> ## Documentation Index > Fetch the complete documentation index at: https://docs.oryx.mechatronstudio.com/llms.txt > Use this file to discover all available pages before exploring further. # Modbus > RTU, ASCII and TCP, with the function codes and exceptions translated. Modbus builds on the UART reading: **every byte is one character on the serial line**. That is why it has serial settings of its own, independent from UART: it is not unusual to read a loose line at 8N1 and a Modbus bus at 8E1 at the same time. ## Settings | Setting | Options | Default | | --------------- | --------------------------------- | ------- | | **Mode** | RTU, ASCII, TCP | RTU | | **Direction** | Auto, Request, Response | Auto | | **Serial line** | The same character format as UART | 8E1 | ### The three modes > In **RTU** frames are split by a 3.5-character silence; in **ASCII** by `:` > and CR LF; in **TCP** by the length the MBAP header declares. For **Modbus TCP** what is read is the framing: the MBAP header and the PDU. The layers below —TCP, IP, Ethernet— are not drawn: the bytes are written as serial characters. ### The direction The data layout depends on the side of the conversation. In **Auto** it is inferred from the frame length and the previous request; it is worth forcing it to draw a single frame on its own. ## Fields | Field | What it is | | -------------------- | -------------------------------------------------------------------------------------------------- | | **Slave address** | Which device the master is talking to. Address 0 is broadcast: everyone listens and nobody answers | | **Function** | What is being asked. With bit `0x80` set, it is an exception response | | **Starting address** | The first register or coil of the request | | **Quantity** | How many registers or coils it covers | | **Value** | What gets written | | **AND / OR mask** | In the masked write | | **CRC / LRC** | The check, depending on the mode | In TCP the MBAP header fields are added: | Field | What it is | | -------------------------- | --------------------------------------------------------------------- | | **Transaction identifier** | Pairs the response with its request | | **Protocol identifier** | Always 0 for Modbus | | **Length** | How many bytes follow, the unit byte included | | **Unit identifier** | Which device behind the gateway; 255 is the one on the network itself | ## Function codes it recognizes | Code | Function | | ---- | -------------------------------- | | `01` | Read coils | | `02` | Read discrete inputs | | `03` | Read holding registers | | `04` | Read input registers | | `05` | Write single coil | | `06` | Write single register | | `07` | Read exception status | | `08` | Diagnostics | | `0B` | Get comm event counter | | `0C` | Get comm event log | | `0F` | Write multiple coils | | `10` | Write multiple registers | | `11` | Report server ID | | `14` | Read file record | | `15` | Write file record | | `16` | Mask write register | | `17` | Read/write multiple registers | | `18` | Read FIFO queue | | `2B` | Encapsulated interface transport | A code outside the list is still shown, as *Function N*. ## Exceptions | Code | Exception | | ---- | ---------------------------------- | | `01` | Illegal function | | `02` | Illegal data address | | `03` | Illegal data value | | `04` | Server device failure | | `05` | Acknowledged, it will take a while | | `06` | Server device busy | | `08` | Memory parity error | | `0A` | Gateway path unavailable | | `0B` | Gateway target failed to respond | ## Errors it detects | Label | What happened | | ---------------------------- | -------------------------------------------------------- | | **Short frame** | Not enough to make a frame | | **Wrong length** | The frame is not the size it should be for that function | | **CRC error** | The RTU check does not match | | **LRC error** | The ASCII check does not match | | **Unknown function** | The function code is not in the list | | **Exception** | It is an exception response | | **Silence inside the frame** | There is a gap in the line within a single frame | | **Line error** | The serial character reading failed (framing or parity) | | **Quantity out of range** | The requested quantity is not valid for that function | | **Wrong byte count** | The count byte does not match the data that follows | | **Coil value** | The value written to a coil is not valid | | **Reserved address** | The slave address is outside the allowed range | | **No CR LF** | The ASCII frame does not close with CR LF | | **Not a hex digit** | In ASCII, a character that is not a hex digit | | **Protocol not 0** | The MBAP protocol identifier is not 0 | | **MBAP length** | The declared length does not match the frame | | **Outside a frame** | Bytes that belong to no frame | ## The summary Each frame is summed up with the device and the function, and an arrow giving the direction: * `0x01 → Read holding registers` for a request * `0x01 ← Read holding registers` for the response * `0x01 ✗ Read holding registers` for an exception * `Broadcast → Write multiple coils` for address 0 * `Incomplete frame` or `Outside a frame` when it does not add up At the end, a `✓` or a `✗` depending on the check. ## How to start With nothing drawn, the status box explains: > *No frame yet. Every byte is one character on the serial line: in RTU and > ASCII a frame starts with the slave address, and in TCP with the MBAP header.*