diff --git a/Makefile b/Makefile
index 67f3fa87..00bbf7b9 100644
--- a/Makefile
+++ b/Makefile
@@ -99,6 +99,7 @@ SRCS = \
 # RTL837x
 SRCS += \
 	rtl837x_bandwidth.c \
+	rtl837x_cabletest.c \
 	rtl837x_flash.c \
 	rtl837x_igmp.c \
 	rtl837x_init.c \
diff --git a/cmd_parser.c b/cmd_parser.c
index 9e226dbf..28965072 100644
--- a/cmd_parser.c
+++ b/cmd_parser.c
@@ -20,6 +20,7 @@
 #include "rtl837x_lldp.h"
 #include "rtl837x_storm.h"
 #include "rtl837x_trap.h"
+#include "rtl837x_cabletest.h"
 #include "sfp.h"
 #include "dhcp.h"
 #include "syslog.h"
@@ -903,6 +904,7 @@ void parse_port(void)
 					 "\nport <port> [show|on|off]" \
 					 "\nport <port> [10m|100m|1g|2g5|duplex] [half|full]" \
 					 "\nport <port> lldp [block|permit]" \
+					 "\nport <port> cabletest [show]" \
 					 "\nport <port> name [custom port name]\n");
 		return;
 	}
@@ -945,6 +947,11 @@ void parse_port(void)
 		}
 	} else if (machine.is_sfp[phy_settings.port]) {
 		print_string(" is SFP no PHY information available.\n");
+	} else if (cmd_compare(2, "cabletest")) {
+		if (cmd_words_len > 3 && cmd_compare(3, "show"))
+			cabletest_show();
+		else
+			cabletest_run();
 	} else if (cmd_compare(2, "10m")) {
 		print_string(" 10M\n");
 		phy_settings.speed = PHY_SPEED_10M;
diff --git a/rtl837x_cabletest.c b/rtl837x_cabletest.c
new file mode 100644
index 00000000..120e87fa
--- /dev/null
+++ b/rtl837x_cabletest.c
@@ -0,0 +1,216 @@
+#include <stdint.h>
+#include <stdbool.h>
+#include "rtl837x_common.h"
+#include "rtl837x_sfr.h"
+#include "rtl837x_cabletest.h"
+#include "rtl837x_phy.h"
+#include "phy.h"
+#include "machine.h"
+
+#pragma codeseg BANK3
+#pragma constseg BANK3
+
+#define RTCT_CTRL		0xa422
+#define RTCT_CTRL_START		0x00f1
+#define RTCT_CTRL_DONE		0x8000
+#define RTCT_SRAM_ADDR		0xa436
+#define RTCT_SRAM_DATA		0xa438
+#define RTCT_SRAM_STATUS	0x8026
+#define RTCT_SRAM_LEN		0x8028
+
+#define RTCT_ST_BUSY		0x01
+#define RTCT_ST_MISMATCH	0x06
+#define RTCT_ST_OPEN		0x08
+#define RTCT_ST_SHORT		0x10
+#define RTCT_ST_OK		0x20
+#define RTCT_ST_DONE		0x40
+#define RTCT_ST_CROSS_SHORT	0x80
+
+#define RTCT_LEN_ZERO		150
+#define RTCT_LEN_PER_50CM	43
+
+#define PHY_POWER		0xa610
+#define PHY_POWER_OFF		0x0800
+
+#define BMCR_PDOWN_1G_FULL	0x0940
+#define BMCR_1G_FULL		0x0140
+#define BMCR_ANRESTART		0x0200
+
+#define CT_IDLE			0
+#define CT_SILENCE		1
+#define CT_RUN			2
+#define CT_POLL_TICKS		(SYS_TICK_HZ / 20)
+#define CT_POLLS		100
+
+extern volatile __xdata uint32_t ticks;
+extern __code const struct machine machine;
+extern __xdata struct machine_runtime machine_detected;
+
+__xdata uint8_t ct_state = CT_IDLE;
+__xdata uint8_t ct_port;
+__xdata uint8_t ct_last;
+__xdata uint8_t ct_polls;
+__xdata uint8_t ct_valid;
+__xdata uint8_t ct_have = 0;
+__xdata uint8_t ct_pair;
+__xdata uint8_t ct_flags;
+__xdata uint16_t ct_bmcr;
+__xdata uint8_t ct_result[4];
+__xdata uint16_t ct_len[4];
+
+void rtl_phy_cable_test_start(uint8_t port) __banked
+{
+	phy_write(port, PHY_MMD31, PHY_MMD31_FEDCR, BMCR_1G_FULL);
+	phy_write(port, PHY_MMD31, RTCT_CTRL, RTCT_CTRL_START);
+}
+
+bool rtl_phy_cable_test_get_status(uint8_t port) __banked
+{
+	phy_read(port, PHY_MMD31, RTCT_CTRL);
+	if (!(SFR_DATA_U16 & RTCT_CTRL_DONE))
+		return false;
+
+	for (ct_pair = 0; ct_pair < 4; ct_pair++) {
+		// The flags are the low byte of the word, not the high one.
