 		erq.w_pos = 0;
 	}
 
+	// error type status flags
+	if (errno >= -499 && errno <= -400) {
+		SCPI_REG_SESR.QUERY_ERROR = true;
+	} else if ((errno >= -399 && errno <= -300) || errno > 0) {
+		SCPI_REG_SESR.DEV_ERROR = true;
+	} else if (errno >= -299 && errno <= -200) {
+		SCPI_REG_SESR.EXE_ERROR = true;
+	} else if (errno >= -199 && errno <= -100) {
+		SCPI_REG_SESR.CMD_ERROR = true;
+	}
+
+	// update the error queue bit and propagate the above flags
 	scpi_status_update();
 }
 
@@ -96,15 +108,7 @@ uint8_t scpi_error_count(void)
 
 // ---- table ----
 
-
-typedef struct {
-	const int16_t errno;
-	const char *msg;
-} SCPI_error_desc;
-
-
 static const SCPI_error_desc error_table[] = {
-	{ 0, "No error"},
 	{ -100, "Command error"},
 	{ -101, "Invalid character"},
 	{ -102, "Syntax error"},
@@ -226,23 +230,38 @@ static const SCPI_error_desc error_table[] = {
 	{ -600, "User request"},
 	{ -700, "Request control"},
 	{ -800, "Operation complete"},
-	{ -9999} // end mark
+	{0} // end mark
 };
 
 
-const char * scpi_error_message(SCPI_error_t errno)
+const char * scpi_error_message(int16_t errno)
 {
-	for (int i = 0; error_table[i].errno != -9999; i++) {
-		if (error_table[i].errno == errno) {
-			return error_table[i].msg;
+	if (errno == 0) {
+		// ok state
+		return "No error";
+
+	} else if (errno < 0) {
+		// standard errors
+		for (int i = 0; (i == 0 || error_table[i].errno != 0); i++) {
+			if (error_table[i].errno == errno) {
+				return error_table[i].msg;
+			}
+		}
+
+	} else {
+		// user error
+		for (int i = 0; scpi_user_errors[i].errno != 0; i++) {
+			if (scpi_user_errors[i].errno == errno) {
+				return scpi_user_errors[i].msg;
+			}
 		}
 	}
 
-	return NULL;
+	return "Unknown error";
 }
 
 
-void scpi_error_string(char *buffer, SCPI_error_t errno, const char *extra)
+void scpi_error_string(char *buffer, int16_t errno, const char *extra)
 {
 	const char *msg = scpi_error_message(errno);
 
@@ -252,8 +271,6 @@ void scpi_error_string(char *buffer, SCPI_error_t errno, const char *extra)
 	len = offs = sprintf(buffer, "%d,\"", errno); // <code>,"
 	buffer += offs;
 
-	if (msg == NULL) msg = "Unknown error";
-
 	offs = sprintf(buffer, "%s", msg); // Error message
 	len += offs;
 	buffer += offs;
diff --git a/source/scpi_errors.h b/source/scpi_errors.h
index 2b30fa5..c2914f9 100644
--- a/source/scpi_errors.h
+++ b/source/scpi_errors.h
@@ -2,8 +2,16 @@
 #include <stdint.h>
 #include <stdbool.h>
 
+typedef struct {
+	const int16_t errno;
+	const char *msg;
+} SCPI_error_desc;
+
+/** User error definitions */
+extern const SCPI_error_desc scpi_user_errors[];
+
 // SCPI error constants
-typedef enum {
+enum {
 	E_NO_ERROR = 0,
 	E_COMMAND_ERROR = -100,
 	E_CMD_INVALID_CHARACTER = -101,
@@ -126,11 +134,11 @@ typedef enum {
 	E_USER_REQUEST = -600,
 	E_REQUEST_CONTROL = -700,
 	E_OPERATION_COMPLETE = -800,
-} SCPI_error_t;
+};
 
 
 /** Get SCPI error message (alone) */
-const char *scpi_error_message(SCPI_error_t errno);
+const char *scpi_error_message(int16_t errno);
 
 
 /**
@@ -139,11 +147,11 @@ const char *scpi_error_message(SCPI_error_t errno);
  *
  * extra can be NULL to skip the optional part.
  */
-void scpi_error_string(char *buffer, SCPI_error_t errno, const char *extra);
+void scpi_error_string(char *buffer, int16_t errno, const char *extra);
 
 
 /** Add error to the error queue */
-void scpi_add_error(SCPI_error_t errno, const char *extra);
+void scpi_add_error(int16_t errno, const char *extra);
 
 
 /** Get number of errors in the error queue */
diff --git a/source/scpi_regs.c b/source/scpi_regs.c
index 9322fb3..e4cc456 100644
--- a/source/scpi_regs.c
+++ b/source/scpi_regs.c
@@ -13,7 +13,7 @@ SCPI_REG_QUES_t SCPI_REG_QUES_EN = {.u16 = 0xFFFF};
 SCPI_REG_OPER_t SCPI_REG_OPER;
 SCPI_REG_OPER_t SCPI_REG_OPER_EN = {.u16 = 0xFFFF};
 
-SCPI_REG_SESR_t SCPI_REG_SESR;
+SCPI_REG_SESR_t SCPI_REG_SESR = {.POWER_ON = 1}; // indicates the startup condition
 SCPI_REG_SESR_t SCPI_REG_SESR_EN;
 
 SCPI_REG_STB_t SCPI_REG_STB;
diff --git a/source/scpi_regs.h b/source/scpi_regs.h
index 7fff32e..65657e1 100644
--- a/source/scpi_regs.h
+++ b/source/scpi_regs.h
@@ -52,8 +52,8 @@ typedef union {
 
 typedef union {
 	struct __attribute__((packed)) {
-		bool OPC: 1;
-		bool REQ_CONTROL: 1;
+		bool OPC: 1; // operation complete - only for instruments with overlapping commands
+		bool REQ_CONTROL: 1; // GPIB-only
 		bool QUERY_ERROR: 1;
 		bool DEV_ERROR: 1;
 		bool EXE_ERROR: 1;