Siemens SIMATIC TI505 Manual de usuario descargar pdf (Pagina 38)

Modbus Commands

2-7

SIMATIC TI505/TI500 MODNIM User Manual

The

MODNIM uses the absolute position of the data for the address. That

is, if you want to address the 3066th coil, then that is the address you pass

with the command (as hexadecimal value BF

A). The maximum address is

65535 (FFFF hex). All current PLCs are within this limit.

For example, if you want to access 1000 coils starting at the 703rd coil, the

Function Code 01 would be as follows:

: 01 01 02 BF 03 E8 52 CR LF

[ASCII mode]

01 01 02 BF 03 E8 0C E8

TU mode]

Modbus imposes a buffer size limit of 256 bytes. Because of this restriction,

MODNIM requests have been limited to the following values.

Function Code Quantity

2000 coils

2000 discrete inputs

125 output registers

125 input registers

800 coils

100 registers

the process of mapping Modbus commands against those used by

SIMA

TIC TI PLCs, when an appropriate command was absent, it has

become necessary to make multiple passes with a single command. In

practical terms, this means that when the maximum amount of data for a

particular command is requested, the MODNIM must wait for several PLC

cycles before sufficient data can be collected. This latent time increases for

addresses greater than 1024 (extended addressing).

The command checking will reject requests for data greater than the limits

shown above. What the checking does

not

do is check the request against

the available PLC memory

. When a request goes beyond the memory of a

particular PLC, the command will be rejected by the PLC and the MODNIM

will notify the host computer with Exception Code 02.

Addressing

Addr

ess Limits

1 2 ... 33 34 35 36 37 38 39 40 41 42 43 ... 90 91

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Siemens SIMATIC TI505 Manual de usuario Pagina 38