HA243821
Issue 5 Mar 98
Section5
Page 5 - 31
MODEL 346 INSTALLATION AND OPERATION MANUAL
5.7.4 PROTOCOL
Protocol defines the control codes, data format, sequencing etc. that is required for the successful implementa-
tion of a serial communications link. The EI bi-synch protocol used by the 340 series of recorders is derived from
ANSII standard X3.28-2.5-A4. This is a character-oriented protocol which supports multi-drop linkage to allow a
host (computer or ‘dumb terminal’) to communicate with a number of recorders.
Certain control characters are used as shown in the table below. The use of these control codes is described as
and when they arise in the following polling and selection sequence description.
Table 5.7.4a Control character codes
TERMINOLOGY
POLL
This refers to the sequence in which a particular recorder is interrogated and responds with data. Recorder
configuration and data values are not altered.
SELECT
This refers to the sequence in which the data values or configuration of a particular recorder are changed by
instructions from the host.
POLL SEQUENCE
Note...
The spaces between ‘characters’ in the poll sequence message below are inserted for clarity. These spaces
must not be inserted into the data sent to the recorder, or the message will not be understood.
The sequence of characters used to poll a parameter is EOT G G U U CN C1 C2 ENQ where the various parts of
the sequence have the following meanings:
EOT
End of text. This character resets the data lines and causes all the recorders to examine the next four
characters to see if they match the address set up in the configuration pages.
G
Group address. This should match the Group ID of the relevant recorder, as set up in that recorder’s
communication configuration pages, but see the Unit Address note below.
U
Unit address. This should match the Unit ID of the relevant recorder, as set up in that recorder’s commu-
nication configuration pages.
Note...
As can be seen, G and U are each only a single character, and a numeric value cannot therefore be entered
for addresses 10 to 15. The characters shown in table 5.7.4b below, are used instead:
CN
Channel number. This character is used to address a channel, derived variable, alarm etc. within the
relevant recorder. A full list of CN values is given later in this document.
C1 C2
Command mnemonic. These two characters form a command code which tells the recorder which
parameter the host wishes to access. For example, to read the measured (present) value, C1 and C2
would be P and V respectively. A full list of parameters appears later in this document.
STX
0 2
0000 0010
CTRL B
ETX
0 3
0000 0011
CTRL C
EOT
0 4
0000 0100
CTRL D
ENQ
0 5
0000 0101
CTRL E
ACK
0 6
0000 0110
CTRL F
NAK
0 7
0000 0111
CTRL U
ASCII
Hex
Binary
Control key
