Last updated: 4 February 2000

Serial Computer Connections

From: Dick Seymour (rseymour@wolfenet.com)

First: when i say "PC" i do NOT mean Macintosh. (Mike's a Mac person and "PC" is ambiguous) I'll try to say "IBM PC" or Intel or Windows or WinTel... but if i slip and say PC i mean them.

Next: "rs232" and "serial" are the -same-. They're (poorly used) synonyms. The (or each) 9-pin connector (about an inch wide, right?) is (more properly) an RS-232 Serial Port.

"rs232" means "Radio Standard number 232"... and refers to one (number 232) of a set of radio industry standards set/agreed-upon/published throughout the 20th century. It (number 232) defines the voltages that shall appear on the pins, and the direction (from low to high or high to low) the signal goes, and how fast it goes (no faster than one volt per microsecond, in this case). The Telephone Company (AT&T) defined the standard to control/define how to connect data devices (teletypes) to various other pieces of telephone or signalling equipment (modems). It usually showed up as a 25-pin connector.

There are (at least) 4 variants: the original RS232... the signal flips from (at least) positive 25 volts to negative 25 volts. Then there's RS232A, rs232B and rs232C... lowering the acceptable voltage from 25 to 12 to 5 as technology developed better receiving circuits. (at that point, for other reasons, the industry switched to rs422 and rs423, but that's another story).

One a classic IBM PC (or, in current parlance: a WinTel machine), you could expect to see two serial ports (the data comes out one bit at a time, at the "baud rate"), and one parallel port (the data comes out 8 bits at a time).

The serial port used to have 25 pins (like the parallel port still does). Nowadays there are 9. Of those 25 (or 9) pins, only 3 are needed by the Autostar:
(1) data -to- the autostar,
(2) data -from- the autostar,
(3) and a ground reference wire (against which the 5 volts are measured).

PC's refer to their serial ports as COM1, COM2 and higher... "Com" for communications. Modems used to be external boxes which had a cable which plugged into a serial port (the -other- pins dealt with detecting whether the phone was ringing, whether the modem heard another computer, and speed control).

Devices on a serial port are (usually...99.9% of the time) dead ends for that cable. Hence you can only have as many serial devices as you have COM ports.

A "USB" port (Universal Serial Bus) is a little flat connector (imagine a telephone connector that's been squashed flat). Unlike the rs232 serial, the USB is designed to be passed -through- devices... so it can go to the keyboard, then on to the mouse, then on to the printer, then on to the...

Yes, it's got "Serial" in the namr... but they don't mean "rs232serial" ... it just means only one bit at a time is going out the wire...

Before 1996, the USB didn't exist. Keyboards plugged into dime-sized round connectors, and mice plugged into either 0.4" inch diameter round connectors, or plugged into a serial (yep..25 or 9 pin) port. Back in Ye Olde Ancient Times... IBM dictated the "AT" standard box... and -all- it had was the keyboard connector and one parallel port. Serial and mouse connections were optional.. and achieved by plugging an expansion card into one of the card slots on the motherboard (where you may now have a modem, or video card or sound card).

Around 1997, the PC industry declared the "ATX" standard... an arrangement of motherboard features which included -two- 0.4" round connectors (for the keyboard and mouse), one or two serial (9-pin) ports, a parallel (25-pin) port, and (optional at the start) two USB ports (flat holes... about 1/4 inch tall, 5/8's wide).

Now there's the PC99 standard. I haven't had to fight one yet, but it's probably USB with optional serial and parallel ports (since you can hang serial and parallel-adapter lumps on the USB cable).

I digress...

The Autostar has an "rs232" connector on the bottom. That means that the voltages and signals it understands conform to the pre-1950 rs232 standard (well, they don't... they conform to the mid-70's rs232c standard... loose talk again). If i remember correctly (it's been decades since i -read- the rs232 standard), the standard says -nothing- about connector shape or which pins do what. Hence it -is- "legal" to say that that telephone-jack-shaped-thing is rs232, and so is the 9-pin (official term: subminiature-D) connector on the back of your PC.

So: the #505 connector set provides:
(a) a looks-like-a-telephone-handset cable (4 pins on each end). This can be used for having two Autostars talk to each other in a CLONE operation. (kinda like Furbies?)
(b) a cable with 4 pins on one end, and a wider 6- or 8- pin connector (there may only be 3 or 4 pins -in- it) at the other,.
(c) an adapter: one end is the 9-pin serial COM port adapter to plug onto the PC (if the PC has a 9-pin port, Macs and iMacs don't), and the other end is the 6- or 8-pin hole for the wide end of cable (b) above.

If your modern PC does NOT have a 9-pin port fitting adapter (c), stop here. You do not have a serial port the Autostar can use.

If your ancient (pre-1997) PC only has 25-pin serial ports, then you also need a 25- to 9-pin adapter. (like me!)

*be careful* ... the PC parallel port is -also- 25 pins. (may they burn forever for that)

The serial and parallel ports were differentiated by their pins (or holes) [careful word choice to avoid NetNanny screening]... the serial port has -pins- on the computer, the serial cable (or adapter, in the 505 case) has holes for those 9 (or 25) pins to poke into. The parallel port has 25 tiny -holes- on the computer, and the parallel cables have 25 tiny pins.

The cleanest PC-to-Autostar connection is: 9-pin adapter (c) into back of PC... wide-end-of-telephone cable (b) into that adapter... narrow end of that cable into narrow hole on bottom of AUTOSTAR (not base of telescope). Autostar's normal cable connecting it to telescope. Then: turn on telescope (this powers the Autostar... get past the Date/Time/Daylight questions). Do NOT go to "download" mode yet. Then: turn on the computer. If you're trying to download stuff with Meade's Autostar Updater program (take a stiff drink)... start it. It will search for the Autostar. After it finds the Autostar... the Updater program will -disappear- from the screen! After a (heart-stopping) moment or two, a box will appear telling you to put the Autostar to Download mode. You do -that- my using the MODE and ENTER and Scroll keys to get to the Setup: Telescope: Download choice, then hit enter until it tells you not to turn it off. Click on the [ok] box on the PC's screen, and the Updater will start, er, messing up your life...

From: shannon@more.net (Shannon Spurling)

In a correction to the serial computer connections tech notes: RS232 means recomeded statndard, not radio standard. The AT configuration had two serial ports in most cases, a 9pin and the older 25 pin.

The 4 variants mentioned are the result of the trigger level of the recieving circuitry, not specified voltages in those standards. In most cases, low power devices will not have enough power to provide a full voltage swing, so they have been designed to take advantage of the low trigger levels. Any time the siginal goes above 3.? ( or 4.? I can't remember) volts a zero is siginaled, and every time a siginal drops below zero volts a 1 is siginaled. The letter designations actually refer to changes to the physical charicteristics reccomeded by various manufacturers.

Also, the serial port on the Mac can be used as an RS232 for straight serial applications like connecting to the Autostar. You just need to get one of the Mac-To-PCmodem connecting cables, and possibly a genderchanger. Changes the type of plug from male to female, or vice-versa. So all of this would also hold for a Mac user with the right cable.


Shannon Spurling

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