SEI Absolute Encoder Communications Protocol
SEI Absolute Encoder Communications Protocol Data Sheet
Each encoder on the bus is assigned a unique address between 0 and E. The host computer is the master of the bus, while the encoders are slaves. Encoders cannot initiate communication, rather the host sends commands, and the encoders respond. The first byte of the command always includes the address of the encoder to be selected. Address F is used to communicate to all devices on the bus at once. An encoder responds by activating the busy line if a valid command is received with the correct address. If the address is incorrect or the command is invalid (including framing errors), the encoder ignores the command. If the address is F, all encoders activate the busy line until the command is processed by everyone (wired-OR). If an encoder has the busy line active while processing a command, other encoders ignore all data.
In a single-byte command, the host only sends one byte, which contains the address and the command. The selected encoder activates the busy line, sends the response, and then releases the busy line. In the case of a multiple-byte command, the host first sends a byte that contains the address and an indication that a multiple-byte command is in process. The selected encoder activates the busy line to acknowledge the selection. The host sends the rest of the command, and the busy line remains active until the particular command is completed. If factory default modes don't need to be changed in the simplest single encoder configuration, only single-byte commands are needed. 16-bit and 32-bit numbers are transmitted with the most significant byte first (big-endian convention).
Single-Byte User Commands:
Request Command:
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| cmd3 | cmd2 | cmd1 | cmd0 | addr3 | addr2 | addr1 | addr0 |
Addr3-0:
Address of encoder to be selected (0 to E). Transmitting an address between 0 and E inclusive causes a single encoder to respond; address F selects all encoders on the bus, and when used in a command, it will cause every encoder on the bus to respond to the transmitted command simultaneously (it does not cause the encoders to serialize their responses). If the host sends a command that returns data with an address of F over an SEI network with multiple encoders, the data received by the host from the encoders will likely be garbled because all of the encoders will attempt to send the requested data at the same time. Suppose the primary purpose of a command is to transmit parameters (such as setting resolution, see below). In that case, the operation will succeed on multiple encoders when address F is used. However, the multiple encoders will still attempt to send back their checksum bytes simultaneously, causing the checksum to be garbled when the host receives it. This may or may not be a problem, depending on the application (the application can choose to ignore the garbled checksum and assume that the operation was successful).
Here are some examples involving the use of address F. In the first example, it makes no sense to read the resolution from multiple encoders using address F; the encoders will all send data at slightly differing times, and it will appear to the host that only one encoder is responding (and responding with garbled data). On the other hand, it is possible to set the resolution on all encoders on a network with address F; all the encoders will set their resolution to the requested value, but the checksum received by the host will likely be garbled because they will all send their responses at slightly different times.
In practical terms, the only commands that should be used with address F in a multi-encoder network are the strobe command, the check serial number command, the sleep command, and the wakeup command. If there is only one encoder on the bus, address F can be used for all operations.
| cmd3-cmd0 | request type |
| 0000 | (reserved for control codes) |
| 0001 | position |
| 0010 | position + 1-byte status |
| 0011 | position + 2 bytes time + status |
| 0100 | strobe |
| 0101 | sleep |
| 0110 | wakeup |
| 0111 | reserved |
| 1000 | reserved |
| 1001 | reserved |
| 1010 | reserved |
| 1011 | reserved |
| 1100 | reserved |
| 1101 | reserved |
| 1110 | reserved |
| 1111 | (multiple byte command) |
Position:
The encoder sends its current position. The format is either 1 or 2 bytes in single-turn mode, 4 bytes in multi-turn mode, most significant byte first (see "Change Mode Command" for details).
Time:
The encoder sends the 2-byte value of its free-running counter at the time of the position reading - the most significant byte first. The hardware counter in encoders with version 3 firmware increments at 1.843MHz (1 count per 0.54 usec), while the counter in encoders with version 4 firmware increments at 7.373 MHz (0.136 usec per count).
