You could implement a finite state machine in your tx isr that formats the modbus message on a character by character basis. My usual method, though, is to have a ram buffer for the largest message and have the tx isr read from that. If I'm doing modbus ascii, I have a state machine in the tx isr that converts the binary modbus message (rtu format) to ascii. There's many ways to do this - it really depends on how much ram you have, how the modbus register information is stored (I have both ram and eeprom based registers) and what else you want to do.

My normal procedure is:

rx isr collects the receive packet into buffer, sets a flag when it has a packet (rtu = timeout, ascii - cr/lf).

main line code tests flag - if set, checks the crc (if rtu),length,modbus addr == our slave addr and modbus command.

each command has a subroutine that decodes the packet and does what is necessary and creates the response packet in ram then fires off the tx isr to acutally send the packet

do other stuff.

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