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Remote Modem Are you a ‘control freak’? Want to control and measure things from a distance? If you have an old modem sitting idle, here’s the ideal project. It simply connects to a standard modem and the flexible interfacing makes it suitable for a vast array of control and measurement applications. By Leon Williams H ave you ever been away from home, say on holidays, and wished that you could turn on the lights, feed the dog, see what the temperature of the tropical goldfish tank is or measure the voltage of your burglar alarm battery? Or ever been at the office and forgotten to water the garden, or worse, forgotten to turn the sprinkler off? Well, the answer is to build the Remote Modem Controller (RMC). Together with a PC and modem, the RMC you can turn things on and off, monitor inputs, measure voltages, measure the temperature and count events whether you are next door or on the other side of the world. You don’t need any fancy software, as it interfaces with just about any PC running terminal emulation software. The RMC is housed in a standard plastic instrument case and has room for extra interfacing or transducer circuits. The rear of the unit has a DC input socket, a 9-pin male D connector for the modem and a 25-pin female D connector for the serial cable to the PC. Operation is simple. Before you leave home, you connect the RMC FEATURES                Easy to build single sided PC board PIC16C73A microcontroller 6-digit password login security Idle timeout protection Individually turn on/off four outputs Monitor four opto-isolated digital inputs Measure temperature between -10°C to +60°C in one-degree steps Measure two separate voltages from 0 to 20V DC in 100mV steps Capture or count slow-occurring events (maximum count of 255) 9V DC input at low power No special software required Easy interactive menu operation; single key-stroke commands Simple 3-wire RS232 control interface (4-wire for modem) EEPROM stores password and system data Operated remotely via a modem or by direct connection to a PC 16  Silicon Chip to your modem which is plugged into the telephone line. Later on, when you are far away, you dial into your RMC from a remote PC and modem. The modem answers the call and the RMC asks for a password. If the password is accepted, you are presented with the main menu, from where you select the various options. When you are finished you select logoff and the RMC commands the modem to terminate the call, ready for the next one. As well as doing all this remotely, m Controller the RMC can be directly controlled by your PC to measure and control things locally. Inputs/Outputs The RMC’s inputs and outputs can be used for an unlimited number of The outputs are open-collector transistors and incorporate a clamping diode which is used if relays are being controlled. If a logic output is required, then a pull-up resistor can be connected between the respective collector and the positive DC rail. With these features it is simple to interface with logic I/O, switches, relays, sensors and transducers. Fig.1 shows some input and output circuits. PIC microcontroller applications. The inputs and event counter are isolated by optocouplers and only require a few milliamps to operate. The analog voltage inputs, while not being electrically isolated, are protected against over-voltage and reverse polarity. A resistive divider has an input impedance of 10kΩ and converts the maximum measurable input voltage to a safe voltage for the microcontroller’s A-to-D converter. The temperature sensor input is designed to match a LM335 temperature sensor, which has an output voltage of 2.73V at 0°C. The star of the show is undoubtedly the PIC16C73A microcontroller. It comes in an unusual 28-pin skinny (0.3") DIP package but it has a lot packed inside it. Some of the internal features are: * 4K OTP (One Time Programmable) program memory * 192 bytes of user RAM * 22 I/O pins * 3 timer/counters * a full duplex UART * a 5-input A-to-D converter Circuit description Fig.2 shows the complete circuit diagram. IC1 is the PIC16C73A micro- controller. Pins 9 and 10 provide the crystal oscillator using X1, C10 and C11. The frequency of 3.6864MHz was chosen to allow the internal Baud Rate Generator to provide accurate baud rates. Pin 20 is the positive supply input while pins 8 & 19 are connected to 0V. Pin 1 is the reset input, connected to +5.12V via a 10kΩ resistor. The PIC has an intelligent internal power up reset circuit and as long as Vcc rises quickly, no extra power reset circuit is required. Pins 21 to 28 are assigned to general purpose port B with pins 21 to 24 configured as outputs and pins 25 to 28 as inputs. Pins 11 to 18 are assigned to port C. Pin 18 is the UART Receive data input, pin 17 is the Transmit data output and pin 15 is the DTR output. Pin 16 is used to control the LED. Pins 11 to 14 are used to interface to the EEPROM (IC3). It is used to hold the password, speed setting and other system data in case of a power failure. It can store 64 by 16 bit words. Data is written to and read from the EEPROM in serial form synchronously with the rising edge of the clock input. The Chip Select (CS) input, pin 1, must be high for any read or write commands to be accepted. A 10kΩ resistor (R4) is used to pull the CS line low when powering up or down to avoid possible data corruption. Data to the EEPROM is at pin 12, data from the EEPROM is pin 11, the clock signal is pin 13 and the Chip Select signal is pin 14. The last port, port A, is associated with the Analog to Digital (A-to-D) converter. Pins 2 and 3 are configured as A-to-D inputs 0 and 1. Pin 4 is a digital input that is sampled at power up to determine if the Default pins are shorted. More on this later. Pin 5 is the temperature sensor input and the voltage on this pin is directly proportional to the temperature. Pin 6 is not connected to the A-to-D converter but instead is connected to the clock input of an internal counter (Timer 0). Pin 7 is not used. The referAUGUST 1999  17 Fig. 1: some of the input and output circuits which could be used with the Remote Modem Controller. There are many more which could be devised. ence voltage for the A-to-D converter is internally connected to the +5.12V rail (Vcc). IC2, an MAX232, is a standard RS232 transceiver used to interface the 5V logic signals in and out of the PIC to the modem and serial ports. It only requires a +5V power supply and produces the required plus and minus RS232 voltages by an internal inverter which employs capacitors C1 to C4. IC2 has two receivers and two transmitters but one receiver (pin 8) is not used here. Pin 13 is the receive data input, pin 7 is the transmit data output and pin 14 is the Data Terminal Ready (DTR) output. P1 is a 9-pin male D connector and J1 is a 25-pin female D connector. In case some communications packages running on the PC require active CTS, DSR and DCD signals, they have been looped to PC outputs so that they will be on whenever the PC is connected. LED1 has the following states: Off when power is disconnected or off for five seconds when clearing a call. It flashes at a slow rate when powered up and the unit is attempting to match the interface speed with the modem or PC. When the unit has matched the modem or PC and is waiting for a connection, it flashes at a faster rate of around 1Hz. When a call is in pro18  Silicon Chip gress, LED1 is permanently on. Connector J2 is the connection point for the analog inputs. Each input has an attenuator made up of three resistors, 1.8kΩ, 6.2kΩ and 2kΩ. This unusual combination is used to allow easy software manipulation of the A-to-D value. The A-to-D converter has a resolution of 8 bits or a maximum value of 255. With this attenuator, an input voltage of 20V gives 4V at the A-to-D input pin and a conversion value of 200 – making the software task a lot simpler. Zener diodes ZD1 & ZD2 protect the PIC inputs from over-voltage and reverse voltage, although reverse voltage inputs should be avoided. The maximum voltage that can be measured is 20V, however the inputs can withstand overvoltages up to say 50V for a short duration. A .01µF capacitor is connected across each PIC input to filter out noise and minimise A-to-D conversion errors. Op amp IC5b, one half of an LM358, is the temperature sensor interface and intended to be used with an LM335 temperature sensor. This gives an output voltage relative to zero degrees Kelvin and which increases by 10mV/°C. At 0°C the calibrated voltage is 2.7315V. IC5b has a gain of -4 which results in the output voltage at pin 6 changing at a rate of 40mV/°C, decreasing with increasing temperature and vice versa. Trimpot VR1 acts as the calibration control. A 4N28 optocoupler, IC6, is used to interface the Events input (J4) to pin 6 of IC1. Resistor R18 limits the current passing through the internal LED in the optocoupler, while diode D2 protects it against reverse voltages. The value of R18 (560Ω) is chosen to provide about 5mA of input current when interfaced to a 5V logic output. Higher voltage input signals will require an external resistor – see Fig.2. R17 pulls up the open collector output of IC6 and provides a high when the input is off (no input current) and a low when the input is on (current flows into the optocoupler). Capacitor C13 acts as an integrator, filtering out any high frequency edges which may occur if switches without debouncing are used as inputs. The Event input connects to an edge-sensitive counter within IC1 and any transitions other then the one wanted will result in false Event readings. However if the inputs are very noisy, an external debouncing circuit will be needed. IC7, IC8, IC9 and IC10 and associated resistors and diodes are the optocoupled inputs. They operate in the same way as the Events input. IC4 is a ULN2003A which interfaces four