Strobe:
An SEI absolute encoder runs in one of two modes: free-running (also referred to as asynchronous mode) or strobe mode. The choice of which mode is active depends on bit "stb" in the Mode byte (the Mode byte bits are described later in this document under the "Change Mode" multi-byte command). When the "stb" mode bit is cleared to zero in the mode byte, the encoder is free-running, and the single byte strobe command (described in the table above, command nibble equal to 0100 binary) has no effect. In free-running mode, the encoder continuously cycles through the computation of position; when queried, the position value returned is the result from the last valid computation, and there is no correlation between the acquisition time of different encoders. The cycle time for the computation is 7 msec for firmware version 4 and up, while the cycle time for firmware version 3 is 4 msec. Thus a synchronous read of position in free-running mode will return a result that is up to one cycle time old (7 or 4 msec, depending on firmware version).
When the strobe mode bit "stb" in the user mode byte equals 1, the encoder operates in strobe mode: the encoder enters a waiting state, waiting for a single byte strobe command to be received before computing position. Thus, when multiple encoders are in strobe mode, they are all waiting for the single-byte strobe command, and they will all start computing position synchronously after all of them receive the single-byte strobe command (the strobe command should be sent with address F so that all the encoders are addressed simultaneously). The host must still wait at least one cycle time (7 or 4 msec) after issuing a strobe command in order to allow the encoder to finish the computation before attempting to read position (otherwise, the position read during the computation will be from a previous computation). Once the encoder finishes the computation, it will go back to waiting for the next strobe command; during this period, the host can read out position from multiple encoders in separate position queries to specific addresses, and the data that will be returned is all from the same instant that the last strobe command was issued to all the encoders.
Free-running mode is the default mode as sent from the factory, and in free-running mode, the single-byte strobe command described in the table above has no effect. The user-mode byte can be programmed to power up in strobe mode using the "Change Default Mode" multi-byte command described later in this document.
Sleep:
The encoder goes to sleep (without responding) after the command is executed, and the current consumption drops below 2 mA. If there are multiple encoders on the bus, use address F to put them all asleep at once. Any activity on the data bus wakes up all encoders (use the wakeup command and address F to wake them synchronously).
Wakeup:
However, this command wakes up all encoders on the bus; the encoders will not send any response to this command. Wait at least 5 msec after this command to send the next command.
Status:
The encoder sends 1-byte status as follows:
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| err3 | err2 | err1 | err0 | sum3 | sum2 | sum1 | sum0 |
| err3-err0 | meaning |
Corresponding SEI Error Code |
| 0000 | no error | n/A |
| 0001 | not enough light | 28101 |
| 0010 | too much light | 28102 |
| 0011 | misalignment or dust | 28103 |
| 0100 | misalignment or dust | 28104 |
| 0101 | misalignment or dust | 28105 |
| 0110 | hardware problem | 28106 |
| 0111 | EEPROM error | 28107 |
| 1000 | multiturn pos. not initialized | 28108 |
Sum3-0
Exclusive OR of all 4-bit nibbles of the request byte received and data bytes returned by the encoder, excluding the status byte. This is a good check of data integrity, especially in a bus with multiple encoders.
Note: In multi-turn mode, the error 1000 binary (0x8 in the upper hex nibble) is sent until a "set origin" or a "set position" command is received. This allows the host to know a reset or power loss has occurred, and the multi-turn position is no longer valid.
Error codes between 1001 and 1111 indicate that a "multiturn position not initialized" error occured in addition to an error in the range 0001 to 0111
Multiple Byte Commands:
All multiple byte commands start with the request byte F0+addr; after receiving this byte, the addressed encoder will acknowledge by activating the busy line. After the acknowledgment, the encoder is ready to receive the rest of the command. Some commands may require the use of address F to select all encoders on the bus (like check serial number, baud rate change, etc...). When addressing multiple encoders, after the first busy is received, wait for 5 msec to send the rest of the command to make sure they are ready. When the command is completed, the encoder sends a checksum byte and then releases the busy line. If the command is invalid or failed, the encoder releases the busy line without sending a checksum. The checksum byte is the exclusive OR of the request byte, command bytes, and bytes returned by the encoder.
