
Rotel RS232 Serial Bus Communication
1. 1 Byte Waveform format
5V
START
STOP
0V
2. Bus Status Test
2~10ms
MessageMessage
A. Test starts to see if line is empty for 2~10msec before transmitting new
message.
B. Line test time is fixed in 2~10msec randomly.

3. Message Structure
META Encoding
COUNT
START IDCOUNT TYPE CHKSUMDATA
CHKSUM
Start: 0xFE
• Whenever 0xFE is received this indicates the start of a new message. The 
start code is always 0xFE.
Count
• This byte indicates the total number of bytes from the ID byte to the Data 
byte(s).
ID
• The device ID byte of the model being controlled. The protocol for each 
model will specify the correct ID.
Type
• The message type - 0x10 for Command (Transmit) messages, and 0x20 for 
feedback status (Receive) messages.
Data
• The data or commands to transmit.
Checksum
• The total value of all bytes except Start. Calculate by adding each byte. In 
the case that the total is larger than 2 digits, drop the most significant bit. 
...
Example 1: Transmit command Volume Up to RSP 1098
0xFE 0x03 0xA0 0x10 0x0B 0xBE
Start: 0xFE Always 0xFE 
Count: 0x03 Indicates 3 bytes to follow - (ID, Type, 1 Data byte) 
ID: 0xA0 ID byte for RSP 1098 
Type: 0x10 Indicates command type 
Data: 0x0B Data value for Volume Up 
Chk: 0xBE Checksum - calculate 0x03 + 0xA0 + 0x10 + 0x0B = 0xBE

Example 2: Transmit command Power On to RSX 1055
0xFE 0x03 0xC3 0x10 0x4B 0x21
Start: 0xFE Always 0xFE 
Count: 0x03 Indicates 3 bytes to follow - (ID, Type, 1 Data byte) 
ID: 0xC3 ID byte for RSX 1055 
Type: 0x10 Indicates command type 
Data: 0x4B Data value for Power On 
Chk: 0x21 Checksum - calculate 0x03 + 0xC3 + 0x10 + 0x4B = 0x121
The most significant bit of 1 is dropped leaving 0x21.
4. Meta Encoding
To keep the device from encountering the start byte 0xFE in any position other than as 
the start byte, any occurrence of the bytes 0xFE or 0xFD in the command string must be 
converted to either 0xFD 0x00 (for 0xFD) or 0xFD 0x01 (for 0xFE). This will allow the 
string to pass while masking any occurrence of the byte 0xFE except as the start byte. 
Example 2: Transmit command Power On to RSP 1098
0xFE 0x03 0xA0 0x10 0x4B 0xFE
In order for this command to function, the string must be meta encoded to 
instead look like this:
0xFE 0x03 0xA0 0x10 0x4B 0xFD 0x01
Start: 0xFE Always 0xFE 
Count: 0x03 Indicates 3 bytes to follow - (ID, Type, 1 Data byte) 
Note the count byte does not change to reflect the extra
byte from Meta Encoding. 
ID: 0xA0 ID byte for RSP 1098 
Type: 0x10 Indicates command type 
Data: 0x4B Data value for Power On 
Chk: 0xFE Checksum - calculate 0x03 + 0xA0 + 0x10 + 0x4B = 0xFE
This is then changed to 0xFD 0x01 to avoid sending the
byte 0xFE to the device.
Note: When sending data, send up to 16 bytes maximum per string. Any larger 
strings should be broken up into 2 strings when transmitting.

RT 1080 RS232 Communication Session Example
Figure 1: RT 1080 DISPLAY
Message sent to RT 1080 “Frequency Down”
Transmitting a frequency down command:
RT 1080 RS232 Controller
|  | 
|---------------- 6 bytes sent from controller  ---------Æ| 
|  | 
|  | 
|←-------------------RT 1080 feedback---------------------| 
|  |
Byte number 
and value (HEX) Description
Byte 1: FE Start Code - Always the same 
Byte 2: 03 Count Byte - Amount of bytes to follow 
Byte 3: 21 Device ID - RT 1080 always 21 
Byte 4: 10 Message Type - Always 10 for command 
Byte 5: 18 Command Key - In this case frequency down 
Byte 6: 4C Checksum of all bytes except start code byte
RT 1080 feedback to RS232 port:
Each feedback is a result of a request by either an RS232 command sent or by front panel or 
remote selections selected. In each case the controller will receive 21 bytes, always starting a 
new message with “FE”.

Breakdown of message 1:
Byte Value Description
Byte 1: FE Start code 
Byte 2: 12 Byte count of this message 
Byte 3: 21 Device ID - 21 for RT 1080 
Byte 4: 20 Message Type - Always 20 for receiving a message 
Byte 5: 00 Flag word 1 - Empty no bit set 
Byte 6: 00 Flag word 2 - Empty no bit set 
Byte 7: 34 Flag word 3 - Bit 3 A, Bit 4 Ant, Bit 5 FM are set 
Byte 8: 00 Flag word 4 - Empty nothing set 
Byte 9: 40 Flag word 5 - One bit set 
Byte 10: 20 Display 1st digit space 
Byte 11: 39 Display 2nd digit Symbol 9 
Byte 12: 30 Display 3rd digit Symbol 0 
Byte 13: 2E Display 4th digit Symbol . 
Byte 14: 31 Display 5th digit Symbol 1 
Byte 15: 30 Display 6th digit Symbol 0 
Byte 16: 4D Display 7th digit Symbol M 
Byte 17: 48 Display 8th digit Symbol H 
Byte 18: 1E Display 9th digit Symbol RS 
Byte 19: 20 Display 10th digit Symbol “space” 
Byte 20: 32 Display 11th digit Symbol 2 
Byte 21: E4 Checksum of message excluding start code byte
Byte 1 - Start - Always 0xFE
Byte 2 - Count
Byte 3 - ID - Unique ID for each model
Byte 4 - Type - Always 0x20 for feedback
Bytes 5-9 - Flag Words - Each flag word contains 8 bits which will be turned on or off to 
indicate the currently displayed items on the display of the unit.
Bytes 10-20 - Display characters - Each byte contains the hex equivalent value of an 
ASCII character that is on the display of the unit.
Byte 21 - Checksum - Total value of all bytes excluding the Start byte.