When I've done this sort of thing in the past I've used "Kermit" ob the sending-end because it has a great range of options for specifying things like idle-timers between sending each character/line - important to prevent data-overrun if the receiving CP/M [or RSTS/E or RT/11...] system's UARTs have only limited buffering ability, or if [heaven forfend!] you're doing the serial-to-parallel conversion on the receiving-end by software 'bit-banging' on a single binary-input line...
Kermit is freely available:
http://www.kermitproject.org/kermit.html
[In the 1980s I did some development with Joe Doupnik and Chris Giannone on Kermit for what was then available via the SIMTEL20 free-software repository]