the only occurence for "bit serial voltage signal" paired with a mention of the protocol RS-232C is in the instruction manuals for the Sysmex CA-50 Blood Coagulation Analyzer (and CA-500 and CA-1500)

"External Input/Output
Equipped with an RS-232C serial port (bit serial voltage signal)."
[http://iserver.netlab.com.ec:8080/sgcWeb/SGC ISO 9001-2008/M...]

"External Input/Output. Equipped with an RS-232C serial port (bit serial voltage signal)." for CA-50

"External Input/Output. bit serial voltage signal (RS-232C)." for CA-500

but for the model CA-7000 that part ("bit serial voltage signal") was dropped, and there's simply a mention of a "Bi-directional RS-232C serial port".

This document makes it clear what was the intended meaning:
"Software
•Interface: ASTM, CA-1000 protocol
•Host connection: Bi-directional RS-232C serial port"
[http://www.sante.ro/produse/siemens/hhs/manuale/CA1500Th.pdf]

"bit serial voltage signal" is simply a rather clumsy description/translation from German(?) for a RS-232C serial port, written at the time when the RS-232C serial port was a recent technological marvel and someone felt the need to add an explanation, but put a clumsy one (or the translation was clumsy).

This port is used to connect the analyzer to a PC, and transfer information both ways. It's used to transmit "signals" "bit by bit" ("serial" link) by varying the "voltage" on various pins, surely, but the whole of "bit serial voltage signal" is a meaningless combination.

Nowadays a serial port is no longer a technological marvel that needs explaining and "serijski port" is more than enough; if you want to keep the "antiquated flavour" of the ST you could add "digitalni".