Specifications

C.2.2 Synchromous communication

If you want to use the entire functional extent of the PCS, then the access to the

data in the programmable controller must be synchronized, i.e. programmable

controller and PCS access alternately. Therefore, a synchronization word is

transferred to the programmable controller. The handling software examines this

word and enables the programmable controller user program access. If the user

program is finished with the processing of the data words, then the synchronization

word is changed and the PCS accesses the data area. While the PCS processes

the data, the user program may not access to the data. This Ping-Pong game

offers the possibility to realize a time-out monitoring also in the PCS. Whenever

the PCS reads the inverted synchronization word, the timer is restarted. If the timer

expires then a time-out exists.

Using this alternating access, actual and preset values can be mixed, bit varia-

bles used, erase behavior 2 realized, etc. Therefore, the whole intelligence of the

PCS is available. The disadvantage for you is that the reaction speed between

PCS and programmable controller is lowered. Furthermore, before accessing the

data the programmable controller program must always scan whether access is

allowed or not.

The time-out time, i.e. the time that passes since the last writing of word 3 up to

the time-out message in the programmable controller, should be set to a minimum

of 2 seconds. In the PCS, the time-out time is set via the AA driver variable or

COM_TIMEOUT.

C2.2.1 Procedure

To realize synchronous communication between the PCS and programmable

controller you must:

 select the SYNC setting on the PCS,

 load the handling software into the programmable controller.

This handling software is described in the following. Of course you can also solve

these tasks differently in your software. It is only important that you keep to the

following procedure:

1. The used data area assigned to the PCS and programmable controller must

be determined in the programmable controller (e.g. 255 flag words) and com-

municated to the PCS via the AL and AM driver variables. The programmable

controller flag word area should be initialized.

2. Thereupon, the PCS sends an order number (byte-by-byte, every time incre-

mented by one) in PCS word 3, e.g.: 01. This is the signal for the programmable

controller, that the data area can be processed. Furthermore, a time-out timer

can be restarted (if this is used).

3. If the processing of the data area is finished, then the inverted sync word of

PCS word 3 is copied to PCS word 2, e.g. with FE. This is the signal for the

PCS to access the data area. Nothing more may be changed now in the data

area by the programmable controller program!

From now on, step 2 and 3 will be cyclically executed. If a time-out appears,

then processing is resumed with step 2 with order number 01 again.

PLC handling software