+		phy_write(port, PHY_MMD31, RTCT_SRAM_ADDR, RTCT_SRAM_STATUS + (ct_pair << 2));
+		phy_read(port, PHY_MMD31, RTCT_SRAM_DATA);
+		ct_flags = SFR_DATA_U16;
+		if (!(ct_flags & RTCT_ST_DONE))
+			return false;
+		if (ct_flags & RTCT_ST_OK)
+			ct_result[ct_pair] = CABLE_RESULT_OK;
+		else if (ct_flags & RTCT_ST_OPEN)
+			ct_result[ct_pair] = CABLE_RESULT_OPEN;
+		else if (ct_flags & RTCT_ST_SHORT)
+			ct_result[ct_pair] = CABLE_RESULT_SAME_SHORT;
+		else if (ct_flags & RTCT_ST_BUSY)
+			ct_result[ct_pair] = CABLE_RESULT_UNSPEC;
+		else if (ct_flags & RTCT_ST_CROSS_SHORT)
+			ct_result[ct_pair] = CABLE_RESULT_CROSS_SHORT;
+		else if (ct_flags & RTCT_ST_MISMATCH)
+			ct_result[ct_pair] = CABLE_RESULT_MISMATCH;
+		else
+			ct_result[ct_pair] = CABLE_RESULT_UNSPEC;
+
+		phy_write(port, PHY_MMD31, RTCT_SRAM_ADDR, RTCT_SRAM_LEN + (ct_pair << 2));
+		phy_read(port, PHY_MMD31, RTCT_SRAM_DATA);
+		ct_len[ct_pair] = SFR_DATA_U16;
+		ct_len[ct_pair] = ct_len[ct_pair] > RTCT_LEN_ZERO ? ct_len[ct_pair] - RTCT_LEN_ZERO : 0;
+		ct_len[ct_pair] = ct_len[ct_pair] * 5 / RTCT_LEN_PER_50CM;
+	}
+	return true;
+}
+
+static void cabletest_print(void)
+{
+	print_string("Cable test port ");
+	print_phys_port(ct_port);
+	if (!ct_valid) {
+		print_string(": timed out\n");
+		return;
+	}
+	write_char('\n');
+	for (ct_pair = 0; ct_pair < 4; ct_pair++) {
+		print_string("Pair ");
+		write_char('A' + ct_pair);
+		switch (ct_result[ct_pair]) {
+		case CABLE_RESULT_OK:
+			print_string(": ok\n");
+			continue;
+		case CABLE_RESULT_OPEN:
+			print_string(": open at ");
+			break;
+		case CABLE_RESULT_SAME_SHORT:
+			print_string(": short at ");
+			break;
+		case CABLE_RESULT_CROSS_SHORT:
+			print_string(": short to another pair at ");
+			break;
+		case CABLE_RESULT_MISMATCH:
+			print_string(": impedance mismatch at ");
+			break;
+		default:
+			print_string(": unknown\n");
+			continue;
+		}
+		itoa_short(ct_len[ct_pair] / 10);
+		write_char('.');
+		write_char('0' + ct_len[ct_pair] % 10);
+		print_string(" m\n");
+	}
+}
+
+void cabletest_run(void) __banked
+{
+	if (ct_state) {
+		print_string("Cable test running on port ");
+		print_phys_port(ct_port);
+		write_char('\n');
+		return;
+	}
+	if (!machine_detected.isRTL8373 && (phy_settings.port < 4 || phy_settings.port > 7)) {
+		print_string("No cable test on this PHY\n");
+		return;
+	}
+
+	// A test started on a port that is off never ends and blocks the PHY until reset.
