diff --git a/fw/Core/Src/modbus.c b/fw/Core/Src/modbus.c
index 9c06664..fdf1513 100644
--- a/fw/Core/Src/modbus.c
+++ b/fw/Core/Src/modbus.c
@@ -65,20 +65,28 @@ int8_t modbus_copy_reply_to_buffer(uint8_t *buffer, uint8_t *msg_len, modbus_tra
 	if (transaction->function_code | MODBUS_ERROR_FLAG) {
 		/* sending error reply */
 		buffer[2] = transaction->exception.exception_code;
-	}
-	switch (transaction->function_code) {
-		case MODBUS_READ_HOLDING_REGISTERS:
-		case MODBUS_READ_INPUT_REGISTERS:
-			byte_count = transaction->register_count * 2;
-			buffer[2] = byte_count;
-			*msg_len = byte_count + 5;
-			for (int i = 0; i < transaction->register_count; i++) {
-				// TODO endianness handling
-				/* buffer16b is alias for both holding and input register buffers */
-				buffer[3 + 2*i] = transaction->buffer16b[i] >> 8;
-				buffer[4 + 2*i] = transaction->buffer16b[i] & 0xff;
-			}
-			break;
+	} else {
+		switch (transaction->function_code) {
+			case MODBUS_READ_HOLDING_REGISTERS:
+			case MODBUS_READ_INPUT_REGISTERS:
+				byte_count = transaction->register_count * 2;
+				buffer[2] = byte_count;
+				*msg_len = byte_count + 5;
+				for (int i = 0; i < transaction->register_count; i++) {
+					// TODO endianness handling
+					/* buffer16b is alias for both holding and input register buffers */
+					buffer[3 + 2*i] = transaction->buffer16b[i] >> 8;
+					buffer[4 + 2*i] = transaction->buffer16b[i] & 0xff;
+				}
+				break;
+			case MODBUS_WRITE_SINGLE_REGISTER:
+				buffer[2] = (uint8_t) (transaction->register_address >> 8);
+				buffer[3] = (uint8_t) transaction->register_address;
+				buffer[4] = (uint8_t) (transaction->holding_registers[0] >> 8);
+				buffer[5] = (uint8_t) transaction->holding_registers[0];
+				*msg_len = 8;
+				break;
+		}
 	}
 	crc16 = modbus_CRC16(buffer, *msg_len - 2); /* last two bytes is the checksum itself */
 	buffer[*msg_len - 2] = crc16 & 0xff;