The LCD Serial Backpack is a small daughterboard that mounts on the back of an alphanumeric LCD. It converts the LCD's parallel interface (6 to 11 I/Os with peculiar timing and initialization requirements) into a one-wire serial hookup.

What kind of serial input does the Backpack accept?

It is switch-configurable for 2400 or 9600 bps with the common N81 format (no parity, 8 data bits, 1 stop bit).

How do I install the Backpack to an LCD?

The Backpack has connections for 1x14 and 2x7 LCD pin arrangements. You install the appropriate header (supplied) to the LCD, then mount the Backpack onto that. Requires minor soldering. See the LCD Serial Backpack spec sheet or instruction manual for more details.

Does the LCD Serial Backpack support Brand X Model Y LCDs?

Virtually all modern alphanumeric LCDs now use the same chipset or a compatible version (H44780, KS0066, etc.), so the Backpack can support them all (up to 80 on-screen characters).

Can I get it already installed to an LCD?

Sure. We carry 2-line by 16-character supertwist LCDs with a Backpack-style interface preinstalled. Not only does this save you installing the unit, but the quality of these LCDs is also vastly better than the LCDs commonly sold in small quantities through distributors, which are often older surplus units. We also stock a variety of larger LCDs with more advanced serial interfaces; see the selection guide.

How do I hook up the serial input?

Just connect the serial TxD (transmit-data) and SG (signal ground) lines to Backpack pins SER (serial) and GND (ground). These are standard header pins on the Backpack circuit board. The Backpack accepts either RS-232 levels or TTL/CMOS.

What about taking data straight from a UART?

The output from a UART is non-inverted serial. It's upside down with respect to the RS-232 logic that the Backpack expects (where a logic 0 is +2.5 to +15V and a logic 1 is -15 to +0.5V). So it has to be inverted before being fed to the Backpack. If you are writing your own software serial-out routines or using something like the BASIC Stamp, just invert the bits you send to the I/O pin that drives the Backpack. If you are using a hardware UART, you'll have to invert in hardware. (If you invert the data bits, the start and stop bits will still be wrong and you'll get gibberish.) You can either use one section of an inverter IC, like a 7404, or a simple one-transistor inverter.

What are the power requirements?

The Backpack itself uses less than 1mA; LCDs typically draw 2mA. Both require regulated 5 volts.

How do I display text on the LCD?

Just send it serially at the appropriate baud rate. If you send the string "Hello" to the Backpack, "Hello" appears on the LCD.

What about instructions like clear-screen, cursor position..?

To help the Backpack distinguish instructions from ordinary text, its program looks for an 'instruction prefix' in the data you send it. When you send this character (ASCII 254), the Backpack regards the next byte as an instruction. Instructions correspond to the standard LCD instruction codes, and are listed in the manual. For instance, ASCII 1 is the LCD code for clear screen; so to clear the screen Backpack style, you send two bytes, [254] [1]. (Where the []s indicate that these are individual bytes, not literally brackets.)

What about terminal-style control codes like carriage returns, tabs, etc.?

The Backpack does not recognize these. Sticking to the LCD's own instruction codes lets the Backpack do very little processing of the incoming serial data. As a result, the Backpack controller can run slowly-- 480kHz --to minimize current draw. Of course, the larger the LCD, the more helpful terminal emulation is, so our 4x20, 4x40, and graphics LCDs do understand control codes. But they are still designed for power efficiency.

Where can I find program examples?

The manual contains program examples for the BASIC Stamp I (BS1), BS2, and QBASIC. We also have a growing collection of application notes. Example programs are written in BASIC because it is quite popular, and everyone can at least read and understand it. You may use any language that can output data serially, however.

I see lots of inexpensive surplus LCDs for sale--are they any good?

Surplus LCDs range from excellent to nearly useless. Many are older TN-type LCDs, which suffer from narrow viewing angle and poor contrast. Try to buy only STN (supertwist) displays. Also try to determine whether the LCD is a normal- or extended-temperature unit. Extended temperature sounds good, but these units typically require a negative supply voltage for proper operation (not included on the LCD Serial Backpack). Your best bet is to simply buy a new, STN LCD. All of the serial LCD modules we make use STN LCDs. We carry 2x16, 4x20, and 4x40 modules ready to go. If you need some other size, check out the Optrex modules available from Digi-Key.

