DigitAx User Guide
Issue code: dgxu4
13-2
Data format
The data field format is selected using parameter
b51.
7-bit plus parity (bit 51 = 0)
One start-bit
Seven data bits
One even parity bit
One stop bit
8-bit, no parity (bit 51 = 1)
One start bit
Eight data bits
One stop bit
13.5 Message structure
Serial messages consist of the following:
Control characters
Serial address code
Parameter identifier
Data field
Block checksum (BCC)
Control characters
If a message is initiated from a keyboard, control
characters may be entered by holding down the
CTRL key and pressing the key shown in the table
below.
Character Purpose ASCII
code
(HEX)
CTRL +
key
EOT Reset
Message begins
04 D
ENQ Enquiry
Interrogating the Drive
05 E
STX Start of text 02 B
ETX End of text 03 C
ACK Acknowledge
(go to next parameter)
06 F
BS Backspace
(go to previous
parameter)
08 H
NAK Negative acknowledge
(repeat same
parameter)
15 U
CR Carriage return 0D M
Serial address code
Each Drive on a serial link must have a unique
address. This allows the host controller to
communicate with each Drive individually. The
Drive address code is entered in Pr22, and can be
any value between 1 and 32. (Drive address 00 has a
special use, see below) Each digit of the address is
transmitted twice, so to address Drive 08, the
following must be transmitted:
Drive address
0088
It is possible to send an instruction to all Drives on
the serial link using Drive address 00. All Drives will
listen to the an instruction, but none will
acknowledge. This can be useful for ensuring that
all Drives on a flow line get the START signal at the
same instant.
Parameter identifier
To address an individual parameter, the host must
send a code that relates to that parameter. There
are two types of parameter in the DigitAx:
Numerical parameters Pr00 to Pr99
Parameter identity
000 to 099
Bit parameters b00 to b99
Parameter identity
100 to 199
Data field
Data is sent as a numerical value with sign and
decimal point. The data field can consist of up to six
characters, two of which must be the sign and
decimal point. The data field can be shortened
when transmitting data to the Drive, since the ETX
control character (see above) indicates the data is
complete. The response of the Drive, when
interrogated, will always have six data characters.
Block checksum
The block checksum character (BCC) is used to
check the received message has not been corrupted
during transmission. The BCC is a value that is
calculated from the ASCII codes of the characters in
the parameter identifier and data fields. The BCC is
calculated by using the XOR function, and starts
with the first character following the STX (ctrl B)
control character. The following example shows
how to calculate the BCC.
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