Instruction Manual

Logic Functions - Book 1
MODCELL SERIAL COMMUNICATION BLOCK
4-21
Function Maximum Values
Code Description to Read/Write
05 Write one discrete point 1 attribute
06 Write one register 1 attribute
08 Loopback diagnostic test
(only "Return of query" supported).
15 Write multiple discretes 16 attributes
16 Write multiple registers 16 attributes
Modbus data types are bit (discrete) and word (count, 16 bit positive value). One Count
occupies one register. MODCELL data types can be mapped into the pure Modbus data
types of Discrete and Count. (MODCELL data types can also pass through without being
mapped, for those users who can already support these data types or who modify their
Modbus drivers. The multiple register read/write and multiple register read only lists are
intended for this purpose.) The following table describes how MODCELL data types are
mapped into Modbus data types. Refer to the Modbus List (ML) block for more detail on how
MODCELL data is mapped into pure Modbus data.
MODCELL Data Type Modbus Data Type Comments
Discrete Discrete
Discrete Count
Short State Count
Long State Count
Count Count
Date Count Packed in 16 bits
Date Date Takes 2 registers
ASCII ASCII Takes as many registers as needed
Hex Hex Takes as many registers as needed
Floating Point Count Top and bottom range required; values out
of range will be mapped to 0 or 65535 as
appropriate
Floating Point Floating Point Takes 2 registers
Msec-time Count Scaling factor required; value of 65535
from MODCELL indicates value out of
count range
Msec-time Msec-time Takes 2 registers
When a write message comes from the master to MODCELL, a response is immediately
returned by MODCELL to the master. This response indicates that MODCELL received the
message and that it will be processed internally by the MODCELL. It does not indicate that
the actual attribute or attributes have been changed. For example, if a write is made to the
output of a PID block and the output mode is AUTO, the write does not occur. Writes to the
Modbus network do not take control of the loop; they are one-shot writes. Note that on a
multiple write, all writes may not occur at the same time.