What is the operating temperature range of standard LCDs?

The standard operating temperature range is 0 to 50 degrees C (32 to 122 degrees F). Extended-temperature LCDs operate from -20 to 70 degrees C (-4 to 158 degrees F). Both types' contrast varies with temperature--the display darkens with heat and fades with cold. LCDs are most appropriate for office and benchtop applications, or protected handheld equipment. Frankly, LCDs are not the technology of choice for unprotected portable gear or vehicle dashboards. The LCD Serial Backpack uses commercial-temperature-range components, and is not designed for use with extended-temperature LCDs.

But I've seen LCDs used in dashboards--what about those?

There are two broad categories of LCDs--static drive and multiplexed. Static-drive LCDs are typically seven-segment numeric or bargraph displays. They're called static because every liquid-crystal segment receives a constant driving voltage. This means that there must be a connection--a wire--to drive every visible element of the display. It's not practical to make graphic or dot-matrix alphanumeric displays that way, since a 2x16 display would entail at least 1120 separate connections. Instead such displays are multiplexed, with each element (pixel or group of pixels) driven part of the time, typically 1/16^th. The trouble arises when multiplexed LCDs get cold. The liquid-crystal material takes longer to change states until eventually the 1/16 duty cycle doesn't allow enough time for pixels to turn on.

To make a long story short, simple static-drive numeric LCDs are sometimes used in wide-temperature situations; multiplexed alphanumeric LCDs, even the extended-temperature types, almost never are.

How do I mount an LCD in a box, panel..?

LCDs have mounting holes at the four corners. The general procedure is to mount the LCD on standoffs behind a rectangular hole in your box, panel, etc. Cover the hole with a transparent window and you're done. We offer a simple mounting kit for our 2x16 and 4x20 LCDs. Unfortunately, standardized mounting hardware for all of the various LCDs does not exist. The LCD manufacturers themselves don't make it. Manufacturers of electronic enclosures, like Pactec, can help with semi-standard cutouts and windows for production purposes.

What kind of OEM pricing can you offer on the Backpack?

The LCD Serial Backpack daughterboard in 100 qty, OEM packaging, is $19.00/unit. We maintain sufficient stock to ship orders up to several hundred pieces same/next day (no lead time).

My application is cost sensitive; can you save me more?

Yes. We offer the LCD Serial Backpack controller IC for $3.96 ea. in order multiples of 25 (rail of dual-inline-package ICs). Support components are minimal and inexpensive--see the schematic. Orders up to 1000 of the DIP ICs can generally be delivered directly from stock. The SMT (SOIC) version is $4.96 ea. in rail-multiples of 42 pieces. Typical lead time for SOICs is 2 weeks. (We do not maintain large stocks of SMT parts, as storage degrades solderability. We obtain them as needed to meet our production and sales requirements. This ensures that you get factory-fresh parts for best production yield.)

Can you customize the Backpack for an OEM application?

To a limited degree, yes. We manufacture the Backpack in pretty large quantities, currently about 10k pieces a year, in order to achieve reasonable economy of scale. Recently, we began manufacturing a small percentage of units without the 5-pin straight-male header normally used to connect power, ground, and serial input. So if you need, say, wire pigtails, or a right-angle header, or a female connector, we can use these units to meet that need. If your application requires major changes to the design, we will probably recommend that you purchase the chips for incorporation into your own board layout. Call or e-mail your requirements.

Others are now offering serial-LCD products; why should I go with yours?

Build a better mousetrap, and every rat will copy it. Seriously, when we introduced the LCD Serial Backpack in 1994, we struck a nerve. Once we demonstrated the market potential, lots of other players jumped in. They took one of two approaches--copy us outright, or add bells and whistles. Most of the copies were ignored in the marketplace and are gone now. The bells-and-whistles units are hobbyist labors of love designed to impress other techies with their virtuosity. But they're typically too costly in quantity, and the stability of their garage-op developers is too iffy for serious consideration.

*LCD Serial Backpack is a registered trademark of Scott Edwards Electronics, Inc.

Scott Edwards Electronics Inc.
1939 S. Frontage Rd. #F, Sierra Vista, AZ 85635
phone 520-459-4802; fax 520-459-0623
e-mail info@seetron.com

LCD Serial Backpack® and Serial LCD Modules

What is the LCD Serial Backpack?