+	phy_read(phy_settings.port, PHY_MMD31, PHY_POWER);
+	if (SFR_DATA_U16 & PHY_POWER_OFF) {
+		print_string("Port is off\n");
+		return;
+	}
+
+	ct_port = phy_settings.port;
+	phy_read(ct_port, PHY_MMD31, PHY_MMD31_FEDCR);
+	ct_bmcr = SFR_DATA_U16;
+
+	// Forcing the speed leaves the link up; only a power-down silences the partner.
+	phy_write(ct_port, PHY_MMD31, PHY_MMD31_FEDCR, BMCR_PDOWN_1G_FULL);
+	ct_last = ticks;
+	ct_have = 0;
+	ct_state = CT_SILENCE;
+	print_string("Cable test started\n");
+}
+
+void cabletest_show(void) __banked
+{
+	if (ct_state)
+		print_string("Cable test running\n");
+	else if (ct_port != phy_settings.port || !ct_have)
+		print_string("No cable test result\n");
+	else
+		cabletest_print();
+}
+
+void cabletest_tick(void) __banked
+{
+	if (ct_state == CT_SILENCE) {
+		if (!TICKS_DUE(ct_last, SYS_TICK_HZ))
+			return;
+		phy_read(ct_port, PHY_MMD31, PHY_POWER);
+		if (SFR_DATA_U16 & PHY_POWER_OFF) {
+			phy_write(ct_port, PHY_MMD31, PHY_MMD31_FEDCR, ct_bmcr);
+			ct_state = CT_IDLE;
+			print_string("Cable test aborted, port is off\n");
+			return;
+		}
+		rtl_phy_cable_test_start(ct_port);
+		ct_polls = CT_POLLS;
+		ct_last = ticks;
+		ct_state = CT_RUN;
+		return;
+	}
+	if (ct_state != CT_RUN || !TICKS_DUE(ct_last, CT_POLL_TICKS))
+		return;
+	ct_last = ticks;
+
+	ct_valid = rtl_phy_cable_test_get_status(ct_port);
+	if (!ct_valid && --ct_polls)
+		return;
+
+	phy_write(ct_port, PHY_MMD31, RTCT_CTRL, 0);
+	phy_write(ct_port, PHY_MMD31, PHY_MMD31_FEDCR, ct_bmcr | BMCR_ANRESTART);
+	ct_have = 1;
+	ct_state = CT_IDLE;
+	cabletest_print();
+}
diff --git a/rtl837x_cabletest.h b/rtl837x_cabletest.h
new file mode 100644
index 00000000..5846b82f
--- /dev/null
+++ b/rtl837x_cabletest.h
@@ -0,0 +1,21 @@
+#ifndef _RTL837X_CABLETEST_H_
+#define _RTL837X_CABLETEST_H_
+
+#include <stdint.h>
+#include <stdbool.h>
+
+#define CABLE_RESULT_UNSPEC		0
+#define CABLE_RESULT_OK			1
+#define CABLE_RESULT_OPEN		2
+#define CABLE_RESULT_SAME_SHORT		3
+#define CABLE_RESULT_CROSS_SHORT	4
+#define CABLE_RESULT_MISMATCH		5
+
+void rtl_phy_cable_test_start(uint8_t port) __banked;
+bool rtl_phy_cable_test_get_status(uint8_t port) __banked;
+
+void cabletest_run(void) __banked;
+void cabletest_show(void) __banked;
+void cabletest_tick(void) __banked;
+
+#endif
diff --git a/rtlplayground.c b/rtlplayground.c
index 4a8b4ec7..a69778ee 100644
--- a/rtlplayground.c
+++ b/rtlplayground.c
@@ -19,6 +19,7 @@
 #include "rtl837x_bandwidth.h"
 #include "rtl837x_init.h"
 #include "rtl837x_lldp.h"
+#include "rtl837x_cabletest.h"
 #include "dhcp.h"
 #include "cmd_parser.h"
 #include "cmd_editor.h"
@@ -1301,6 +1302,7 @@ static void handle_tick(void)
 	health_phase(HEALTH_PH_STP);
 	lldp_tick();
 	health_phase(HEALTH_LLDP);
+	cabletest_tick();
 }