Super Systems AC20 RS485 Modbus - Manual (Page 12)

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TITLE: MODBUS/J-BUS Protocol for SSi’s- AC20 ENG. 824E REV. 1
NOTES:
1. “Broadcast” address
When using the writing codes (5, 6, 15 and 16) the slave address 0 is permitted: in this
case all the slaves connected accept the command but do not give any reply.
2. Words format
Every time the information transfer is performed by using 2 bytes (1 word of 16 bits), the
first byte transmitted is the most significant one. For the negative numbers the “two
complement” format is used.
3. Reply time
The slave will start to send a reply from 2 ms to 250 ms after the end of the request
detected by counting the received bytes
4. Decimal digits
The decimal point that may be present in the value is ignored.
Example:
The value 204.6 is transmitted as 2046 (07FEh)
The value -12.50 is transmitted as -1250 (FB1Eh)
5. Local/remote status
At power up, the slave will be in local mode if it was in control mode at power off. In the
other cases, it will maintain the condition it was at the power off.
In order to enable a slave to be controlled from the master, it is necessary to set the
local/remote status bit (ModBus bit 218).
For a slave to remain in remote status, it is sufficient to detect line activity.
If there is no line activity for more than 3 sec. every slave will automatically return to
local mode.
Local mode: the communication between master and slave is limited to
transferring data from slave to master without possibility of modifying
any parameter from the master itself (with the exception of the
local/remote status bit). Therefore from the local keyboard,
parameters can be displayed and modified.
Remote mode: the instrument parameters can be modified by the master. Therefore,
from the local keyboard the parameters can be only displayed but not
modified.