Editing Modbus (section)

=== Modbus ASCII ===
Modbus ASCII makes use of [[ASCII]] characters for protocol communication. The ASCII format uses a [[longitudinal redundancy check]] checksum. Modbus ASCII messages are framed by a leading colon (":") and trailing newline (CR/LF).

A Modbus ASCII frame includes:{{Sfn|MODBUS over Serial Line protocol|2006|p=17|ps=, "2.5.2.1 MODBUS Message ASCII Framing"}}
{| class="wikitable" style="float:middle; margin:0 0 1em 1em;"
|-
! Name
! Length (bytes)
! Function
|-
! Start
| 1
| Colon <code>:</code> ([[ASCII]] value 3A<sub>16</sub>)
|-
! Address
| 2
| Station address
|-
! Function
| 2
| Indicates the function code e.g. "read coils"
|-
! Data
|''n'' × 2
| Data + length will be filled depending on the message type
|-
! LRC
| 2
|[[Checksum]] ([[longitudinal redundancy check]])
|-
! End
| 2
| Carriage return + line feed (CR/LF) pair (ASCII values 0D<sub>16</sub> and 0A<sub>16</sub>)
|}
Address, Function, Data, and LRC are ASCII hexadecimal encoded values, whereby 8-bit values (0–255) are encoded as two human-readable ASCII characters from the ranges 0–9 and A–F. For example, a value of 122 (7A<sub>16</sub>) is encoded as two ASCII characters, "7" and "A", and transmitted as two bytes, <code>55</code> (37<sub>16</sub>, ASCII value for "7") and <code>65</code> (41<sub>16</sub>, ASCII value for "A").

LRC is calculated as the sum of 8-bit values (excluding the start and end characters), negated ([[two's complement]]) and encoded as an 8-bit value. For example, if Address, Function, and Data are 247, 3, 19, 137, 0, and 10, the two's complement of their sum (416) is −416; this trimmed to 8&nbsp;bits is 96 (256 × 2 − 416 = 60<sub>16</sub>), giving the following 17 ASCII character frame: <code>:F7031389000A60␍␊</code>. LRC is specified for use only as a checksum: because it is calculated on the encoded data rather than the transmitted characters, its 'longitudinal' characteristic is not available for use with parity bits to locate single-bit errors.