6.2 PROTOCOL
6.2.1
Interface and Termination
The serial interface is RS-422/485 compatible 2/4-wire with no receiver biasing option. As the
amplifier dos not contain transmitter or receiver termination, the local serial communications bus
should be fitted with 120R ±1%, ¼W carbon film or similar termination resistors between pin d (RX+)
and pin e (RX-), also between pin b (TX+) and pin c (TX-) of the 41-pin amplifier control connector.
Termination of the serial communications bus is dependent upon the length and type of cable used,
and should follow normal transmission line practice. If used in the redundant mode with another
amplifier, or on a multi-drop system, the bus termination is only required on one equipment, typically
the furthest from the master device.
A screened cable, terminated to the backshell of the user interface connector, should be used to
prevent radio frequency interference from adversely affecting operation. When connecting the screen
of the cable to the backshell, a full 360º electrical connection should be made (pig-tailed connection
should be avoided).
For short cable runs up to approximately 5 m, a suitable data transmission cable, with a characteristic
impedance of 120R, would include a twin twisted pair conductor arrangement, typical conductor size
of 24 to 26 AWG, screened in aluminised tape with an overall tinned copper braid. A pin is provided
on the connector for the connection of a drain wire if available on the cable. For cable runs above 5m
and additional insulated core should be used as a signal return connection.
6.2.2 Data
Format
The format is:
1
start
bit
7 data bits with most significant bit sent as EVEN parity
1
stop
bit
Which can be realised as:
1
start
bit
7
data
bits
EVEN
parity
1 stop bit
6.2.3 Data
Rate
The default data rate is 9600 baud but other baud rates may be selected using the appropriate
command.
6.2.4 Generalised
Message
Format
<delimiter> address command data <delimiter> checksum
Message component
Function
ASCII
Hex
Decimal
Start of text
STX
2
2
End of text
ETX
3
3
Acknowledge ACK
6
6
Delimiter
Not Acknowledge
NAK
15
21
Address
The unique address of the amplifier in
the stated range.
0 - o
30 - 6F
48 – 111
Command
A character specifying a particular
function for the amplifier to perform.
The character can be in the stated
range.
0 - DEL
30 - 7F
48 – 127
Data
A parameter qualifying a command or
data returned in response to a
command. Characters can be in the
stated range.
SP - DEL
20 - 7F
32 – 127
Checksum
Bit by bit exclusive OR of all characters
in message between and including the
delimiters. Characters can be in the
stated range.
NUL – DEL
0 - 7F
0 – 127
© e2v technologies (uk) limited 2010
DAS768892AA
Version
1,
page
28