Set Origin Command:
2 bytes: request byte, 01
Returns checksum if the command is successful.
Sets the absolute 0 at the current position.
In single-turn mode, the new origin is stored in EEPROM; therefore, it will be effective after resets and power-downs until a "Set Origin" or a "Set Absolute Position" command is received.
In multi-turn mode, the 32-bit counter is reset but not stored in EEPROM. This is effective until a reset occurs or a "Set Origin" or a "Set Absolute Position" command is received.
Set Absolute Position Command:
Single-turn: 4 bytes: request byte, 02, pos MS byte, pos LS byte.
Multi-turn: 6 bytes: request byte, 02, 4 bytes pos (MS to LS).
Returns checksum if the command is successful.
Sets the given absolute position (at the current resolution) at the current position.
In single-turn mode, the new origin is stored in EEPROM; therefore, it will be effective after resets and power-downs until a "Set Origin" or a "Set Absolute Position" command is received.
In multi-turn mode, the 32-bit counter is set but not stored in EEPROM. This is effective until a reset occurs or a "Set Origin" or a "Set Absolute Position" command is received.
Read Serial Number:
2 bytes: request byte, 03
Returns 5 bytes: 4 bytes serial number and checksum if the command is successful.
Check Serial Number:
10 bytes: request byte, 04, 4 bytes serial number, 4 bytes mask.
Returns nothing.
The encoder does a logical AND of its serial number with the mask supplied; the result is compared to the serial number supplied. If they match, the busy line is held active until another byte is received. Otherwise, the busy line is released. This command is used to determine if an encoder with a particular serial number is present on the bus.
Fail Serial Number:
10 bytes: request byte, 05, 4 bytes serial number, 4 bytes mask.
Returns nothing.
The encoder does a logical AND of its serial number with the mask supplied; the result is compared to the serial number supplied. If they don't match, the busy line is held active until another byte is received. If they match the busy line is released. This is useful to determine if an encoder, whose serial number is known, is the only one on the bus.
Get Address:
6 bytes: request byte, 06, 4 bytes serial number.
Returns 2 bytes: 1-byte address and checksum if the command is successful, only if serial number matches.
The encoder compares its serial number with the one supplied; if they match, it returns its address (0 to E). Otherwise, it returns nothing.
Assign Address:
7 bytes: request byte, 07, 4 bytes serial number, 1-byte address.
Returns checksum if the command is successful.
The encoder compares its serial number with the one supplied; if they match, it assigns itself the address supplied (must be between 0 and E). The new address is stored in EEPROM; therefore, it will be effective after resets and power-downs.
Read Factory Info:
2 bytes: request byte, 08
Returns 15 bytes: 2 bytes model number, 2 bytes version, 2 bytes configuration, 4 bytes serial number, month, day, 2 bytes year, and checksum if the command is successful.
Read Resolution Command:
2 bytes: request byte, 09
Returns 3 bytes: resolution MS byte, resolution LS byte, and checksum if the command is successful.
A zero value means 16-bit resolution.
Change Resolution Command:
4 bytes: request byte, 0A, resolution MS byte, resolution LS byte.
Returns checksum if the command is successful.
The resolution can be any number between 0 and FFFF; 0 is for the full 16-bit position. However, the accuracy is only guaranteed to 12 bits. The new resolution is stored in EEPROM; therefore, it will be effective after resets and power-downs.
Read Mode Command:
2 bytes: request byte, 0B
Returns 2 bytes: mode and checksum if the command is successful.
Change Mode Command (temporary):
3 bytes: request byte, 0C, mode.
Returns checksum if the command is successful.
