FLEX is a one-way paging protocol developed by Motorola in the 1990s, used by commercial paging networks to deliver messages to pagers. It supports multiple baud rates and modulation schemes, allowing higher capacity than its predecessor POCSAG. FLEX paging networks were widely deployed across North America, Europe, and Asia, and some systems remain in operation today.

The Flex RX app receives and decodes FLEX pager messages, displaying the capcode (pager address) and message content on screen.

This is an external app (.ppma file on the SD card).

The key items that can be selected with the cursor and changed with the encoder knob are:

• Title bar: The usual items may be changed and displayed.
• Frequency: The default frequency is 931.74 MHz. Adjust to match the local paging network channel.
• AMP / LNA / VGA: Gain controls. Start with AMP off, LNA 24, VGA 24 and adjust based on received signal strength shown in the RSSI indicator.
• Config: Opens the configuration screen (logging, capcode filter, display font) — see Config screen below.

FLEX paging frequencies vary by country and operator. Some examples:

Region	Frequency
USA (primary)	931.8125 MHz
USA (secondary)	929.7125 MHz, 931.7375 MHz
USA (various)	929.0375 – 932.0 MHz band
UK / Europe	153.025 MHz, 466 MHz band
Australia	148.0125 MHz

Note: The app default (931.74 MHz) approximates the US 931.7375 MHz channel. Local frequencies depend on the operator and region. Scanning across known paging bands with Looking Glass or Scanner may help identify active channels.

FLEX uses FM modulation with automatic detection of baud rate and modulation level from the sync codeword. The following modes are supported by the Mayhem decoder:

Mode	Baud rate	Modulation	Phases
1600/2	1600 baud	2-FSK	A
1600/4	1600 baud	4-FSK	A, B
3200/2	3200 baud	2-FSK	A, C
3200/4	3200 baud	4-FSK	A, B, C, D

The app demodulates at a sample rate of 3.072 MHz (decimated to 24 kHz for symbol detection) with a channel bandwidth of 1.75 MHz.

Received messages are shown in a scrolling console. Each decoded message entry shows:

• Capcode — the numeric pager address (up to 10 digits)
• Message text — decoded content (alphanumeric, numeric, or "Tone Only")

Up to 20 messages are stored in the display buffer. When the buffer is full, the oldest messages are removed to make room for new ones. A blank line separates each message entry.

The following message types can be decoded:

Type	Description
Alphanumeric	Free-text message (7-bit ASCII)
Numeric	Digits only, using BCD encoding
Tone Only	Alert to the pager, no message text
Secure	Encrypted alphanumeric (content not decoded)
Binary	Raw binary data (not decoded)

Numbered Numeric and Special Numeric variants are also supported.

A Config button at the top of the screen opens a settings page, much like POCSAG RX. The choices are kept in the app settings and restored the next time the app is opened:

• Enable Log — writes each decoded page to LOGS/FLEX.TXT, timestamped.
• Log Raw Data — in addition, logs every packet with all of its fields (the serial output format), including system information (BIW). This is the complete capture and is not affected by the capcode filter.
• Use Small Font — shows the message list in the compact 5×8 font, fitting more lines on screen.
• Hide Tone Only — tone-only pages carry no message text and are hidden by default. Uncheck to list them as well.
• Filter Mode — None, Drop or Keep, used together with a Capcode value (64-bit, so long addresses fit):
  • None — no filtering (default).
  • Drop — hide the pages addressed to the given capcode.
  • Keep — show only the pages addressed to the given capcode.

Note: The capcode filter affects only the pages shown on screen and the normal log (Enable Log). Log Raw Data and the USB serial output always keep the complete, unfiltered capture.

The top two rows of the screen show network metadata decoded from BIW words. An RSSI meter at the top of the screen shows live signal strength.

Row 1: C<cycle>/F<frame> <bitrate> <polarity> HH:MM:SS UTC+N

Time is reconstructed from BIW TIME as Frame-0 time + frame*1.875s + ext_sec*0.9375s. If the transmitter sends an ExtSec value it is shown as UTC+N es=N.

Row 2: Local ID, coverage zone, country code from SSID BIWs. R flag = roaming supported.

The selected frequency is saved automatically and restored the next time the app is opened. The Config-screen choices (logging, raw logging, small font, hide tone-only, and the capcode filter mode and value) are saved as well. Settings are stored in SETTINGS/rx_flex.ini on the SD card.

Receiving pager transmissions may be restricted in your country. In many jurisdictions, intercepting communications intended for others — even if unencrypted — is regulated under telecommunications or wiretapping law. Check local regulations before use. Transmitting on paging frequencies without authorization is illegal.

• FLEX protocol — Wikipedia
• FLEX paging frequencies — RadioReference