52
Serial Bus Analysis
13 .6 UART/RS-232 BUS
The UART bus (Universal Asynchronous Receiver Trans-
mitter) is a general bus system and the base for many pro-
tocols. One example is the RS-232 protocol. It consists of a
frame with a start bit, 5 to 9 data bits, one parity bit and a
stop bit. The stop bit can assume the single length, or 1.5
or twice the length of a normal bit.
13.6.1 UART/RS-232 BUS Konfiguration
To decode the UART BUS it is necessary to first determine
which channel will be connected to the data line. This
setting is selected after choosing the bus type UART in the
BUS menu and pressing the soft menu key CONFIGURA-
TION. In the menu that opens you can press the top soft
menu key DATA SOURCE to select the desired channel via
universal knob.
The soft menu key ACTIVE can be used to determine if
the data transferred to the BUS are active high (High = 1)
or active low (Low = 1) (for RS-232, choose Low). Use the
soft menu key SYMBOL SIZE and the universal knob to
select a value between 5 bit to 9 bit for the bits that form
a symbol. Another setting can be selected via soft menu
key PARITY. Parity bits are used to detect errors during a
transmission. The soft menu PARITY offers the following
options:
For UART/RS-232 BUS trigger and decoding you need the
R&S®HOO10 resp. R&S®HOO11 option or the upgrade voucher
R&S®HV110 resp. R&S®HV111.
Fig. 13.11: UART bit sequence
Prior to the
BUS configuration it is necessary to set the correct
logic level (threshold). Please refer to chapter 4.5. The default
setting is 500 mV.
If the option R&S®HOO11 resp. the voucher R&S®HV111 is in
-
stalled, it it only possible to select analog channels as source.
If the option R&S®HOO10 resp. the voucher R&S®HV110 is ins
-
talled, both analog and digital channels are available as source.
❙
None:
Use no parity bits
❙
Even:
The parity bit is set to “1” if the number of ones in a
specific set of bits is uneven (without parity bit)
❙
Odd:
The parity bit is set to “1” if the number of ones in a
specific set of bits is even (without parity bit)
The bottom soft menu key STOP BITS allows you to define
the length of the stop bit (1 = single, 1.5 = 1 1/2 or 2 =
double). On page 2|2 of the UART BUS setup menu, you
can select the BIT RATE (symbol rate) via universal knob
in the CURSOR/MENU section. The bit rate defines how
many bits are sent per second. The soft menu key BIT
RATE allows you to select standard numeric values. Press
the soft menu key USER if you wish to define customized
rates via universal knob or numeric input (KEYPAD button).
The IDLE TIME describes the minimum time between the
stop bit of the last data and the start bit of the new data.
The sole purpose of the idle time is to define the start of a
transmission and consequently the exact start of a frame
(one or more symbols, most commonly bytes). Only this
information can guarantee correct decoding and triggering
(regardless of the trigger type). A start bit within the idle
time will not be recognized. You can enter the value via
universal knob in the CURSOR/MENU section or numeric
input (KEYPAD button).
Certain portions of the UART messages will be displayed
in color to distinguish between the different elements:
❙
White
: Start / End of complete frame
❙
Red
:
Incomplete word that is not completely
contained in the acquisition; change the
horizontal scale or move the“Time Reference“ to
get a longer acquisition
❙
Cyan
: Decoded words
13 .6 .2 UART/RS-232 BUS Triggering
After the
BUS configuration, it will be possible to trigger
on various events. Press the TYPE button in the TRIGGER
section of the control panel and choose the soft menu
key SERIAL BUSES. Then press the SOURCE button in
the TRIGGER section and choose UART. This will only be
available if it was configured earlier.
Fig. 13.13: Page 2|2 UART BUS setup menu
Fig. 13.12: Trigger menu UART data