outputs of IC1 to connector J6. It has open-collector transistors which can each sink 350mA and have a maximum collector voltage of 50V. IC4 is suitable for driving relays as well as providing a logic output by connecting a resistor between the collector of the transistor and the +5V supply. Connector J5 has a +5V point which can be used for this purpose. Note that when using the digital output option, the output is low (0V) when it is on and high (+5V) when the output is set to off. The +5V point on J5 is only intended for this purpose and the internal power supply is not designed to power multiple relay coils. The power supply consists of an LM317T 3-terminal adjustable voltage regulator (REG1). The DC input is filtered by C14 and protected against reverse voltages by diode D1. The output voltage is adjusted to 5.12V by trimpot VR2. It is set to 5.12V rather than 5V to provide the correct reference voltage for the A-to-D converter within IC1. Fig. 2: the circuit diagram for the complete controller. Calibration of the temp-erature input (based around IC5b and using VR1) is not covered in the text but is menu-driven and will be self-explanatory when this menu is displayed. Voltage setting (using VR2) is important and is fully covered in the text. AUGUST 1999  19 Parts List 1 PC board, code 07408991 1 Plastic instrument case 200mm x 160mm x 70mm 4 3 way PC board mount screw terminals 5 2 way PC board mount screw terminals 1 9 pin male right angle PC board mount D connector (P1) 1 25 pin female right angle PC board mount D connector (J1) 1 DC chassis socket (to match plug pack) 2 200 ohm horizontal trimpots (VR1, VR2) 1 3.6864MHz crystal (X1) 1 28 pin 0.3" IC socket (can be 2 14 pin sockets) 6 PC stakes 4 No 4 x 6mm self tapping screws 1 LED mounting clip Hookup wire Semiconductors 1 PIC16C73A-04/P pre-programmed microcontroller (IC1) 1 93LC46B 64 x 16 bit EEPROM (IC3) 1 MAX232 RS232 transceiver (IC2) 1 ULN2003A solenoid driver (IC4) 1 LM317T 3-terminal regulator (REG1) 1 LM358 dual op amp (IC5) 5 4N28/4N25 optocoupler (IC6 - IC10) 1 5mm green LED (LED1) 1 1N4004 1A diode (D1) 5 1N4148 signal diode (D2 - D6) 2 18V 1W zener diode (ZD1,ZD2) Capacitors 1 470µF 25VW PC electrolytic 6 10µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 4 0.1µF monolithic (code 100n or 104) 2 .01µF ceramic (code 10n or 103) 2 22pF ceramic (code 22p or 22) Resistors (0.25W, 1%) 1 120kΩ 1 30kΩ 9 10kΩ 1 1.5kΩ 1 1.3kΩ 5 560Ω 2 6.2kΩ 1 620Ω 3 2kΩ 1 330Ω 2 1.8kΩ 1 240Ω A pre-programmed PIC16C73A microcontroller and a 93LC46B EEPROM are available for $30.00 including postage (cheque or money order) within Australia from L. Williams, 14 Powell Street, Bungendore NSW 2621. email lmwill<at>alphalink.com.au, http://www.alphalink.com.au/~lmwill blocks, noting that there are 2-way and 3-way types. Follow these with the PC stakes and finally the two D connectors. With the PC board complete, place it on the pillars in the bottom righthand half of the case. You will find that some of the pillars are directly under soldered connections and the PC board does not sit flat. The solution is to remove the offending pillars with a large pair of side cutters or drill them out. Now comes the tricky part: you need to mark and cut out the rear panel so that the two D connectors protrude through it with enough room around them to clear the mating cables. Slide the rear panel into its slot, place the PC board up against the rear panel and mark where the rectangular cutout should be. Take your time with the cutout. Although the plastic is soft, forcing the cuts can easily break the panel. Before installation, drill a suitable hole and mount the DC power supply socket on the rear panel. You will also need to drill a 10mm hole in the rear panel and fit it with a grommet so that you can easily run wiring into the case at a later stage. The lefthand side of the case has ample room for interfacing circuits. Once this is done, place the rear panel into the slot in the bottom half of the case and screw the PC board down with four self-tapping screws. Drill a hole in the front panel for the LED, install it with a mounting clip and then slide the panel into the bottom case half. Wire the LED and the DC input socket to the PC board stakes with hookup wire, noting that they are polarised. DC supply adjustment The power supply input voltage is nominally 9V DC but a 12V DC plugpack will be acceptable. The circuit draws about 40mA at 9V and REG1 should not normally require a heatsink with this voltage. Construction Before you start assembly, check the PC board for any faults, especially where the tracks run between IC pads. Check also that all the PC board holes are correctly drilled. Refer to the component overlay diagram of Fig.3 and solder the resistors in first. If in doubt use your 20  Silicon Chip multi-meter to check each value. Install the IC sockets and the trimpots, followed by the capacitors, double checking that the