SEND_REQ & READ_VAR

[rajsiyer 7]

Hi gang; Now I'm back to a Premium PLC project. The scene is like this.... Thre are 8 nos load cell instruments on MODBUS (serial) hung out, as slaves on the SCP111 card.This port, channel 1 being configured as Modbus master. I need to poll them for data and use them ASAP in the logic. There are 2 options (at least!) i.e. one could do a READ_VAR or send out a SEND_REQ to reqest these Modbus slaves to speak up with what they sense. I find that you could do this with either instruction. READ_VAR or SEND_REQ. My question therefore is... when do you use READ_VAR and when do you need SEND_REQ. really grateful if anyone has been here and done this, and is happy to share the experience with us. Regards, Best wishes, Raj S. Iyer

[undercontrol 1]

READ_VAR and WRITE_VAR are a couple, the purpose is definitely to read and write messages (Modbus?? - not sure). SEND_REQ on the other hand is to send a request to another party. The request might be a read request, a write request, a diagnostic request, .... depends on the request code you put in the syntax's parameters. Hope this helps