Que es lo que quieres hacer realmente con este circuito?
EDITO: he buscado algo respecto al circuito, pero no hay mucha info...
A ver si tengo mas suerte luego.
He encotrado una pagina (creo que es de donde has sacado el circuito) que salen unas fotos de osciloscopios y hablan sobre unas tensiones, pero no comentan nada de esa V+ y V-.
A no ser que sea +12v, -12v que es con lo que trabaja el puerto serie..
EDITO (de nuevo): leete esto :
HP49 models starting with an ID below 94xxxxxxxx have a hardware bug in the serial buffer. You can correct
the error by opening your calculator, and patching the circuit. There’s a document on
www.hpcalc.org on how
to do so. This document describes a different way, without opening the calc. It is an external buffer that forms a
high impedant load to the HP49. It ‘steals’ power from the PC and amplifies the signal, depending on the
voltages supplies by TX, RTS and DTR. The negative voltage is taken from TX and the positive one is taken
from both RTS and DTS. Fortunately HPCOMM sets these signal high. If you are using a different terminal
program to communicate, make sure you set hardware handshaking on. Also connect pin 7 and 8 (RTS and CTS)
at the bottom of the PCB (See schematic drawing).
The current design is made with SMD components. It can made with through-hole components as well.
If you need gerber-files, let me know.
Good luck, dont’t fry your calc,
A ver si hay suerte ;-)