Mbus_Slave Instruction - Siemens SIMATIC S7 System Manual

Libraries
9.3 Modbus RTU library
Parameter MaxIQ sets the number of I and Q points available to Modbus addresses 0xxxx
and 1xxxx at values of 0 to 256. A value of 0 disables all reads and writes to the inputs and
outputs. The suggested value for MaxIQ is 256.
Parameter MaxAI sets the number of word input (AI) registers available to Modbus address
3xxxx at values of 0 to 56. A value of 0 disables reads of the analog inputs. The suggested
value for MaxAI to allow access to all of the CPU analog inputs, is as follows:
● 0 for CPUs CR20s, CR30s, CR40s, and CR60s
● 56 for all other CPU models
Parameter MaxHold sets the number of word holding registers in V memory available to
Modbus address 4xxxx or 4yyyyy. For example, if you want to allow Modbus master access
for 2000 bytes of V memory, set MaxHold to a value of 1000 words (holding registers).
Parameter HoldStart is the address of the start of the holding registers in V memory. This
value is generally set to VB0, so the parameter HoldStart is set to &VB0 (address of VB0).
Other V memory addresses can be specified as the starting address for the holding registers
to allow VB0 to be used elsewhere in the project. The Modbus master has access to
MaxHold number of words of V memory starting at HoldStart.
When the MBUS_INIT instruction completes, the Done output is turned ON.
The Error output (Page 493) byte contains the result of executing the instruction. This output
is only valid if Done is ON. If Done is OFF, the error parameter is not changed.
9.3.3.3

MBUS_SLAVE instruction

Table 9- 10 MBUS_SLAVE instruction
LAD / FBD STL
CALL MBUS_SLAVE, Done, Error
Table 9- 11 Parameters for the MBUS_SLAVE Instruction
Parameter
Done
Error
The Done output is ON when the MBUS_SLAVE instruction responds to a Modbus request.
The Done output is OFF, if there was no request serviced.
492
Data Type Operands
BOOL I, Q, M, S, SM, T, C, V, L
BYTE VB, IB, QB, MB, SB, SMB, LB, AC, *VD, *AC, *LD
Description
The MBUS_SLAVE instruction is used to ser-
vice a request from the Modbus master and
must be executed every scan to allow it to
check for and respond to Modbus requests.
The instruction is executed on each scan when
the EN input is ON.
The MBUS_SLAVE instruction has no input
parameters.
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART