Linecorders & Laptops

Comms problems have become more and more common over the last few years with the emergence of laptops whose com ports don't hold to the RS232 spec but comply with the new, less power hungry EIA-562. This spec claims the output need be no more than ±3.3V which is rather a far cry from the ±12V of the original RS232. In fact, the new spec is almost what the old spec accepted as the noise level!

The incompatibility may not only manifest itself through a definite comms failure. It is also strongly suspected that "strange programming" of the unit, indicated by settings not being what was requested by the user, is attributable to the lower voltages from the laptop's com port.

Not only is the new spec a headache, but it would appear there is also a daft move among some designers to not connect Pin 5 i.e. to not provide a signal ground. Instead, they rely on the casing of the plug to offer any ground reference point. Some com ports are also 'cut down' versions with some signals strapped directly to the supply voltage via resistors to limit any short circuit current.

Linecorder comms interfaces are optically isolated (hey!, do you want the voltages being measured fed into your laptop? - thought not!). They operate on power drawn from the com port connected to it and therefore requires both a solid ground pin and a fully RS232 compliant com port (minimum of ±7V) for the comms to be reliable. As a result, new style laptops are posing a challenge to IT purchasing departments.

A few simple tests will reveal if the laptop is a potential headache. All the tests are best carried out with an old fashioned moving coil meter that provides a small amount of load on the port. If you don't have such a meter, you can provide a load using a resistor of 4k7 to 10k strapped across the leads during the voltage measurements. If such a resistor is not available, 'unloaded' test voltages are given but it must be noted that they are not as reliable as the loaded readings.

Testing should also be done directly on the com port to remove any doubt as to the cable's integrity or possible wiring changes (the Telog cable is one such cable). It is also best done with the DOS program invoked to ensure the RTS is in a Set state.

pin numberspin view
Above is a com port as viewed when looking at the pins.

TX (PIN 3):
Measure between the casing of the com port connector and Pin 3. This should exceed -7V loaded, -8V unloaded (please note the polarity is negative with respect to the casing).

RTS (PIN 4):
Measure between the casing of the com port connector and Pin 7. This should exceed 7V loaded, 8V unloaded.

GND (PIN 5):
We will be testing the continuity of Pin 5 to Ground. Some modern digital multimeters can be affected by just a few mV that may exist between Pin 5 and the casing. It is suggested that the PC be shut down for this test.

Switch the meter to ohms and measure between the casing and Pin 5, this should be below 10W.

If the com port passes all three of the above tests then it should work satisfactorily with Telog Linecorders. We have not covered the issues regarding operating systems which is a whole subject on its own. As a guideline, if you are using Windows95 you'll be ok. From Win98 onwards 'Telogers for Windows' will work fine although it has been known for the DOS program to have trouble working correctly.

If the com port fails the above test then a solution, if handy with a soldering, is given here.

If not wishing to exercise soldering techniques then a very acceptable solution lies with using a PCMCIA com-port card as a cure for the EIA-562 problem. These tend to still have full RS232 voltage output (i.e. ±12V). Remember to change the port number to the PCMCIA com-port.

Analogue ICs for 3V systems

Note: The following has been taken from MAXIM's website, the manufacturers of the most reliable communications ICs I have ever designed with.

Three-volt digital ICs have quickly become popular for the power savings they offer in portable equipment. And to complement these digital ICs, the industry has created a new generation of low-voltage analog ICs, also offering the benefit of lower power consumption.

Single 3V operation is available for many op amps, comparators, and microprocessor supervisors, and for some RS-232 interface ICs. For A/D and D/A converters, analog switches, and multiplexers-which often require minimum supply voltages of 5V or ±5V-the choice is more limited. You can, however, easily provide the required voltages with a local switching regulator or charge-pump converter.

Though 3V designs are beginning to appear across the board, the switch to low voltage is most notable in systems for which size, weight, and power consumption are especially critical-palmtop computers and wireless phones, for example. And, with the increasing demand for small size and longer battery life, it is likely that blood analyzers, barcode scanners, data loggers, and other portable equipment will also follow suit.

The switch from 5V to 3V also benefits line-powered systems, because the lower power dissipation associated with 3V operation allows smaller power supplies, heatsinks, and fans. The change from 5V to 3V also means that higher-density, higher-speed logic can operate at the same level of power dissipation.

The following discussion covers 3V analog ICs, the power savings inherent in their operation, and the problems associated with low-voltage operation. It also presents methods for generating 5V from 3V, and methods for generating 3V from inputs that range above and below 3V (such as the terminal voltage of a 3-cell alkaline battery).

Power savings from 3V operation
The power saved by lowering VCC from 5V to 3.3V can be dramatic. For resistive and capacitive loads, power saved is proportional to the voltage squared: 1 - (3.3/5)2 = 56%. For constant-current loads such as references and op amps, the savings is linear: switching from 5V to 3.3V saves 34%. For constant-power loads such as hard-disk drives, the switch to 3V doesn't save power; it merely requires the device to operate at a lower input voltage.

Many new op amps, microprocessor supervisors, and interface ICs (along with a handful of A/D and D/A converters, voltage references, and switches) are now specified for 3V operation. The following sections discuss these product types in detail.

Interface transceivers
As design improvements reduce the overall power required by a system, power dissipated by the serial-data interface becomes increasingly significant. Fortunately, the serial interface is an area that is still amenable to power reduction in most cases. One need only switch from the old RS-232 serial-interface standard to the newer EIA/TIA-562 standard.