The mode is changed temporarily and will be effective until the encoder is reset, power down, or another mode change command is received. It is not stored in the EEPROM. Mode byte as follows:
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| reserved | /256 | reserved | incr | size | multi | stb | rev |
rev:
rev = 1, the position increases counterclockwise.
rev = 0, the position increases clockwise.
stb:
stb = 1, the encoder operates in strobe mode: it waits for a strobe request before computing the position; this mode is used to synchronize multiple encoders. After entering this mode, wait at least one computation cycle time before sending the first strobe command.
stb = 0, the encoder operates in asynchronous mode: the encoder cycles through computing position continuously and sends the most current valid position when requested. The data can be from 0 to one cycle time old (version 3 firmware cycle time is 4 msec, and version 4 firmware cycle time is 7 msec).
multi:
multi = 1, multi-turn mode: a 32-bit counter keeps track of the position (it increases or decreases over multiple turns, i.e., 3 1/2 turns at a resolution of 100 would be 350). This counter is cleared at reset or power-up.
multi = 0, single-turn mode: position is between zero and the max resolution, according to the shaft angle.
Note: in older versions (V1.X), this bit indicated a fast mode (3 msec update rate) with 9-bit accuracy.
Also, any other command besides position inquires can corrupt the multi-turn position.
size:
only effective in single-turn mode:
size = 1: the encoder always sends the position in 2 bytes, even if the resolution is 256 decimal or less.
size = 0: the position is sent as 1 byte if the resolution is up to 256 decimals or as 2 bytes if above 256 decimals.
In multi-turn mode, the position is always 4 bytes, and this bit is ignored.
incr:
only effective in multi-turn mode:
incr = 1: the encoder sends the position change since the last request as a 32 bit signed number.
incr = 0: the encoder sends the position as a 32 bit signed number.
/256:
only available for analog version
only effective in multi-turn mode:
/256 = 1: the encoder position is divided by 256.
/256 = 0: the encoder position is normal.
Change Power Up Mode Command:
3 bytes: request byte, 0D, mode.
Returns checksum if the command is successful.
Same as "Change Mode Command" described above, except the mode is stored in EEPROM; therefore, it will be effective across resets and power cycles.
Reset Command:
2 bytes: request byte, 0E
Returns checksum if the command is successful.
After releasing the busy line, the encoder does a software reset (the baud rate returns to 9600 after the checksum byte is sent). The encoder requires 35 msec after reset to be ready to receive new commands. The other ways to reset are: turning the power off or sending a break condition (dataL low, dataH high) for at least 1 second. Note that the reset threshold of the power input is between 4.5 and 5.5 volts. The EEPROM parameters are not affected, but any temporary mode byte changes or baud rate changes are lost.
User Diagnostic Commands:
Loopback Mode:
2 bytes: request byte, 10
Returns no checksum.
After receiving this command, the encoder returns every byte it receives. This is useful to test the SEI bus. The busy line remains active during this test. The test is aborted if a delay of 350msec or more occurs. (rev 1.01 firmware was 35 msec). The host must release the data lines after each byte sent to allow the encoder to respond.
Offline Command:
2 bytes: request byte, 11
Returns checksum if the command is successful.
After releasing the busy line, the encoder does not respond to any command. It must receive a break condition (dataL low, dataH high) for at least 1 second or power down and up again to place it back online. We do not recommend using this command. Not all firmware revisions support this command.
Change Baud Rate Command (temporary):
3 bytes: request byte, 0F, baud rate byte
Returns checksum if the command is successful.
The rate will be changed as follows and will be effective after the checksum is sent until the encoder is reset or another baud rate change command is received. At reset, the baud rate always defaults to 9600. Note: If multiple encoders are on the same bus, use address F to set them all to the same baud rate.
| Baud Rate Byte | 0x00 | 0x01 | 0x10 | 0x11 | 0x12 | 0x13 | 0x14 | 0x15 |
| Baud Rate | 115200 | 57600 | 38400 | 19200 | 9600 | 4800 | 2400 | 1200 |