2019-05-01_Linux_Format

58 LXF249 May 2019 http://www.linuxformat.com

TUTORIALS Debug serial comms

Theeasiestwaytodetermineif theoperating system

detectstheUSBcable/adaptordeviceistoinstall the

deviceaftera reboot.Installingit beforea reboot

causesthedevicedetailstobeconsolidatedinside

otherbootmessagecontent.

Froma commandlineinterfacetype dmesg. If the

cable/adapterwaspluggedintothePCbefore bootup,

thedetailswewantareembeddedinthedmesg listing.

Forthenoviceit’smucheasiertopluginthecable/

adapteraftera rebootandthenrundmesg. The new

devicedetailswillbeattheendofthelisting.

Fromtheinformationreturnedyoucanextract

whichchipset/UARTisinthecable/adapter.The

detailswillalsoidentifywhichcommunication port

hasbeenassigned.Youarelookingforsomething

like‘ttyUSB’,suffixedwithanassignmentnumber –

forexample,ttyUSB0.

Forthenextstepintroubleshootingweare going to

useanapplicationcalledPutty– itsUImakesit easy to

configuretheports.Thegoalistoconfirmthat software

onthehostPCcancommunicatethrougha USB-to-

serialadapter.OncePuttyisinstalled,openthe

application. Select ‘Serial’ in the ‘Connection type’

choices. In the ‘Serial line’ field, enter the COM port

details you have recorded previously. Proceed to the

serial connection screen. The most common, default

setup is 9600baud 8N1. The number sequence

translation means: eight bits of data, no stop bits, one

parity bit.

Give the session configuration a name and save it.

The session configuration is not saved by default when

you leave the application; if you don’t save the session,

you’ll need to input the configuration again next time.

heitchforanarticleontroubleshootingUSB-

to-serialconnectionsneededtobescratched,

afterworkingona requestforassistancefrom

a localmodelrailroadgroup(seeLXF239). Thistutorial

willprovidetheknowledgetotroubleshoot

communication(comms)issuesfromanappona PC

toa serialdevice,usinga USB-to-serialcable/interface.

Inthediagramshownbelow,thereisa PCrunning

anapplicationthatrequiresa serialinterface.ThePC

isconnectedtotheserialdevice,shownonthefarright

sideofthedrawing,usinga USB-to-serialcable/

interface.Thedrawingcontainsa fewadditionaldetails

thatwillbeintroducedasourdiscussionexpands.

HardwareinthePCsupportedserialcomms;atone

pointduringtheevolutionofthePC,a minimumoftwo

DB9ports(the9-pinCOMport)wasconsidered

standard.Theredhighlightedtextinthediagramare

someexamplesofCOMportcommunicationneeds.

Butallthingscometoanend,andtheubiquitousUSB

portsoonreplacedCOMports.Toconsolidateallthe

differentserialdevicecommsrequirements,a USB-to-

serialcable/adaptorcameintoplay.Unfortunately,this

generic serial comms doesn’t always work, and it can be

frustrating trying to isolate the root cause of serial

communication failure.

After connecting the USB-to-serial cable to the host

PC, the first step is to determine if the OS recognises

the USB-to-serial adaptor. In Linux, the dmesg

command reveals information regarding hardware

detection, driver assignments and communication port

allocation. It is important to take note of what COM port

(COM1, 2, 4 or so on) is used – this will be needed to

support our efforts, as well as set up the application.

Serial connection

path mapping,

with additional

landmarks shown.

Debug serial comms

SeanD.Conway
is a retired IT
security specialist
who now
exercises
his brain by
mining knowledge
through
Raspberry Pi
projects.

T

COMMS

OUR

EXPERT

Sean Conway walks you through the process of troubleshooting

communication problems between a PC application and a serial device.

Serial Application

(Putty)

Data

Data

DB9Connector

(jumperfortesting)

+

PC

CH341 Chip

USB Connector Device Connector

Serial Connection

(USB to Serial cable/adapter)

CD 11 CD

RxD 22 RxD

TxD^33 TxD

DTR^44 DTR

SG^55 SG

DSR^66 DSR

RTS 77 RTS

CTS 88 CTS

RI 99 RI

Serial Device

NULL Modem (DCC)

(Optional)

UART/RS232/

RS485/RS422

Parallel printer to

USB printer

EPP parallel and

MEM parallel

synchronous serial2wire/4wire

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