As said, the TTY2PI PCB is not just an 20mA to RS232 converter. We have more signal sources:

  • the ASR-33 20mA ("TTY")
  • the RS232 DSUB-9 male ("DSUB")
  • the RPis internal hardware serial port ("RPi")
  • and further more RPi WLAN and USB ports, all capable of routing RS232 traffic outside


RS232 DSUB-9

Not much to say: its a 9-pin male connector, conforming to PC-standard.

Pinout: 2 = RxD, 3 = TxD, 5 = GND.
Additionally, DTR on Pin 4 is wired to the READER ENABLE.

RPi serial hardware "ttyAMA0"

RPis TxD and RxD Pins have 3.3.Volt levels, we use a MAX232 converter chip to interface to +/-9V RS232 voltage levels.

READER ENABLE is driven via RPi GPIO6 (header pin 31).

RPi Zero small

(click to enlarge)

And there's a challenge with the on-board UART: I had to learn the hard way that RPi Zero under Raspian has in fact *two* serial hardware ports, and different names for these ... here my research results:

  • /dev/ttyS0 is connected to the RxD and TxD header pins by default. However, its a "limited" hardware UART called "mini UART", not capable of running at 110 Baud, as the ASR-33 needs.
  • the "good" UART is /dev/ttyAMA0. It is 16550 compatible, but connected to the onboard Bluetooth module by default.
  • the one of these UARTs which is used for serial communication is linked to the fix name /dev/serial0

To use the good "AMA0" UART on TxD and RxD pins, edit file "/boot/config.txt" :

$ sudo nano /boot/config.txt[all]
# Switch serial ports

Then do all interfacing to the ASR only via device "/dev/ttyAMA0".

Three-way hardware signal flow

There are 3 possible serial signal routings, settable by a DIP switch bank.

Manual selection is necessary, because you cannot just wire all RxDs and TxDs together ... short cuts everywhere.

tty2pi signal route


1. ASR --- DSUB

ASR-33 is connected to DSUB-9, tty2pi is a typical 20mA - RS232 converter then.

tty2pi signal route TTY DSUB

You can plugin a Nullmodem-cable to the DSUB-9 to any other device.

Cave-at: when using a USB-RS232 adapter, it likely it will not support 110 bauds.
Rumor has: beware of FTDIs, test with Prolific chipsets.

2. ASR --- RPi

ASR is connected to the RPi. You can do any signal processing or forwarding there ... see next page.

tty2pi signal route TTY RPi


3. DSUB --- RPi

ASR-33 is not in use, RPi just gets a full RS232 port. Useful if you want to switch your "TTY" setup from an 20mA device to a RS232 device, perhaps for replacing the ASR-33 with a DECwriter or VT100.


tty2pi signal route DSUB RPi



No routing needed here. READER ENABLE is just a diode-wired-OR between RPi GPIO 6 and DSUB-9 DTR (pin 4): activate one of both, and READER is running on START.