ADS-sum command: Read or Write a list of variables with one single ADS-command
Download: ADS Sum Command : Read or Write multiple variables
This sample describes how to read multiple single variables with one single ADS API call.
![]() | Mind the ADS requirements Note that ADS is just a transport layer, but there could be important side effects: So read these requirements and take care on limitations. |
Background :
ADS offers powerful and fast communication to exchange any kind of information. It's possible to read single variables or complete arrays and structures with each one single ADS-API call.
This new ADS command offers to read with one singale ADS call multiple variables which are not structured within a linear memory.
As a result the ADS caller application (like scada Systems etc.) can extremly speed up cyclic polling :
Sample :
- Until now : Polling 4000 single variables which are not in a linear area (like array / structure / fixed PLC address ) would cause 4000 single Ads-ReadReq with each 1-2 ms protocol time.
As a result the scanning of these variables take 4000ms-8000ms. - New Ads-Command allows to read multiple variables with one single ADS-ReadReq : 4000 single variables are handled with e.g. 8 single Ads-ReadReq (each call requesting 500 variables) with each 1-2 ms protocol time.
As a result the scanning of these variables take just few 10ms.
Requirements and important limitations:
Note that ADS is just a transport layer, but there could be important side effects. So read these requirements and take care on limitations:
- Version of target ADS Device:
ADS itself is just the transport layer, but the requested ADS device has to support the ADS-Command. - Bytes length of requested data:
Requesting a large list of values from variables is fine, but the requested data of the Ads-response (the data-byte-length) have to pass the AMS-router (size by default a 2048kb)
So the caller has to limit the requested variables based on calculation of requested data-byte-length. - Number of Sub-ADS calls: Highly recommended to max. 500!
If the PLC is processing one ADS request, it will completely work on this single ADS request BEFORE starting next PLC cycle.
As a result one single ADS request with 200.000 sub-Ads-requests would cause that PLC would collect and copy 200.000 variables into one single ADS response, before starting next PLC.
So this large number of ads-sub-commands will jitter the PLC execution!
// This code snippet using ADSIGRP_SUMUP_READ with IndexGroup 0xF080 and IndexOffset as number of ADS-sub-commands
// Demonstrates how to read a list of variables, see full demo-code
// Use ADS-ReadWrite request : "Write" the requested data down to ADS device and "Read" the received answer
nErr = AdsSyncReadWriteReq( pAddr,
0xF080, // Sum-Command, response will contain ADS-error code for each ADS-Sub-command
reqNum, // number of ADS-Sub-Commands
4*reqNum+reqSize, // number requested bytes in the sample two variables each 4 bytes. NOTE : we request additional "error"-flag(long) for each ADS-sub commands
(void*)(mAdsSumBufferRes), // provide buffer for response
12*reqNum, // send 12 bytes for each variable (each ads-Sub command consist of 3 * ULONG : IG, IO, Len)
&parReq);
// This code snippet using ADSIGRP_SUMUP_WRITE with IndexGroup 0xF081 and IndexOffset as number of ADS-sub-commands
// Demonstrates how to write a list of variables, see full demo-code
// Use ADS-ReadWrite request : "Write" the send a list of data to list of variables down to ADS device and "Read" the received return codes
nErr = AdsSyncReadWriteReq( pAddr,
0xF081, // ADS list-write command
reqNum, // number of ADS-Sub commands
4*reqNum, // we expect an ADS-error-return-code (long) for each ADS-Sub command
(void*)(mAdsSumBufferRes), // provide space for the response containing the return codes
16*reqNum, // cbyteLen : in THIS sample we send two variables each 4 byte
// --> send 16 bytes (IG1, IO1, Len1, IG2, IO2, Len2, Data1, Data2)
&parReq); // buffer with data