RS-232 appeared in the days of mainframe and mini computers, at a time when ±12V power supplies were common in such systems. Not surprisingly, the first RS-232 transceivers required ±12V for operation. Voltage drops internal to the IC reduced the output swing to about ±9V, so the required minimum was set still lower, at ±5V. Now (32 years later), the RS-232 standard is still around, with the official name of EIA/TIA-232-E (or 232E for the sake of brevity).

The advent of portable and low-voltage equipment has spawned a new serial-interface specification that can replace the 232E standard. Called EIA/TIA-562 (562 for brevity), this new standard became effective in 1991. The 562 and 232E standards are electrically compatible, so the new 562 designs will mate with existing 232E equipment and vice versa.

For a comparison of certain 232E and 562 specifications, see Table 1. Note that the driver output swings differ (±5V vs. ±3.7V), but the receiver input thresholds are the same (±3V). The 562 devices' ±3.7V minimum output swings allow them to communicate with 232 receivers, which have input thresholds of ±3V. The noise margin, however, is only 0.7V. By comparison, the 232 drivers' ±5V minimum swings guarantee a noise margin of 2V.

Table 1. Comparison of 232E and 562 Interface Standards
Mode of operation
Single ended
Single ended
Allowed number of transmitters and receivers per data line
1 Tx, 1 Rx
1 Tx, 1 Rx
Maximum cable length

C<2500pF for data rates<20kbits/sec,
C<1000pF for data rates>20kbits/sec

Maximum data rate
Driver output voltage, loaded
Maximum driver short-circuit current
Transmitter load impedance
3kW to 7kW
3kW to 7kW
Instantaneous slew rate
Receiver input threshold (sensitivity)
Receiver input resistance
3kW to 7kW
3kW to 7kW
Receiver input range

The 562 standard cuts power consumption by specifying a minimum output swing of ±3.7V (vs. ±5V for 232E). The resulting power consumption for 562 drivers is only 55% of that required for 232E drivers. Note that line drivers (not the receivers) consume most of the power. Therefore, a palmtop computer containing 562 interface ICs provides power savings whether it connects to a 562 receiver or a 232E receiver.

Maxim has four 3V interface ICs that comply with the 562 standard. Each includes a charge-pump converter for generating the required output-voltage levels. The charge pump doubles VCC to create the positive level, then inverts that voltage to create the negative level. For a given IC, the required external charge-pump capacitors (a set of four) have values of either 0.1µF or 1.0µF, with the larger value supporting a larger number of drivers and receivers.

The MAX563, for example, has two drivers and two receivers, and operates with four 0.1µF capacitors. Its 116k bits per second (116kbps) data rate makes it compatible with LapLink™ software. It also provides a 10µA shutdown mode in which the receivers remain active.

This feature-active receivers during shutdown-extends battery life in portable applications. It enables the computer to monitor external devices such as the ring indicator of a modem, via the serial interface, with minimal power consumption. In remote data gathering, for example, the computer may spend much of its time waiting for a ring signal or other external stimulus. If the computer and the interface IC have no access to AC power, both can remain shut down until "awakened" by the external signal.

Maxim also offers RS-232 transceivers that operate from 3V. These chips include special high-efficiency DC-DC converters for generating the higher output swings specified by EIA/TIA-232E. High efficiency is attractive because RS-232 loads can consume several hundred milliwatts at high data rates.

Some manufacturers include charge-pump voltage triplers in their 3V interface ICs, but these ICs dissipate considerable power, and are unable to sustain the ±5V minimum outputs at higher data rates. Though effective in compensating for voltage drops in themselves and in their driver-output stages, voltage triplers are less efficient than the doublers used in 5V ICs. Miniature on-chip switching regulators are the most efficient at generating RS-232 voltages. That's why the new 3V RS-232E transceivers from Maxim contain efficient switching regulators rather than voltage triplers.

Switchers draw 50% less current than do charge-pump triplers. They also provide outputs suitable for powering mice and supporting high data rates (such as 116kbps for LapLink™). Other vendors' charge-pump-tripler ICs can't necessarily meet the drive requirements of a mouse (10mA at 5V and 5mA at -5V). Nor can they necessarily provide the minimum output levels (±5V) required by 232E at high data rates (Figure 1).

Figure 1. Maxim's 3V RS-232 transceivers, which derive their output-signal levels from a low-cost switching regulator, maintain valid levels at high data rates. Those with charge-pump triplers (from other vendors) do not.

Because many receivers have TTL voltage thresholds, it may be acceptable for an RS-232 output to fall below 5V while transmitting to another RS-232 device. Sub-5V RS-232 levels for the mouse, however, may cause it to fail. The mouse steals power from the RS-232 line to supply an internal microcontroller, whose minimum supply voltage in most cases is slightly below 5V.

The components used in the switcher and charge-pump-tripler approaches are equivalent in cost and size.

The 3V MAX212, an RS-232 transceiver with three drivers and five receivers in a 24-pin package, produces ±6.5V with a single-inductor, double-duty switching regulator. The MAX218 employs a different approach. This two-driver/two-receiver IC produces a positive output level with a boost switching regulator, and a negative output level with an inverting charge pump (Figure 2).


Figure 2. This low-voltage interface IC includes a high-efficiency DC-DC converter, which generates the voltages required for RS-232 communications.

The MAX218 operates from 3V VCC or a 2-cell battery (minimum voltage 1.8V), with a guaranteed data rate of 120kbps. Its two receivers remain active during the 1µA shutdown mode, enabling the chip to monitor external devices while consuming small amounts of power. Packages include 20-pin DIPs, SOs, and SSOPs.

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NOTE: This webpage does not form part of any official documentation.
Any information contained herein is used at own risk.

©  M.T.P. - 03.06.03