TTL Adapters

You can also use TTL adapters instead of converting the signal to the +/- voltages required by RS-232. The Pi requirement is that the signaling side (TTL) should be capable of operating at +3.3 V instead of the usual +5 V. Using a +5 V adapter could damage your Pi. Units that can interface +3.3 V will likely have a jumper to select the voltage.

DTE or DCE

A normal USB serial adapter (for a laptop, for example) will present a DTE (male) connector. The wiring of this cable is such that it expects to plug into to a DCE (female) connection. When this holds true for your Raspberry Pi’s adapter, the laptop’s serial adapter can plug straight into the DCE (female) connector, eliminating the need for a crossover cable or null modem.

Consequently, for your Pi, choose a RS-232 converter that provides a female (DCE) connector. Likewise, make sure that you acquire for the laptop/desktop a cable or USB device that presents a male (DTE) connection. Connecting DTE to DTE or DCE to DCE requires a crossover cable, and depending on the cable, a “gender mender” as well. It is best to get things “straight” right from the start.

Assuming that you used a DCE converter for the Pi, connect the RS-232 converter’s 3 V logic TX to the Pi’s TXD0 and the RX to the Pi’s RXD0 data lines.

All this business about DCE and DTR has always been rather confusing. If you also find this confusing, there is another practical way to look at it. Start with the connectors and the cable(s) that you plan to use. Make sure they mate at both ends and that the serial cable is known to be a straight cable (vs. a crossover). Once those physical problems are taken care of, you can get the wiring correct. Connect the TX to RX, and RX to TX. In other words, you wire the crossover in your own wiring between the RS-232 adapter and the Raspberry Pi. The important thing to remember is that somewhere the transmitting side needs to send a signal into the RX (receiving) side, in both directions.

Start Bit

When an asynchronous character of data is to be sent, the line first shifts to a space level for the duration of 1 bit. This is known as the start bit (0). Data bits immediately follow.

Asynchronous lines do not use a clock signal like synchronous links. The asynchronous receiver must have a clock matching the same baud rate as the transmitter. The receiver samples the line 16 times in the bit cell time to determine its value. Sampling helps to avoid a noise pulse from triggering a false data read.

Data Bits

Data bits immediately follow the start bit, with the least significant bit first. A space is a 0 data bit, while mark represents a 1 bit. Early teletype equipment used 5 data bits sending characters in the 5-bit Baudot code.¹¹ For this reason, serial ports can be configured for 5, 6, 7, or 8 data bits. Before the ASCII character set was extended to 8 bits, it was common to use 7-bit serial data.

Parity Bit

An optional parity bit can be generated when transmitting or can be detected on the receiving side. The parity can be odd, even, or stick (mark or space). The most commonly used setting today is No Parity, which saves 1-bit time for faster communication. Older equipment often used parity to guard against errors from noisy serial lines. Odd parity is preferred over even because it forces at least one signal transition in the byte’s transmission. This helps with the data reliability.

Stop Bits

Asynchronous communication requires synchronizing the receiver with the transmitter. For this reason, 1 or more stop bits exist so that the receiver can synchronize with the leading edge of the next start bit. In effect, each stop bit followed by a start bit provides built-in synchronization.

†Unsupported by the Raspberry Pi

Baud Rate

The baud rate is calculated from bits per second, which includes the start, data, parity, and stop bits. A link using 115200 baud, with no parity and 1 stop bit, provides the following data byte rate:

where

B is the baud rate.

s is the start bit (always 1 bit).

d is the number of data bits (5, 6, 7, or 8).

p is the parity bit (0 or 1).

S is the stop bit (1, 1.5, or 2).

The 115200 baud link allows 11,250 bytes per second. If a parity bit is added, the throughput is reduced:

The addition of a parity bit reduces the transmission rate to 10,472.7 bytes per second.

Break

With asynchronous communication, it is also possible to send and receive a break signal. This is done by stretching the start bit beyond the data bits and the stop bit(s), and eventually returning the line to the mark state. When the receiver sees a space instead of a mark for the stop bit, it sees a framing error.

Some UARTs distinguish between a framing error and a break by noting how long the line remains in the space state. A simple framing error can happen as part of noisy serial line communications (particularly when modems were used) and normally attributed to a received character error. Without break detection, it is possible to assume that a break has been received when several framing errors occur in a sequence. Short sequences of framing errors, however, can also just indicate a mismatch in baud rates between the two endpoints.

Flow Control

Any link that transmits from one side to a receiver on the other end has the problem of flow control. Imagine a factory assembly line where parts to be assembled arrive at the worker’s station faster than he/she can assemble them. At some point, the conveyor belt must be temporarily stopped, or some parts will not get assembled. Alternatively, if the conveyor belt is reduced in speed, the assembly worker will be able to keep up, but at a slower than optimal pace.

Unless the serial link receiver can process every character of data as fast as it arrives, it will need flow control. The simplest approach is to simply reduce the baud rate, so that the receiver will always keep up. But this isn’t always satisfactory. A logging application might be able to write the information quickly, except when writes occur to an SD card, for example.

Which Is in Use?

So this Pi is configured to use the mini UART (/dev/ttyS0).

Disabling the Serial Console

Cursor down to select “Interface Options” and press Return (Figure 10-3).

Then cursor down (Figure 10-4) to select “Serial” and press Return.

Then choose “<No> ” to disable the console and press Return (Figure 10-5).

The dialog completes (Figure 10-6) by prompting if you want to reboot. This is necessary for a console change in the kernel. This prompt may be skipped if you already had the chosen setting.

Raspberry Zero Serial Console

Booting the Raspberry Pi Zero (not W) with the serial console enabled confirms that it uses the PL011 (/dev/ttyAMA0) device for the console:

Raspbian GNU/Linux 8 raspberrypi ttyAMA0

raspberrypi login: pi

Password:

The Raspberry Pi Zero shown in Figure 10-7 has a female header strip soldered into the board. The serial port adapter is a true RS232C converter using the MAX232CSE IC. The connections are:

• P1-01 (+3.3 V) to VCC

• P1-06 to Gnd

• P1-08 (GPIO14) to TX

• P1-10 (GPIO15) to RX

With this arrangement, there is no flow control. Given the high baud rate involved, if improved data integrity is needed, drop the baud rate to something lower like 9600 baud.

PL011 and Mini UART Differences

The mini UART has smaller FIFOs and does not support hardware flow control. Without flow control, it will be prone to losing data at high data rates. The mini’s baud rate is also linked to the VPU clock, leading to other problems.

Avoid the mini UART if you can.

RTS/CTS Access

Hardware flow controls CTS and RTS are available on GPIO 30 (P1-26) and 17 (P1-11), respectively, when configured. By default these are GPIO inputs, but this can be reconfigured. To gain access to the UART’s CTS and RTS signals, configure GPIO 30 and 17 to alternate function 5.