1 physical layer, 2 device addressing, 3 session layer – Despatch P1400+ Temperature Controller User Manual

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SECTION 11: ASCII COMMUNICATIONS

This is simple ASCII protocol provides backwards compatibility with some older products. ASCII is
not available in all models in the range. The Modbus protocol is recommended for future use.
Refer to the relevant Model Group Section for the ASCII and Modbus Application Layer (parameter
address/ident information).

11.1 Physical Layer

The Base address, bit rate and character format are configured via the front panel in Configuration
Mode or by using the PC Configurator software.

Physical layer configuration settings possible are:

Data rate:

1200, 2400, 4800 (default), 9600 and 19,200 bps

Parity:

Even

Character format:

7 bits per character. + 1 stop bit.

The transmitter must not start transmission until 3 character times have elapsed since reception of
the last character in a message, and must release the transmission line within 3 character times of
the last character in a message.

Note:

Three character times = 1.5ms at 19200, 3ms at 9600, 6ms at 4800, 12ms at 2400 and 24ms
at 1200 bps.

11.2 Device Addressing

The instrument is assigned a device address by the user using the

Addr

parameter in Configuration

Mode. The address may be set to any unique value from 1 (default) to 99. This address is used to
recognise ASCII messages intended for this instrument. The instrument does not respond to
messages that do not match the address that has been assigned to it.

11.3 Session Layer

The ASCII protocol assumes half duplex communications. The master device initiates all
communication. The master sends a command or query to the addressed slave instrument and the
slave replies with an acknowledgement of the command or the reply to the query.

Messages from the master device may be one of five types:

Type 1:

{S}{N}??*

Type 2:

{S}{N}{P}{C}* or R{N}{P}{C}*

Type 3:

{S}{N}{P}#{DATA}* or R{N}{P}#{DATA}*

Type 4:

{S}{N}{P}I* or R{N}{P}I*

Type 5:

{S} {N} \ P S S ? *

All characters are in ASCII code. See the following

Parameter Key table

for details of the

parameters in brackets { }.