electrolytics are in the correct way. The diodes can be installed next, again checking their polarity. Install the voltage regulator, the crystal and all the ICs except for the PIC which is plugged into its socket later on. The ICs don’t all face the same way, so study the component overlay closely. Any mistakes here could result in PSD (premature semiconductor death). Install the PC-mount terminal Leave the PIC chip out of the socket at this stage. Adjust trimpot VR2 to its mid position and plug the power supply into the DC socket. Using a digital multimeter, measure the power rail (Vcc); it should be around 5V. If not, switch off immediately and check your work. Look for shorts or components in the wrong way or simply that the DC connector is wired incorrectly. Once everything is OK, adjust VR2 until Vcc is exactly 5.12V. The accuracy of the voltage measurements depends on the setting of Vcc. The expected calibrated accuracy is ±100mV. The resistors in the input atten-uators can also introduce some errors but with 1% values the accuracy of measurements should be acceptable under most circumstances. Turn off the power and insert the PIC micro. Now comes the big moment – turn on the power again. The LED should come on for a few seconds and then flash slowly with a period of about five seconds. The flashing is good news; the microcontroller is alive and well! Turn the power off again. Initial set-up Now that you have your Remote Modem Controller built, it can only be accessed by entering a correct password. The password is stored in the EEPROM along with the speed and other system data. When the unit is powered up for the first time, the EE-PROM contents and hence the password, are unknown. We need a way of bypassing the unknown password and this is done with the default operation. When the unit is powered up it senses the state of the default pins and if they are shorted, the EEPROM is programmed to set the speed to 9600 and the password to 123456. Note that this default operation requires physical intervention at the actual unit and cannot be done remotely. Follow the steps below to set the password and speed for the first time: 1. Power up the PC and run the terminal emulation software. This can be Windows 3.1x or Windows 95 Hyperterminal, for example. Set the communications parameters to 9600bps, 8 data bits, no parity and 1 stop bit. 2. Connect the PC to the RMC with a 9 to 25-pin cable. The 25-pin end plugs into the RMC. 3. Short the default pins and apply power. Note that a default mode message is displayed on the PC screen. You will also be asked to remove the link and press any key to continue. 4. Remove the link on the default pins. If you do not remove the link, the message will be displayed again after a key is pressed. This will continue until the link is removed and is protection against the possibility of leaving the link in place. 5. Press any key as prompted. The Setup menu will be displayed. If a key is not pressed within 30 seconds, the default operation is aborted. 6. Enter P to access the Password sub-menu and then C to change the password. 7. Enter your new password. It must be six characters long, can be any combination of printable ASCII characters and is case-sensitive. Press S to store the password and return to the Setup menu. 8. Press S to access the Speed submenu. Select the desired speed, by pressing 1, 2 or 3 and then M to return to the Setup menu. 9. Press M to go to the Main menu, then L to logoff and finally press Y when prompted. 10. Turn the unit off for 10 seconds and then on again. 11. If you have changed the speed from the default of 9600, change the PC speed to suit. You will now see the letters AT appear on the screen. This will be the unit training the connected device to the correct speed. Press the letters OK within two seconds of AT appearing. If you are too slow, AT will be displayed again in five seconds. 12. Type in the word LOGIN (upper or lower case) to log into the unit. You will be asked to enter the password. 13. Type in the new password exactly as entered above. 14. If the password and speed are correct, the RMC opening banner will be displayed. Press any key as prompted. 15. The Main menu will be displayed. Note the message indicating that a power reset has occurred. This is only displayed if there has been a power off/on cycle since the last login time. 16. Enter L to logoff but enter N to cancel when prompted. The Main menu is displayed again and note that the power reset message has disappeared. The password and speed settings can be changed anytime in the future, once you are logged on. Modem and cables The RMC should work with just about any modem as long as it is AT command set compatible. This means that you ‘talk’ to the modem by sending it commands preceded by the AT attention sequence and the modem responds to these commands. Although modems that are AT-compatible are basically all the same, there are differences