I have test equipment with optically isolated RS232 ports. Seems modern RS232 ports do not support +/-12V. They all seem to be less than +/-8V, many only +/-5V.
Anybody know of a cheap source for +/-12V RS232 PCI serial boards?
What ever happened to 1488/1489 serial drivers?
A +/-5 V transmitter is not RS-232, a transmitter generating +/-6 V to
+/-25 V might be RS-232, a receiver with threshold levels between +/-3 V is acceptable.These chips require some uncomfortable voltages like +/-15 V, which are not readily available in current 5 or 3.3 V systems.
Paul
RS232 minimum voltages have progressively got lower with each new revision of the standard. Now +/-5V is the minimum. From memory that is at the receiver end though, so if a card is only giving 5V that doesn't allow anything for losses but it is still within spec.
Have you considered dropping in an active buffer between the PC and the isolator? For moderate speed RS-232 this needn't be difficult. Back of an envelope calculations show a TL084 used as a comparator will slew plenty fast enough over 24V for 38400, though you will probably want to artifically lower the input impedance to give a degree of noise resistance.
Of course, the driver will be fairly simple to knock up, but providing a +/-15V supply is going to be cumbersome.
-- Andrew Smallshaw andrews@sdf.lonestar.org
B&B Electronics
A lot of designs use the MAX232 class of devices, with on-chip charge pumps (using external capacitors) to generate the RS232 mark and space voltages. Technically, +/- 5volts is acceptable (barely - the spec allows down to
+/-3), but most of these charge pump devices seem to run around +/- 9 volts or so.-- Mark Moulding
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