Pretty much I have no interchar delay as the data is output via interrupts. I think the interchar delay spec is so you don't mistakenly interpret an interchar delay as an end of message. I haven't got the spec to hand, but I would think the 1.5char time value is a maximum, not a minimum.

I just had a quick glance at an old modbus spec - the 1.5char delay is for a packet restart. I can't say I've ever considered this and I've not seen it actually implemented. As to why you want to abort a packet, I'm not sure. It all comes down to how compliant you need to be and what you expect to connect to. My stuff has connected up to large Emerson and Honeywell modbus networks as well as a number of ethernet to modbus converters with no ill effects.