Hameg HO88, Manual

Page: 15 / 48

15
Sous réserve de modifications / Reservado el derecho de modificación
of the cable), and the maximum cable length for any single run is limited to 20 meters.
The devices are connected in parallel, with a cable assembly consisting of cables with
both plug and receptacle connectors at each end, ideally as „piggy-back“ connectors,
i.e. with male and female connectors molded one above the other, the latter taking the
plug connector of the next cable
(Figure 2) . This design allows devices to be linked in
either a daisy-chain or star configuration, or a combination of the two.
Function of the bus lines
Data transfer is performed by consecutive transmission of 8-bit bytes, each of which
is sent in parallel over the data lines DIO1 through DIO8. The handshake lines designated
DAV, NRFD and NDAC are used to control data transfer. The 5 interface management
lines ATN, IFC, REN, SRQ and EOI are separate from the handshake lines, and are
used by the System Controller to maintain order and initiate proceedings within the
bus. 6 of the 7 ground return lines are twisted together with the lines DAV, NRFD,
NDAC, ATN, SRQ and IFC (EOI in the IEC-625 bus). The shield drain line SHIELD is
connected to ground.
Data lines DIO1 through DIO8
Each data line (DIG stands for „DATA IN OUT“) is used for transfer of one bit of each
8-bit data word. Most data use the 7bit ASCII or ISO code set. Each letter or character
is represented as a : 7-bit word
(Figure 4) .The 8th bit (DIO8) is either unused or is
used for parity.
Binary, decimal, octal and hexadecimal coding systems are also used in practice for
representing bit combinations. The ASCII code is now in general use by all
manufacturers of test and computer equipment; consequently, the IEEE-488 and
computers utilize the same character set. As a rule. It is therefore no longer necessary
to convert character sets between computers and the IEEE-488 bus.
The handshake lines DAV, NRFD and NDAC
The handshake lines control the transfer of message bytes among devices over the
data lines according to a scheme defined in the standard
(Figure 3) .
Figure 3: Handshake lines DAV, NRFD and NDAC
The user of an IEEE-488 bus-controlled system normally has no way of influencing the
sequence of signal sent over the handshake lines. Even when this is possible, a trough
understanding of the workings of the IEEE-488 bus is essential. The following
explanations of the DAV, NRFD and NDAC lines are provided by way of information;
they are not, however ,required for a general understandinf of the functions of the
IEEE-488 bus.
DAV (data valid)
A device with data or commands to send first checks the NRFD line to verify whether
all devices are ready to receive (NRFD must be in its logic 0 state). It then transfers