I've got an Epson LX-300+ that needs a serial printer cable. I'm
using the serial port because that machine, a CNC Milling machine with
a FANUC 18im control, only has a serial port(25pin F), and the port is
working fine. I've also set the parameters on the printer to match
the parameters of the controller.
You say the port is working fine - how have you determined this?
Be aware that Standard COM ports - 9-pin or 25-pin - are invariably MALE on the
machine.
What kind of cable do I need, and what are the differences in all the
serial cables?? I looked at Startech.com, and there are so many
different types of serial cables. The Epson manual just says serial
cable, with no specs on pinout or wiring diagram. I also Googled for
"serial printer pinout" and got different answers. Some say straight
through, some say a Serial Modem cable, some say Null Modem cable,
some showed the pinouts with pins 2 and 3 crossed...I'm confused.
Do I need a Null modem cable, straight through cable, a modem cable or
what???
Someone with experience please let me know which serial cable I need
to connect a printer to the serial port on my controller. I would
also assume it should work on COM1 from a PC.
There are whole books written on the subject of how to connect serial devices
via COM ports. The reason is that the "standard" was derived before the PC was
dreamt of, and computers were mainframes with (often remote) terminals. The
standard defined two types of equipment:
Data Terminal Equipment (DTE) - the remote terminal, and the mainframe port
(unless it had a modem built in)
Data Communications Equipment (DCE) - the modems used at each end of the
intervening comms line.
'pooter DTE <--> modem <----- (line) -------> modem <--> DTE terminal.
Each of these types has a different pinout. In 25-pin numbering, transmitted
data (TxD) is on pin 2 of a DTE. Pin 2 of a DCE is receive data (RxD).
Then came the PC. It was (rightly) designated as DTE. People who wanted to
connect their remote terminal (DTE) directly to the PC COM port needed a cable
that correctly crossed over not only the data lines (2-3 and 3-2) but also
correctly configured all the necessary handshake lines to convince both devices
that the modems were there and on-line.
Manufacturers then complicated the scene by introducing serial printers
configures as a DCE so that it could be connected with a "straight" cable. And
some produced machines that could be configured as DTE or DCE at the flick of a
switch.
SO the short answer is that there is no simple universal answer to questions
like yours. You can try a number of cable types to see if you have success.
Bear in mind that the standard describes the elctrical properties of the ports,
and requires that any line can withstand an indefinite short to ground OR
indefinite contention. (Contention is when two outputs - one HIGH and one LOW -
are connected). So if the ports are compliant ( and the majority of laptops and
PC's of the last decade aren't truly compliant, but IMOE meet the above
safeguard) then you can safely test - with cables or a multimeter.
If both ports are 25-pin, then you can fairly simply determine the DCE/DTE sense
of the port with a multimeter and a resistor, or just a multimeter, as follows.
Connect the multimeter (volts scale) between pin_under_test (PUT) (i.e. 2 or 3)
and pin 7 (signal ground). Note the voltage, then apply a load of say 1K ohms
or less between the probes. If the voltage collapses the PUT is an INPUT (RxD)
but if it holds up above 3 volts it is likely an output. After testing both 2 &
3 on each machine you have at least determined their port sense and this tells
you whether you need a null modem cable or not - two the same will need a null
modem cable of some kind.
The harder part is identifying what handshake line connections the ports require
to happily Tx and Rx. That is a bit long to describe here. I suspect that's
why they produce whole books on the subject.
If you do find the ports require a null modem after testing as above, google for
"null modem pinout" and the most common ways of defeating hadshake requirements
should appear.
HTH
