>
> Hi,
> I am working on a home brew SBC and am trying to build
a serial cable for
> it. Are there any RS-232 serial port gurus who could
help me plumb out a
> custom cable?
>
> The SBC serial connection works but as it currently is
configured it only
> works as 3 wire serial with no flow control (ie, RD,
TD, & GND pins).
>
> I would very much like to make the serial cable and SBC
support hardware
> flow control. Having the DSR/DTR pins connected should
allow it. However
> the PC is expecting CTS/RTS hardware flow control.
I find it better to think in terms of 'handshake in' and
'handshake out'
pins for things like this. Sure it's completlely mangling
the RS232 spec,
but any use of hardware flow control does that (at least
according to the
spec I've read).
In your case :
On the PC, Handshake out is RTS (prssuambly asserted whrn
the PC is ready
to receive). Handshake in is CTS (the external device
asserts this when
it is readt for the PC to send something to it)
On the SBC, handshake out is DTR (asseted when the SBC is
ready to
receive I guess), handshake in is DSR (asseted by the PC
when it is ready
to receiv soemthing from the SBC).
>
> My SBC UART is wired so that the RD, TD, DTR, DSR, and
GND pins are brought
> to the serial connector. I probably should have
brought CTS & RTS out but
> it is too late to change it now.
>
> I have wired a cable like this which works for 3 wire
serial (no flow
> control)
>
> PC DB25 (9 to 25 pin cable) SBC (25 pin female)
> 2 TD----------------------RD
> 3 RD----------------------TD
> 7 GND---------------------GND
> 6 DSR---------------------DTR
> 20 DTR---------------------DSR
That would, I think, work for hardware flow control if the
PC was using
DSR/DTR flow control, not RTS/CTS.
>
> Obviously this cable does not support hardware flow
control because when I
> try to connect with hardware flow control enabled, I
can see the SBC boot
> message on the terminal screen but cannot send
characters from the PC to the
> SBC.
>
> So I have been experimenting trying to figure out how
to fix this. I got
> the data sheet for the 16C550 and a serial port
breakout box. After much
> combinations, I discovered that if I connect a jumper
between the PC serial
> port pins 5 (PC CTS) and pin 20 (SBC DSR) then the
serial port *DOES*
> support hardware flow control. At least it appears to.
I get the feeling
> though that I am mixing dissimilar control signals and
it is confusing me.
>
> My question is, does the above configuration with the
PC CTS and SBC DSR
> connected really use hardware flow control or is it
just "faking out" the PC
> and/or SBC UART? As a follow on, are both sides
actually getting hardware
> flow control or is it just one side or the other?
>
> This is my new and improved serial cable with *appears*
to support hardware
> flow control. Is there any way to test this?
>
> PC DB25 (9 to 25 pin cable) SBC (25 pin female)
> 2 TD----------------------RD
> 3 RD----------------------TD
> 7 GND---------------------GND
> 6 DSR---------------------DTR
> 20 DTR----+----------------DSR
> 5 CTS----+
Alas that's 'Faking it' I think. DTR is an output on the PC,
it's being
sent back to the PC's CTS (which is the handshake in line),
so the PC is
always being told it can transmit (I assuem DTR is aserted
all the time
by the PC). And by connecting that to DSR on the SBC, you're
telling the
SBC that it can send any time it likes.
How about the following :
PC SBC
TxD ---------- RxD
RxD ---------- TxD
Gnd ---------- Gnd
RTS ---------- DSR
CTS ---------- DTR
DSR--+
DTR--+
-tony
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