tashtalk/documentation/prototype.md

TashTalk Prototype Details

Raspberry Pi

A Raspberry Pi 3 Model B was used. Note that these instructions are specific to this model and may differ for other models.

UART Setup

raspi-config

Under Interface Options > Serial Port, answer "No" to "Would you like a login shell to be accessible over serial?" and then "Yes" to "Would you like the serial port hardware to be enabled?".

/boot/config.txt

Add the following lines:

• dtoverlay=miniuart-bt
  • Ordinarily, the Bluetooth device uses the PL011 UART (/dev/ttyAMA0); this line forces it to use the mini-UART instead. We want the PL011 UART available because it's the only one that supports hardware flow control.
• gpio=16-17=a3
  • This line switches GPIO16 and GPIO17 to be CTS and RTS for hardware flow control.

Connections

PIC12F1840 Pin	RPi 3 Pin
01 Supply	01 3v3 Power
02 RA5/LocalTalk Out
03 RA4/Driver Enable
04 RA3/LocalTalk In
05 RA2/UART CTS	36 GPIO16/CTS
06 RA1/UART RX	08 GPIO14/TXD
07 RA0/UART TX	10 GPIO15/RXD
08 Ground	06 Ground

Driver/Receiver

While the LocalTalk bus is half-duplex, LocalTalk and PhoneNet dongles use a hybrid transformer to accommodate the SCC's separate TxD and RxD lines. Because a PhoneNet dongle was used for the prototype, a pair of SN65HVD08 were used, one as a receiver and one as a driver.

Other RS-422/485 drivers may be suitable, but it is important to experimentally verify that their receivers output a logic '1' when the bus is not being driven.

Connections

Receiver SN65HVD08 Pin	PIC12F1840 Pin	LocalTalk/PhoneNet
01 R	04 RA3/LocalTalk In
02 !RE	08 Ground
03 DE	08 Ground
04 D
05 GND	08 Ground	SGnd
06 A		RxD+
07 B		RxD-
08 Vcc	01 Supply

Transmitter SN65HVD08 Pin	PIC12F1840 Pin	LocalTalk/PhoneNet
01 R
02 !RE	03 RA4/Driver Enable
03 DE	03 RA4/Driver Enable
04 D	02 RA5/LocalTalk Out
05 GND	08 Ground	SGnd
06 A		TxD+
07 B		TxD-
08 Vcc	01 Supply