RESISTOMAT
®
Model 2302-V001
Page 30
Introduction &
applications
Description of functions
Brief guide
Operation
Calibration
Test and offset
instructions
RS232 data
output
Accessories and Circuit diagram
RS232 data
output
burster devices therefore allow instructions in a bus system to be executed not only at individual
stations but also globally. Special instruction codes (e.g. for global measurement start) are thus
neither required nor provided.
The protocol list selected for our devices is described in subcategory 2.5 of the ANSII standard.
Its complete designation is ANSII-X3.28(2.5-A4)-1976. This protocol allows information - or
messages - to be sent to a device immediately after the header without having to wait first for an
acknowledgement signal from the device. This type of information transfer is known as "Fast
Select“. In this case, the header is sent in a slightly modified form:
<Start character><Address><B1><B2><Separator><Message>
<Separator>
The "Start of Text“ character STX (Control-B: C
B
, 0x02) is used as a
separator;
<Message>
= <String><Message end><Block check> in acc. with subcategory A4;
<String>
This contains the actual information in the form of ASCII characters;
no control characters are involved here;
<Message end>
The ASCII character "End of Text“ ETX (Control-C: C
C
, 0x03) is used to end
the message.
<Block check>
A block check consists of a control character allowing the device which
receives it to check the integrity of the message. It is formed through a
bit-by-bit exclusive OR operation on the entire message, including the
end character.
—> To prevent the block check character from being mistaken for an ASCII control character (0x00
to 0x1f), our devices always send the block check character with bit 7 (MSB in the byte) active.
The standard V.24 driver programs for PCs can be used then.
—> burster devices also allow the transmission and anticipation of block check characters to be
deactivated by means of a special instruction. The block check function can be activated again
with another instruction. This option greatly facilitates the testing of our devices; in particular, it
easily allows manual communication with the devices from any terminal. Further details on this
operating mode are found in the description of the instructions db and eb.
Note:
Power-on state: block check on.
If the addressed station is requested by the master computer to transmit data, it responds with:
<Separator><String><Message end><Block check>
The tokens have already been defined above. The master computer responds with one of the
acknowledgement signals (see above). If an NAK is sent, the device repeats the last data
transmission. If measurement values are involved, a new value might be transmitted. If an actual
transmission error occurs, this could cause a loss of data. However, an advantage here is that,
by constantly responding with NAK, the master computer can call up the latest measurement
values from the station which was addressed.
