C type control products

The purpose of this document is to discuss the simple, fixed address control devices (CTR, CTA and the CX families) and their supported hardware.

Rolling code (K series) units will be covered in a different document.

I have updated the document to detail the products which we have designed, built and sold, but never actually promoted. These are listed in bold italic. I have not included any “theoretical” designs or anything built only as a lab prototype

The fixed address control products share a number of common features:

Simple biphase data formats.

Short burst structure (150mS or less)

Single address, user defined.

Narrowband compatible data rates and preamble timings.

Parallel I/O

One-way link

Several operating modes.

These products exist as a piece of firmware (the ‘chip’) and various applications boards using it.

CTA88. This controller uses a 1kbit/sec biphase datastream, but it is not compatible with CTR44 bursts. The basic format uses an 8 bit address field and an 8 bit data word, although the burst structure is easily expandable. Multiple operating modes are provided at transmit and receive ends.

The chip is a 28 pin device, 8 address inputs, 8 data I/O, 3 mode select bits. There is a 10% duty cycle, faster data rate version (CTA88F), usable with FPX3 radios, and the underlying coder/decoder routines are used elsewhere, such as in the CTA version of FOB1, and the CTA-to-serial interface chip used in M48 type modems.

The mk2 version includes a ‘receive address learn’ mode, and uses a more recent version of PIC (16F883 …. same as the Bull products)

The CTA88 has a wide range of interface boards designed for it:


  • 8 input (opto isolators) transmitter. LMT or BiM_T radio
  • 2 input (opto isolators) transmitter. LMT or BiM_T radio (uses inputs 1 and 4)
  • 8 relay (8 amp 240v) receiver. LMR or BiM_R radio
  • 2 relay (8 amp 240v) receiver. LMR or BiM_R radio (uses outputs 1 and 4)
  • 8 input (opto isolators) transmitter. 500mW.  LMT2 plus AFS2 modules
  • 8 input (opto isolators) transmitter. 500mW.  FPX3,  HMT2 or SHX1
  • 8 relay (8 amp 240v) receiver. FPX3 radio (rx only version)
  • 4 relay (8 amp 240v) receiver. LMR or BiM_R radio
  • 1 relay (8 amp 240v) receiver. LMR or BiM_R radi

The CTA88 is a highly adaptable byte-wide radio ‘port’

CXT,CXR   This is a firmware option offered for the LM family of radios (including the high power HMT transmitter). It is a simple, single function “actuator” device, employing a 600bit/sec biphase burst, with a 16 bit address. The address is programmed via a simple serial RS232 port.

Operating modes are controlled by the two I/O pins: input A latches output A on, while input B resets output A and also (momentarily) activates output B.

This is not offered as a “bare chip” product, but is used in the low cost (SIL package radio module) DXT/DXR two input/single relay boards.

The actual control radio modules CXT/CXR have two ‘demo/apps’ board pairs for them:  basic two button battery powered transmit / one relay receive boards and an EAS80 cased pair.

The CX code can be used on any LM type radios (HF, VHF including MURS band, or UHF)


There is also a “simple alarm” version of this product (AXT, AXR) which has two momentary inputs and adds a multiple burst “p/n dithered” transmit function to handle collisions in multiple transmitter systems.

These devices are available as simple modules, but there are also the “red button” (battery powered, cased wall mounted alarm button) and “black button” (pocket sized personal alarm) products using the AXT module. 

The CX code is fundamentally an educated doorbell (or lightswitch)

CTR44 (etc)   This was the first of the Radiometrix control products. It uses a basic 1kbit/sec data format, containing a 6 bit address field and up to two data bytes. Modes supported are momentary (output is active while input is active) and latched (output holds value last sent).

Versions are CTR44 (4 address inputs, 4 data I/O, 18 pin device),  CTR124 (4 address, 12 data, 28 pin) and CTR160 (address fixed to zero, 16 data, 28 pins), although the NBEK test device also uses CTR44 bursts (address fixed to zero, 4 data).

Hardware using the CTR44 is limited to a demo board (SIL or BiM radios, 4 relay outputs, or logic inputs), and the NBEK.

To all intents and purposes, the CTR44 should be considered obsolescent. (even though small numbers will continue to sell to certain existing users)

The chips and boards described above provide a powerful toolbox of products for industrial control tasks, but the basic parts are only half of the subject. There is a lot more that these devices can achieve, with just a little customization:


Clock rate.  All the standard fixed address control products use the same 3.58MHz ceramic resonator clock, but this is not set in stone.

Reducing the clock frequency will proportionately slow down all the timings, but will enhance range (s/n improves with the lower speed data) although extra filtering on the receiver output may also be needed to benefit from this if the clock rate is below 500KHz

Increasing clock frequency will require a re-programmed chip (HS oscillator mode) and radios capable of handling the faster switching times and higher bitrate (wideband modules) but will speed up the response time of the link.

Different radios.  Where a radio module that will not fit the LM or BiM footprint is required, then a board can be supplied with a SIL header replacing the radio module, to allow the user to wire-in an external receiver or transmitter (mounted on a remotely located carrier board or housing).

This also allows Radiometrix ‘active aerial’ devices (radios in polypipes) to be used with (for instance) CTA88 series boards.

Different housings. Most of the control devices we offer are bare boards. Although rarely listed as such, some also fit the EAS series of extrusions, while most can also be “put into” large industrial boxes as desired.

