Serial Interfaces

In a serial interface the ASCII characters (bytes) are output as a string of pulses, one after the other. The computer has to wait until each character is printed before it can output another character.

If you have a Level II 16K machine, this is usually what you would be using to drive a printer. There are exceptions, wherein you can connect an interface directly to the output bus of your keyboard, but for now, we will consider serial interfaces, such as the

TRS 232 from Small Systems Software. The Small Systems interface connects to the auxiliary output of your cassette port. Software (on cassette) comes with the TRS 232 which will rearrange the device control blocks to output to the auxiliary port. The TRS 232 (a small black box) has a DB-25 connector on it into which your printer cable will fit.

The software which comes with the TRS 232 will drive most ASCII printers, such as the ASR 33 Teletype, Texas Instruments Silent 700 series, ASR 38 and others. The software will ask you to input the baud rate and number of nulls after a carriage return (to give the carriage time to return before sending another character). The software is in the form of a BASIC program that will stuff code into high memory, which then must be MEMORY SIZE protected.

If the printer you are using does not accept ASCII code, you must modify this driver (software) program to make the necessary code conversion. This would be necessary for example, if you tried to drive a Selectric, such as the TRENDATA 1000. In that case, you would have to convert from ASCII to the IBM correspondence code. This code would have to be included with the code already in high memory, and would make the conversion from ASCII to the appropriate correspondence code.

Also, if you have a lower case modification in your machine, it too, will need to be up there in high memory. This results in quite a bunch of things to have up there, but a machine language program can be written to include all these features in one chunk, which can then be loaded as a single SYSTEM tape program.

If you have an expansion interface with the RS 232 (don't confuse this with the TRS 232 mentioned earlier) board installed, you will need a different serial driver. The RS 232 operates out of a different output port than the TRS 232, aside from being accessed differently.

In general, all serial type printers will require a piece of hardware and a software program (driver) to operate a printer. Serial is probably the cheapest way to go, but costs memory (to hold the driver routines). Of the serial interfaces available, the TRS 232 from Small Systems is probably the most economical. It comes with the software for around $50. and does not require the expansion interface to be present. Serial driven printers are generally slow. We have used theTRS 232 to interface to the ASR 33 and ASR 38 Teletypes, as well as the TRENDATA 1000 Selectric. These all run at 110 baud (about 10 characters per second).

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