between them Table 1: Resistor Colour Codes             No. 1 1 9 2 1 2 1 1 1 5 1 1 Value 120kΩ 30kΩ 10kΩ 6.2kΩ 2kΩ 1.8kΩ 1.5kΩ 1.3kΩ 620Ω 560Ω 330Ω 240Ω 4-Band Code (1%) brown red yellow brown yellow black orange brown brown black orange brown blue red red brown red black red brown brown grey red brown brown green red brown brown orange red brown blue red brown brown green blue brown brown orange orange brown brown red yellow brown brown 5-Band Code (1%) brown red black orange brown orange black black red brown brown black black red brown blue red black brown brown red black black brown brown brown grey black brown brown brown green black brown brown brown orange black brown brown blue red black black brown green blue black black brown orange orange black black brown red yellow black black brown AUGUST 1999  21 Fig. 3: all components, with the exception of the “data” LED and the DC power socket, mount on the PC board. Terminations are made directly to the screw terminals while the modem and PC connections are via rear-panel sockets. and so you will need to check your modem user manual and ensure that it is configured correctly. The RMC needs to detect the string RING to indicate an incoming call from the modem and the string CONNECT to indicate that the modem has made a connection to the remote PC. As a result, the modem must be configured to enable the call progress results in verbal form (typically ATV1). The RMC forces the modem off-line at the end of a call by turning the DTR line off. The modem must be con-figured to return to command mode when the DTR line is taken low (typically AT&D2). The speed at which a modem talks to the connected PC can be configured in a number of ways. The RMC has the ability to remotely change its speed in the Setup sub-menu. If the modem was set to a permanent speed and the RMC speed was changed, the two 22  Silicon Chip could never communicate. To avoid this situation, the RMC sends the command AT to the modem to train the modem to the new speed. This is done after power up every five seconds until answered with OK and in call-waiting mode every 30 seconds but does not require the OK response. The modem therefore must be programmed so that it monitors the PC data and sets its interface speed to that of the PC (typically AT&I1). Note that this does not refer to the actual data rate between the modems but the speed at which the modem talks to the RMC. This is often referred to as ‘auto-bauding’. The other important modem settings are asynchronous operation (typically AT&M0) and no-flow control (typically AT&K0). The RMC does not provide a modem RTS signal and if the modem is set for RTS/CTS flow control, the modem will not send and receive properly. Also you need to ensure that there are at least two rings, before answering so that the string RING can be detected. This is normally set in S register 0. All these settings will probably be invoked by forcing the modem into its default configuration (typically AT&F), but as indicated, not all modems are the same. Remember to store the modem settings in non-volatile memory with the AT&W command after you have made your changes. The RMC has two ports. The first is the 9-pin male D connector for the modem. Only four signals are used: Receive Data from the modem (pin 2), Transmit Data to the modem (pin 3), Data Terminal Ready (pin 4) and Signal Ground (pin 5). The second port is a 25-pin female D connector which is used to connect directly to a PC. Only three signals are used: Transmit Data from the PC (pin 2), Receive Data to the PC (pin 3) and Signal Ground (pin 7). RTS and CTS are looped and DSR, DTR and CD are also connected together internally. You can easily make up your own cable but the interfaces have been designed to support a standard 9-pin to-25 pin PC to modem cable. The input Data (Pin 2) of each D connector is connected in parallel, so ensure that only one cable is plugged in at a time to avoid loading the RS232 drivers in the modem and PC. Operation and menus Using the RMC is very easy. From the menu system all functions are accessed with a single keystroke, without the need to use the Enter key. As discussed before, when the unit is powered up it sends out the letters AT, waiting for an OK response. This OK will come automatically from a modem if it is powered on and connected, but if you have the PC connected you will need to enter this manually. After a call is finished, the unit polls the connected device every 30 seconds, again with the letters AT. This is done to avoid a problem that could arise if the unit is powered up but the modem is powered off and then on. In this case it could be poss-ible for the unit and the modem to be ‘talking’ at different speeds. The 30-second spaced AT makes sure that the modem is at the correct speed when a call comes in. Incidentally, Looking from the rear to the front panels across the PC board. As you can see, there's plenty of room