Another clueless question...

[toa]

Hi

I'm trying to make sense of an RS232 card that I took from an old PC, which
necessitates my understanding of the PC's ISA bus architecture. I'll be
connecting the card to some custom-made circuitry instead of the ISA bus.
Most of what I've learned about it so far seem straightforward, but the one
thing I'd like to get a tip on, is the power supply. In addition to +5V and
Ground, the card requires a +12V input and, worse, a -12V input. What is the
standard way of providing these voltages? All I have so far is a power
supply offering one output tha can be varied continuously from 0 to 30
volts. So, do I actually need 3 power supplies, or what?

 

[toa]

toa posted:
<< I'm trying to make sense of an RS232 card that I took from an old PC, which
necessitates my understanding of the PC's ISA bus architecture. I'll be
connecting the card to some custom-made circuitry instead of the ISA bus.
Most of what I've learned about it so far seem straightforward, but the one
thing I'd like to get a tip on, is the power supply. In addition to +5V and
Ground, the card requires a +12V input and, worse, a -12V input. What is the
standard way of providing these voltages? All I have so far is a power
supply offering one output tha can be varied continuously from 0 to 30
volts. So, do I actually need 3 power supplies, or what? >>

--

The standard way of providing the voltages is by using the PCs power supply, or
buy or scrounge a computer power supply. They have one or more of: 12V, -12V,
and 5V. These supplies commonly require a minimum load on each output in order
to work correctly.

You should not be surprised at the need for +12V and -12V, as classical "RS232"
require both polarities, where - is mark (1) and + is space (0).

Don

Oh, I'm not surprised, just at a loss as to how to proceeed. Until now, that
is. I guess I'll go butcher my old 386 some more, then. Any idea what that
"mimimum load" might be? I suppose I should wire some heavy-duty resistors
in parallel with the card then?

Anyway, thanks a lot for your input - hopefully it will get me going again.

 

[Dbowey]

toa posted:
<< I'm trying to make sense of an RS232 card that I took from an old PC, which
necessitates my understanding of the PC's ISA bus architecture. I'll be
connecting the card to some custom-made circuitry instead of the ISA bus.
Most of what I've learned about it so far seem straightforward, but the one
thing I'd like to get a tip on, is the power supply. In addition to +5V and
Ground, the card requires a +12V input and, worse, a -12V input. What is the
standard way of providing these voltages? All I have so far is a power
supply offering one output tha can be varied continuously from 0 to 30
volts. So, do I actually need 3 power supplies, or what? >>

--

The standard way of providing the voltages is by using the PCs power supply, or
buy or scrounge a computer power supply. They have one or more of: 12V, -12V,
and 5V. These supplies commonly require a minimum load on each output in order
to work correctly.

You should not be surprised at the need for +12V and -12V, as classical "RS232"
require both polarities, where - is mark (1) and + is space (0).

Don

 

[Dbowey]

toa posted:
(snip)

<< Oh, I'm not surprised, just at a loss as to how to proceeed. Until now, that
is. I guess I'll go butcher my old 386 some more, then. Any idea what that
"mimimum load" might be? I suppose I should wire some heavy-duty resistors
in parallel with the card then?
--

I think the P.S. will be stamped showing minimum loads. The ones do that I've
handled.

Good luck
Don

 

[Willco]

RS232 interfaces require at least plus and minus 5 volts.

- Voltages of -3v to -25v with respect to signal ground are considered
logic '1', whereas voltages of +3v to +25v are considered logic '0'. The
range of voltages between -3v and +3v is considered a transition region
for which a signal state is not assigned.

The +/- 12v supplied by the ISA bus is used to power the line driver
section of an MC1488 (an RS232 Line driver IC) while the +5 volts powers
the TTL input section.


Jack
Willco Electronics

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

Hi

I'm trying to make sense of an RS232 card that I took from an old PC, which
necessitates my understanding of the PC's ISA bus architecture. I'll be
connecting the card to some custom-made circuitry instead of the ISA bus.
Most of what I've learned about it so far seem straightforward, but the one
thing I'd like to get a tip on, is the power supply. In addition to +5V and
Ground, the card requires a +12V input and, worse, a -12V input. What is the
standard way of providing these voltages? All I have so far is a power
supply offering one output tha can be varied continuously from 0 to 30
volts. So, do I actually need 3 power supplies, or what?

If the board uses the 1488 and 1489 RS-232 driver and receiver chips
then it might help if you looked at their specs.