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Serial line: RTU and ASCII

Serial-line Modbus adds an address and an error check to the PDU and sends it between devices connected by RS-232 or RS-485. There are two transmission modes, RTU and ASCII, and every device on one bus must use the same mode and the same line settings. This page is based on sections 2 (data link layer) and 3 (physical layer) of the MODBUS over Serial Line Specification and Implementation Guide V1.02.

Clients, servers and addresses

Section titled “Clients, servers and addresses”

The serial specification calls the client the master and the server the slave. A bus has exactly one client and up to 247 servers (§2.1).

Address Use (§2.3)
0 Broadcast. Every server processes the request and none responds.
1–247 Individual server addresses
248–255 Reserved

A client sends requests in one of two ways (§2.2):

  • Unicast: to one server at address 1–247. The client waits for the response timeout and treats a missing response as an error.
  • Broadcast: to address 0, for write requests only. Because nobody answers, the client waits a turnaround delay that gives the servers time to process the request, then sends the next one.

Character format

Section titled “Character format”

A serial line sends bytes as characters, each framed by a start bit, data bits, a parity bit and stop bits.

Mode Data bits Default format Allowed formats Bits per character
RTU (§2.5.1) 8 8E1 (even parity, 1 stop bit) 8O1, 8N2 11
ASCII (§2.5.2) 7 7E1 7O1, 7N2 10
  • Even parity is the default.
  • Without parity, two stop bits keep the character length the same. PDComm rejects no parity with one stop bit (8N1, 7N1) as Invalid endpoint configuration. If a device is set to 8N1, change it to 8E1 (recommended) or 8N2.
  • Section 3.2 requires 9600 and 19200 bit/s and makes 19200 bit/s the default.

PDComm selects 8 data bits when transport is rtu and 7 when it’s ascii. You set parity and stop_bits in the configuration.

RTU frames

Section titled “RTU frames”

RTU sends bytes as they are. A frame is a one-byte address, the PDU and a two-byte CRC.

+-----------+---------------------+---------+---------+
| address 1B| PDU                 | CRC low | CRC high|
+-----------+---------------------+---------+---------+
  01          03 00 00 00 0A        C5        CD

Character timing: t1.5 and t3.5

Section titled “Character timing: t1.5 and t3.5”

RTU has no start or end characters; silence separates frames (§2.5.1.1).

  • t3.5: frames are separated by at least 3.5 character times of silence.
  • t1.5: if the gap between two characters inside a frame exceeds 1.5 character times, the frame is incomplete and discarded.
  • Above 19200 bit/s, t1.5 is fixed at 750 µs and t3.5 at 1.750 ms.
Rate and format Character time t1.5 t3.5
9600 bit/s, 8E1 1.146 ms 1.719 ms 4.011 ms
19200 bit/s, 8E1 0.573 ms 0.860 ms 2.006 ms
38400 bit/s and above 0.750 ms 1.750 ms

On an RS-485 multidrop bus every server receives every frame. Each server delimits frames by t3.5 silence and answers only requests for its own address. A broadcast write to address 0 is applied by every server, and none of them replies.

USB serial adapters and operating system drivers hand received bytes to applications in batches; an FTDI adapter’s default latency timer is 16 ms. Gaps you observe on a PC are therefore not the t1.5 and t3.5 of the line. PDComm handles this as follows:

  • A client completes a response by the length its function code defines and by its CRC, so a late batch doesn’t make it drop the response.
  • Before sending a request, the client discards leftover input, and it keeps at least t3.5 between frames.
  • Serial entries in trace have timestamp_scope host_io_batch: the time the PC finished a read, not UART bit times.

CRC

Section titled “CRC”

RTU uses CRC-16 (§2.5.1.2), computed over the address and the PDU with the initial value 0xFFFF and the polynomial 0xA001 (0x8005 bit-reversed), and appended low byte first. The CRC of 01 03 00 00 00 0A is 0xCDC5, sent as C5 CD. A receiver drops a frame with a wrong CRC and doesn’t answer, so the client sees a timeout.

ASCII frames

Section titled “ASCII frames”

ASCII sends every byte as two hexadecimal characters (§2.5.2). A frame starts with : (0x3A) and ends with CR LF. Hex digits are the uppercase characters 0–9 and A–F.

:  01  03  00  00  00  0A  F2  CR LF
|  |   |                   |   +-- end (function 08/03 can change the final character)
|  |   +- PDU              +------ LRC
|  +----- address
+-------- start
  • LRC (§2.5.2.2): the two’s complement of the 8-bit sum of the address and PDU bytes. 01+03+00+00+00+0A = 0x0E, so the LRC is 0xF2.
  • Gaps between characters: ASCII allows up to one second between characters. PDComm’s ascii_timeout_ms (default 1000) is that limit; beyond it, the partial frame is discarded.
  • The same PDU takes more than twice as many characters as in RTU. ASCII trades efficiency for readability and relaxed timing.

Physical layer

Section titled “Physical layer”

Section 3 is mostly about RS-485:

Interface Description
RS-485 two-wire Several devices share one pair (D0, D1) plus a common line. It’s half duplex, so the transmitting device switches its driver direction.
RS-485/RS-422 four-wire Separate transmit and receive pairs. Optional.
RS-232 Short point-to-point links. Optional.
  • Terminate both ends of an RS-485 bus, and bias (polarize) the idle line at one point. Follow the device and adapter instructions for values and placement.
  • An RS-232 adapter can’t talk to an RS-485 port directly; you need a converter.
  • Check the installed wiring with the real devices connected.

Configure serial ports walks through connecting a PC to a device.

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