inside the case if you wish to add in other sensor/controller devices. Power is from an external 9V DC plugpack, closely regulated on board. the temperature is read at this point, updating the minimum and maximum temperature settings. If you are directly connected, you need to type the letters LOGIN in upper or lower case to access the unit. If it is connected to a modem and a call is detected, it sends ATA and raises the DTR signal. One or both of these steps may not be necessary, especially if the modem is set to auto-answer, however it is good insurance. In any case, the DTR signal should be used to drop the modem off line at the end of a call. When the unit has answered the call, it waits 30 seconds for the CONNECT message from the modem. If this fails to be recognised within the 30-second period, the call is aborted. When either the login command has been entered from a PC or the connect message has been received from the modem, the password prompt is displayed. Here you are given two chances to enter the password correctly. The password must be entered within 30 seconds and is case-sensitive. If the password is twice entered incorrectly, the call is aborted. Once the password is entered correctly the opening banner is displayed. You are asked to press any key to proceed. Once a key is pressed, the Main Menu is displayed. If a key is not pressed within 30 seconds the call is aborted. If there has been a power failure since the last call or log in, a message will be displayed indicating that there has been a failure, that the outputs are turned off and the Event counter has been reset. This is done to avoid possible dangers that could be caused by turning the outputs on again when the power is restored, if it has been off for a long period. Also the Event counter could provide a meaningless result if lots of events were missed during this period. This message is not displayed again during this call. Each of the sub-menus are accessed from the Main menu by pressing the first letter (in brackets) of the submenu required. There is a programmable idle timeout on the menus –selected in the Setup menu. If you are in a sub-menu and a character is not entered within the timeout period, you will be asked to press any key to continue. If a key is pressed you will be returned to the Main menu, however if a key is not pressed within 30 seconds the call will be aborted. If you are in the Main menu and no characters are entered within the timeout period you will be advised that the idle timer has expired and you are asked if you want to continue. Again pressing any key will restart the timer and if there is no response to this question within 30 seconds the call is aborted. AUGUST 1999  23 The rear panel is positively spartan with a DB25 socket (computer connection), DB9 socket (modem connection) and a 2.1mm DC power socket. The grommeted hole (right side) is for cabling which connects to the internal terminal strips. Fig. 4: this full-size front panel artwork can be copied and glued to the front panel and/or used as a drilling template. In the Volts measurement screen, the voltages on the two analog inputs are measured and the results displayed. Pressing the letter U forces a new measurement and updates the screen while pressing M returns you to the Main menu. If the input voltage is measured 24  Silicon Chip as being over 20V then an OVER V warning message is displayed. The Input screen shows you the state of the four inputs as being ON or OFF. Again pressing the letter U updates the screen and ‘M’ returns you to the Main menu. The outputs sub-menu allows you to individually turn each output on or off by pressing 1, 2, 3 or 4. Each time the number is entered the corresponding output changes state and the screen is updated. As long as the power to the unit is not removed between calls, the outputs will remain unaltered. Pressing M once again returns you to the Main menu. The Event counter is displayed in the Events sub-menu. If the counter has passed 255, the word OVERFLOW! will be displayed. The result can be reset to zero by entering the letter R. Pressing U will update the display and pressing M returns you to the Main menu. Entering T moves you to the temperature sub-menu. Here the current temperature is displayed as well as the maximum and minimum temperature since they were last reset. Pressing R resets the maximum and minimum temperatures to be the same as the current temperature. Pressing U updates the display and M returns you to the Main menu. Entering the letter L tells the unit that you wish to end the call, however it asks if you are sure. If you wish to continue, press the Y key or the N key to end the call. To access the Setup menu, enter the letter S. The same technique is used here as in the Main menu; ie, the first letter of the required function is entered to access that function. Pressing P takes you to the password screen where you are shown the current