null modem signal flash

Thread Starter


Joined Dec 7, 2008
hello, i have a thermal printer that runs on a parallel to serial mull modem cable, im trying to figure out how i could do a simple circuit to put led's in line with the cable and have them turn on and remain on when a print job is sent, then to turn them off i would like to press a button, you know, kind of like a confirm i picked the printed paper up type thing.

im thinking 555, and thats about all i know.... what pins or such should i use and what type of curcit could i use? anything i can lookup, because im not good with doing things from scratch...

thanks a lot everyone!

Thread Starter


Joined Dec 7, 2008
so here is the pinout for my cable i guess ( i have db25 to db9)

Null-Modem Pinouts - DB-25 to DB-9
2 to 2
3 to 3
4 to 8
5 to 7
6 and 8 to 4
7 to 5
20 to 6 and 1

Pin assignments - DB-25
2 TD Transmit Data
3 RD Receive Data
4 RTS Request to send
5 CTS Clear to send
6 DSR Data Set Ready
7 SG Signal Ground
8 DCD Data Carrier Detect
20 DTR Data Terminal Ready

Pin Assignments - DB-9
1 DCD Data Carrier Detect
2 RD Receive Data
3 TD Transmit Data
4 DTR Data Terminal Ready
5 SG Signal Ground
6 DSR Data Set Read
7 RTS Request to Send
8 CTS Clear to Send

would i use pin tx or rx on the null cable?


Joined Dec 10, 2005
im thinking 555

I absolutely despise the 555 timer. It has a reputation for being the most useful electronics component, when really the thing sucks for just about anything anyone ever uses it for. I have a few of them, but I quickly learned they have a lot of problems, and something else is usually a better choice. For example, a 74HC123 can be set up as an astable multivibrator (the most frequent use of a 555 timer seems to be to replace an otherwise too-difficult-to-build oscillator), and unlike a 555, it won't become completely disfunctional when you try to use it above audio frequencies. ...but I digress...

Something like the attached schematic might work, depending on the exact voltages of your serial transmission, and the voltage expectations of your logic chips. You might have to increase the value of that first resistor if the circuit works but your printer doesn't. If it doesn't work at all, you might need something like the 1489 to convert the RS232 levels to TTL levels.

I don't know if you need the inverter or not. The bitstream is inverted so many times between the PC and the cable that I can never remember which way it goes. Anyway, the idea is that the two NAND gates work as a bit of memory, and activity on the serial line sets the bit, and then pushing the button clears it. Hopefully your computer doesn't talk to your printer when it doesn't have anything for it to do.


Thread Starter


Joined Dec 7, 2008
hmm, very interesting.

i just realized that i can simplify everything a BUNCH, being that this printer is a receipt printer, it has a rj-11 jack for hooking up a cash drawer that will open upon printing, and by using the advanced settings there could be a trigger for detecting the open and close status of the drawer.

which is PERFECT for what i want, when i print, turn the led's on (open drawer), and when someone presses a button turn the light off (drawer closed)

its an epson tm-t90


Joined Feb 19, 2009
It could be as simple as an SCR driving an LED, with the gate of the SCR attached to the TX through a diode (to keep the -12V away) and a momentary reset switch.

But, you already found a solution.

Thread Starter


Joined Dec 7, 2008
im actually now gong to not interface to the printer at all, i posted another thread for this but, im going to put some led's and buttons on the serial port and just read the bits like that. which i can incorporate into the program that i wrote to print to the themral printer.

thanks everyone:)