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Commander SE Advanced User Guide

Issue Number: 4

1.3 ANSI Protocol Description

Data is transmitted at a fixed speed or baud rate in the form of a character. A character is comprised of seven bits, and the
baud rate represents the data transmission rate in bits/second.
In order for a data receiver to recognise valid data, a frame is placed around each character. This frame contains a start bit, a
stop bit, and a parity bit. Without this frame, the receiver would be unable to synchronise itself with the transmitted data, as
the data is asynchronous.
A frame is shown here:

The frame consists of 10 bits:
•

1 start bit, followed by

•

7 data bits (starting with the least significant bit (i.e. lsb - bit 0) and ending with the most significant bit (i.e. msb - bit 6)),
followed by

•

1 even parity bit, followed by

•

1 stop bit

The parity bit is used by the data receiver to check the integrity of the 7 data bits it has received.
The 7 data bits are called a character and comprise the low ASCII set. The ASCII character set comprises 128 characters
decimally numbered from 0 to 127. The first 32 (0 to 31) characters in the ASCII set (hexadecimal 00H to 01FH) are used to
represent special control codes. Each control code has a particular meaning (e.g. ASCII character 02 H is called 'STX' the
'start of text').
A message consists of a group of frames or characters. These characters consist of the following types which are used to
construct the different types of messages:
•

Control characters

•

Address characters

•

Parameter characters

•

Data characters

•

Block Checksum character

These character types are described below.

Control characters

Commands and requests are sent to the drive in the form of a set of characters, including control characters as a message
packet. Each message is started with a special control character, and may contain further control characters. A list of all the
relevant control characters in the ANSI protocol used when sending a message, and receiving is as follows:

Address characters

Each drive on an ANSI communications serial link must be given a unique identity or address so that only one target drive will
respond to a transmitted message. This serial address comprises two parts:
•

The Group Address which is the first digit of the 2 digit address, and

•

The Unit Address which is the second digit.

Both the group address and unit address have a range of 1 to 9. A group or unit address of 0 is not allowed (i.e. addresses
01, 10, 20, etc. are invalid). The reason for this is that drives can be grouped together (up to 9 units per group), and a
particular message with an address containing 0 can be sent to all units of the particular group. To address a particular group,
the unit address of zero (0) is used, for example, to address all units of group 6 the full address is 60.
An additional feature of the ANSI protocol is that a message can be sent to all units of all groups simultaneously using the
address 00. For example, the address 00 can be used to send a frequency/speed command to all the drives which are
mechanically coupled together driving a conveyor. All the drives will then change frequency/speed simultaneously.

ASCII 7-bit character (Low ASCII set)

Least significant Hexadecimal digit

Most significant Hexadecimal digit

(3 bits only)

Start bit

Seven data bits

Even

Parity bit

Stop bit

bit 0
(lsb)

bit 1

bit 2