Does Freelance really suppor MODBUS ASCII
Voted best answer
Yes, Freelance supports MODBUS ASCII transmission mode, as well as RTU transmission mode.
This is how it can be configured Modbus master/slave object in hardware structure you can select between Framing ‘RTU’ or ‘ASCII’ as shown here.
The Modicon MODBUS Protocol reference guide “PI–MBUS–300 Rev. J ” depicts that ‘The selection of ASCII or RTU mode represents only to standard Modbus networks. It defines the bit content of message fields transmitted serially on those networks. It determines how information will be packed into the message field and decoded.'
Hence any data transmitted either by RTU mode or ASCII mode transmitted data information should be the same at source and destination point. The way(frame format) of transmission is going to be varied. The main advantage of ASCII mode is that it allows time intervals of up to one second to occur between characters without causing an error. It is best suitable for slow response applications.
Responce to your comment on answer Modbus master/Slave function block do not have any setting about Modbus RTU selection. Because there is no requirement of configuring frame here at software block. Rather it’s our assumption that it is Modbus RTU I guess…
If I understood correctly things are mixing up between frame and data type to be transferred. Via Modbus any data type supported with 16 bit can be communicated with RTU or ASCII transmission mode. Pls check more about data type @ http://www.simplymodbus.ca/FAQ.htm#Modbus.
Now answer to your comment about ASCII signal read\write can be done via use of UFB(User define function block) by creating block which can code and decode ASCII to binary and binary to ASCII data and you can communicate not only via Modbus but even using Ethernet send/receive. I have not done ASCII data type read/write but it can be possible by this way….. (UFB : ASCII to number of bit conversion depends on supported protocol Read/write function block)
Pls find attached here with csv file with ASCII charachter communicated over MODBUS using freelance AC800F, Note that RS232 is used for that to test as loop back you need to connect TB 870F terminal 1-15, 2-14, 5-18 to stablish RW, key in 'Var_In' with ASCII character i.e 'STX', or '#' you will receive 'Var_Out' as same value in ASCII character.
More over you can try with changing frame 'RTU' it works. I have used ASCII to DEC UFB to perform this.