password. Pressing M returns you to the Setup menu, while pressing C opens the change password screen. The password must be six characters long and can be any printable characters. For example &%Re1Z would be acceptable...if you could only remember it! If you make an error press R to clear your entry and try again. When you are finished press S to store it and return to the Setup menu. Only if S is entered at this point, will the currently stored password be overwritten. The speed screen allows you to select 300bps, 2400bps or 9600bps by entering 1, 2 or 3. While this is primarily included to match to the speed of the interface, ideally it should also match the modem line speed. For example, if your modem is an older type with say a maximum speed of 2400bps then select 2400 as your speed. Press M to return to the main menu. Note that the new speed selected does not take effect until the present call is finished. The Event trigger screen allows the selection of the edge that the internal PIC counter is incremented. An on to off signal into the Events optocoupler results in a rising edge input, while an off to on signal results in a falling edge input to the counter. Enter a C to change the trigger and update the screen or M to return to the Setup menu. This feature is handy if you are trying to capture a state change (high to low or low to high) rather then counting a series of pulses. To do this you would select the edge required and then go to the Events menu and enter R to reset the count to zero. A count of 1 would indicate that the edge has occurred. Pressing I takes you to the Idle timer screen. You can choose 1, 15 or 60 minutes by pressing 1, 2 or 3. The Idle timer will disconnect the call if a key has not been pressed within the timeout period. Without the Idle timer, it could be possible for the RMC to hold the tele-phone line in a busy state or not allow you to log in again if you did not log off properly. The default value is one minute. Input/Output testing Access the Input menu and verify that all inputs are off. Connect a power supply of 5V to Input 1. Update the screen and check that Input 1 is on. Remove the power supply, update the screen and check that input 1 is off. Repeat this procedure for the other inputs. Go to the Outputs menu and turn all the outputs off if not already done. Connect a LED in series with a 220Ω resistor between +5V and output 1, with the anode of the LED connected to the +5V terminal. Check that the LED is off. Turn output 1 on and check that the LED comes on. Repeat this procedure for the other outputs. In the Setup menu, select the Event trigger sub-menu. Change the trigger to OFF to ON if not already done. Return to the Events menu and reset the counter. Connect 5V to the Events connector. Update the display and check that the counter has increm-ented to 1. Remove the power supply, update the display and check that the counter has not changed. This verifies that the counter has incremented on the OFF to ON edge. If any of these tests fail, you will need to check the circuit around the faulty input or output. Installation The RMC is intended to sit alongside the modem or PC and connect us- Fig. 5: even if you purchase a commercial PC board, this same-size pattern can be used to check the tracks before assembly. Many readers still make their own boards from these patterns, too. ing a standard PC to modem interface cable. If you connect to a modem and have it powered on and connected in parallel with the telephone line, it will answer incoming calls and override your telephone. To avoid this problem, leave the modem disconnected while you are at home. If you are away and you have the RMC and modem enabled and someone else unknowingly calls your telephone, the call will be automatically answered. The caller will hear the modem tones but the RMC will time-out after 30 seconds and release the call, because the password won’t have been entered. Security & safety The RMC is equipped with a 6-digit password. No commands will be accepted until the correct password is decoded. If the password is entered incorrectly, the caller is offered a second attempt. If this second attempt fails, the call is aborted automatically. The caller has to connect again and retry the password. While this provides a high degree of security, it is not impossible for a ‘hacker’ to eventually crack the password and gain access to the RMC. It is therefore recommended that the RMC not be used in situations where damage to property or personal injury could occur because of unapproved access to the system. The RMC input and output circuits and PC board are not intended to control mains voltages (240VAC) directly. While optocouplers are used for some inputs, they are included for DC isolation and limited overvoltage protection of the PIC inputs only. If control of 240VAC devices is required, then suitably rated external relay circuits will be needed. SC AUGUST 1999  25