diff --git a/README.md b/README.md
index 1236aeb..3c6de2c 100644
--- a/README.md
+++ b/README.md
@@ -139,4 +139,4 @@ This is the list of supported message IDs:
 |------------|------------|------------|--------------|------------|-------------------------|-------------|
 | 0x10       | feedback   | any        | yes          | 2          | 0b00000KJI 0bHGFEDCBA   | Feedback module status report (A..E = end switch D..H, F..J = empty switch D..H, K = reset switch, 0 = off, 1 = on) |
 | 0x11       | poweriface | any        | yes          | 2          | 0bHHHHHHHH 0bLLLLLLLL   | Power module status report (0xHHLL are the contents of the 16-bit shift register) |
-| 0x21       | controller | poweriface | no           | 1          | 0b00000MMM              | Start dispensing from slot M (0-4) |
+| 0x21       | controller | poweriface | no           | 1          | 0b00000MMM              | Start dispensing from slot M (1-5) or stop all dispensers (M = 0) |
diff --git a/poweriface/firmware/main.c b/poweriface/firmware/main.c
index 5cfcab5..eba790e 100644
--- a/poweriface/firmware/main.c
+++ b/poweriface/firmware/main.c
@@ -113,8 +113,11 @@ static void send_status() {
 }
 
 static void fire_relay(uint8_t num) {
-	if (num < 5) {
-		uint16_t out = 0x0080 >> num;
+	if (num == 0) {
+		bang(0);
+		strobe();
+	} else if (num <= 5) {
+		uint16_t out = 0x0080 >> (num - 1);
 		bang(out);
 		strobe();
 	}
@@ -133,6 +136,7 @@ static void loop() {
 				}
 			case 0x21:
 				if (msg.length == 1) {
+					// stop (d0 = 0) or fire relay (d0 = 1..5)
 					fire_relay(msg.data[0]);
 					// also send out the current status
 					send_status();