NORD Drivesystems BU0050 - Manual (Page 21)

1.3 USS - Protocol speciication
BU 0050 GB-3111 Subject to technical amendments 21
1.3.5 Transfer Procedure
The USS protocol functions according to the master/slave principle, whereby the master is the control device
(PC, SPS etc) and the slaves are the frequency inverters
Only one slave can be addressed with each telegram (exception: broadcast telegram without response by
the slaves).
In order to ensure reliable detection of the start of the telegram by the slave, the master must observe a so-
called start pause between the receipt of the slave telegram and transmission of the next telegram. The
master starts to transmit a telegram. After the data package has been sent, the master switches the bus data
direction from transmission to reception. The slave addressed in the telegram must respond within a
specified response time.
The lengths of the master and slave telegrams are the same, i.e. the master telegram determines the length
of the response by the slave.
Telegram traffic can be cyclical a acyclical.
The following time definitions must be observed:
Time Size Meaning
t
SP
Minimum 2 character durations* Start pause time
t
AVZ
Maximum 20 ms Response Delay Time
t
TLZ
1.5 x consecutive telegram duration
= 1,5 x (n+4) x character duration
Max. residual telegram duration
t
ZVZ
Smallest start pause time Character delay time
*Character duration = 11 x (1/Baud rate)
1.3.6 Start Pause Time
The start character STX (= 02 Hex) on its own is not sufficient to enable the slaves to uniquely identify the
start of a telegram, because the bit combination 02/Hex may also occur in the information characters.
Therefore, in advance of the STX a start pause time t
SP
where no characters are sent for at least the
duration of 2 characters is specified for the master. The start pause time is a component of the order
telegram. A valid telegram is only identified by an STX with a preceding start pause.
The exchange of data is always performed according to the pattern described above (semi-duplex
operation):
The minimum start pause time which is to be observed for the various baud rates can be found in the Master
Telegram Time table in the Additional Information section.
Master
BCC LGE ADR 1. n BCC STX
STX LGE ADR 1. n BCC
t
SP
t
AVZ
STX
t
SP
t
TLZ
Slave
x x+1
t
ZVZ