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ASM 421 Technical Description
6GT2 097-3AH00-0DA2
– 19 –
RD: 03/02
6GT2 002-0DA00
ASM 421 / V.24
Subject to change without notice!
6GT2 002-0DB00
ASM 421 / RS422
6GT2 002-0DC00
ASM 421 / TTY
J31069-D0142-U001-A0-7618
Block Transmission Sequence:
Sender
Receiver
STX
DLE
1st user byte
2nd user byte
last user byte
Data
block
DLE ETX
BCC
DLE
.
.
.
.
.
.
.
.
t
z
= Watchdog
timer on ASM
300 ms
t
z
Data security
To protect data against transmission errors, a parity bit is attached to each character transmitted. It is
supplemented by an odd or even quantity of “ones”. In addition, the sum of the respective bits of each
character in a block is supplemented by a further bit to even parity (EXOR operation on all characters).
The resulting block check character (BCC) is itself secured by the character parity principle and is trans-
mitted at the end of the block. All characters in the block are saved except the start control character STX.
Repeat counter
If no acknowledgement or a negative acknowledgement is received, the telegram will be sent again. The
3964R has a repeat counter. If a block cannot be correctly transmitted after a number of repeats, the trans-
mission will be terminated. The telegram is lost or the user receives an error message.
Initiation conflict
Since both stations have equal rights, an initiation conflict can arise if both partners try to transmit at the
same time. To avoid this, one station is given a higher priority (master) than the other (slave). This can be
set using switch 4 (see section 2.3).
Watchdog timers
Two watchdog timers are used to monitor the data transfer:
– Acknowledgement watchdog timer t
Q
= 300 ms
The acknowledgement watchdog timer is started after transmission of the control character STX, or
DLE ETX BCC.
If a positive acknowledgement fails to arrive within t
Q,
the whole data block is sent again. If the trans-
mission has still not been performed correctly after three attempts, an error message is sent to the user
or the telegram is rejected by the ASM and an error indicated by the red LED.
– Character watchdog timer t
Z
= 300 ms
The receiving ASM 421 monitors the arrival of the individual characters with time constant t
z.
If a character fails to arrive within the watchdog period, receiving is aborted and an error indicated by
the red LED.