If a customer requires a specific form factor or casing, it is simple and quick to re-draught these simple module based circuits into whatever new shape is required

Lighting control. For several years, Radiometrix has supplied festive lighting controller equipment to Millennium Quest, for use (mostly) in Northampton. This equipment is based on the CTA88 data communication architechture (albeit expanded to allow simultenious control of 16 “groups” of lights and to accomodate repeater operation). By changing the software, all these receivers and transmitters/ controllers can operate in native CTA88 mode.

The “system” consists of hand-held transmitters, a repeater base (incorporating timer inputs), repeater out-stations (two, maximum) and individual single output “switch” receivers.

Some of the transmitters, the recievers and the especially base unit were lab built one-off custom devices (specific to the Northampton job) and the 2012 generation of receivers were complex and troublesome “tube mounted” devices, but the project has spun off a pair of more general designs:


            FLR2:             1 relay (8 amp 240v) receiver. LMR radio. Mains powered

                                    mounted in “Spelsberg” IP66 case. Internal aerial

FLT2              2 button hand held transmitter. LMT radio. PP3 battery

                                    also mounted in “Spelsberg” IP66 case       


Read-back.  None of these simple control devices are explicitly designed to operate in a bidirectional mode, but with the application of some low cunning it is possible to implement this function. In all cases the master transmit end cannot be set to ‘continuous’ (it is necessary to share the channel between the master command bursts and the replies)

  1. Parallel.   A transmitter board is wired to the outputs of the main receiver, and is triggered from it (by wiring the ‘burst received’ led to the transmitter C0 header pin). A second receiver board (in latched mode) is used to provide the readback port. The two links can share a channel, but must operate on different addresses.
  2. Serial.  The parallel wiring is awkward, and not always feasible. Instead, a code version can be used on the main receiver that outputs a 9600 baud asynchronous byte on the ‘burst received’ pin. The LED is then replaced with a connector, and the byte is wired to a serial-mode CTA88 transmitter. The readback receiver is the same as for the parallel example, and again, different addresses must be used.
  3. Custom. In principle the CTA88 data structure is entirely suitable for a transmit/respond architechture, if used with transceivers. In this case a fully custom design would be implemented, as has been done for Lemar in Canada and Bull (the BD118, BL118 and even ML1144 are fundamentally CTA88 developments)

The entire ML1144 project (a 15 node alarm mesh network, using BL118 hardware) is a whole subject on it’s own. As well as it’s basic function (push any button: all relays activate) it is easily expandable to provide more complex functionality. The OD28 (serial bus controlled parallel I/O) chips are a spin-off of this project.

Serial.  The usual way to use these devices is as a ‘wire replacement’, with a parallel input transmitter linking to a parallel output receiver. This is, however, not the only possible pattern of use. In some cases a master unit (a single board computer, or an industrial PC) communicates with one or more remote units.

The standard CTA88 has a serial interface mode, allowing a single transmitter to be commanded via an RS232 byte, or a single receiver’s status to be monitored. For a more sophisticated system (allowing control of transmit address, and monitoring of receive burst addresses) a ‘Serial to CTA88’ interface chip can be fitted to an NBEK or to any M48 modem board. This device allows transmission and reception of CTA88 compatible data bursts, using a simple ASCII user interface (9600 baud as standard).

Data network ?   A single (serial) master can easily command multiple receivers (each on a different frequency) and can receive databursts from a remote transmitter. Unfortunately, it is frequently necessary to monitor multiple locations, requiring multiple transmitters.

Conflict can be avoided in two ways:

  1. Random/dithered transmit mode. This sets the transmitter to send bursts at frequent but randomly separated intervals. Multiple transmitters can then share a channel, accepting occasional data burst losses due to collisions. If master transmission (to remote receivers) is also required then the master will need to send a series of consecutive bursts, to be sure that at least one gets through
  2. Polled. If the remote units are configured with a receiver as well, a command sent to this unit can be used to trigger a reply transmission from the remote transmitter. This method requires more complex hardware, but is collision immune.

More I/O  The standard range of CTA88 boards only offer up to 8 I/O paths. When more are needed there is a solution:

Boards are stacked, using modified interconnectors. Only the topmost board mounts a radio. A modified chip is used on the topmost board, using a longer burst and outputting or receiving a serial stream on the ‘burst’  led.  A new chip is used on the lower board, which just interfaces the I/O paths to the serial bus.

In this way it is possible to build 16, 24 or even 32 I/O transmitters and receivers from the existing applications boards

Different outputs  Most of the applications boards use the same 8A mains rated change over relay, but that is not compulsory:

If a logic output is needed, the relay can be omitted, and connection made to the relay coil driver. This is a 100mA rated NPN open collector output, with a back-emf protection diode to the internal 12v rail.  If greater than 100mA is needed, the output device can be replaced by a higher rated part (up to 1A). A terminal block is normally used (2 way, O/C output and +12v), but variety of connectors can be fitted.

If much higher relay currents are needed, then the open collector configuration is used, driving the coil of an off-board mounted relay (or contractor) mounted off-board. (Depending on the relay, it may be necessary to up-rate the coil driver). Digikey can supply a 30A C/O relay from stock.

The future

  There seem to be an awful lot of control products, and permutations of them. That is not a reason to stop. A statement that “we already  have everything that could possibly be needed” is both arrogance, and a flashing sign to the competition to “exploit here” !

  • So what is planned ?
  • The S39 bus ‘modular control system’,
  • Adaptors/firmware to use existing boards with S39 architecture.
  • Narrowband, higher spec. “keyfob” transmitters
  • More hand-held devices
  • More capable dedicated bidirectional control links
  • US market (27MHz band) control devices
  • Analogue I/O boards
  • Higher current relay boards
  • LED panel lighting controller/dimmer product(s)
  • Repeaters

And anything else we can think of !