125 lines
7.1 KiB
Plaintext
125 lines
7.1 KiB
Plaintext
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<<<<< On The Topic of 450 Baud! >>>>>
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by Scott Loftesness
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May 28, 1983
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Many users have asked about using 450 baud with this RBBS system.
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Typically, they are asking why such an option exists (the N option on the
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main RBBS menu) when their modem and IBM-PC BASIC don't appear to support
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such a speed.
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They are correct about IBM-PC BASIC not supporting 450 baud. In
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fact, the ROM BIOS routines in the PC don't support 450 baud as an option
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either. It's not that IBM or Microsoft skipped the speed when they developed
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the PC, but the fact that 450 baud is not a standard communications speed
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for the RS-232-C interface. The next higher 'standard' speed above 300
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baud is 600 baud, followed by 1200 baud, etc. As a result, 450 baud is
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not supported on most personal computer systems.
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As it turns out, however, most 300 baud modems will run at somewhat
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higher speeds - up to, guess what!, 450 baud in most cases. Although a
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somewhat higher error rate can be expected when you "push" a 300 baud modem
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to 450 baud, the error rate is typically still low enough to not be any
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kind of serious problem to file transfer and message passing operations.
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In fact, using the XMODEM file transfer protocol, errors which do occur
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are normally detected by the check sum error detection scheme used by XMODEM
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and simply force a re-transmission of the block originally received in
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error.
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Since the speed advantage of 450 over 300 baud can be substantial,
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and since we are all interested in doing the most work in the least amount
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of time with the equipment at hand, many users have implemented 450 baud
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as an option which can used with their "home-grown" communications programs.
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(Note that standard IBM-PC communications packages don't support 450 baud
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either - such as PC-Talk III). I'll describe how 450 baud can be
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implemented from a programming standpoint shortly. But first, let's get
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rid of a few erroneous impressions about modem operation.
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Your Hayes 300 baud SmartModem (or a Hayes 1200 running in 300 baud
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mode) is capable of receiving modem commands and data at up to 1200 baud.
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What this means is that the microprocessor in the Hayes is programmed to
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automatically look for the AT command sequence - and performs automatic
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baud rate detection at the speed you happen to be sending the command
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sequence to the modem. This capability, for example, is used in the RBBS
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code to send the various modem commands at 1200 baud - to minimize the time
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spent sending them!
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Your Hayes 300 will even try to send data across the line at 1200
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baud - if you open your communications line at that speed and attempt to
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go to data mode. However, the 300 is not capable of handling data at that
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speed - in fact, it can only transmit and receive data at up to slightly
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more than 450 baud - with any reasonable error rate. Since the Hayes doesn't
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care at what rate is receives data (anything from 1 to 1200 baud is
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acceptable), it is possible to transmit data to the modem and have the
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modem transmit it to the phone line at any of those speeds. However, 450
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baud is the reasonable upper limit for data communications with the Hayes
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300.
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Now that we understand the workings of the Hayes modem, let's talk
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about how we can get the PC to send data from its RS-232-C Asynchronous
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Communications Adapter at 450 baud. As mentioned earlier, 450 is not an
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acceptable option to either the BASIC OPEN statement or to the direct
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BIOS calls for asychronous communications. However, as always, almost
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*anything* is possible using software! The IBM ACA uses an 8250 chip which
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is capable of running at speeds from 1 to 19,600 baud (officially, only
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up to 9600 baud, but that's another discussion!). The 8250 includes an on-chip
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software programmable baud rate generator - which actually determines the
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speed used to send and receive data across the RS-232-C interface. This
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baud rate generator can be programmed to run at *any* baud rate between
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1 and 19,600 baud. The IBM-PC ROM BIOS routines, however, were programmed
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to only support the "standard" RS-232-C speeds. Since BASIC uses the BIOS
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routines, BASIC only supports those same "standard" speeds.
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However, it is possible for you to program the 8250 to run at any
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speed you like. In this case, we are only interested in programming it to
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run at 450 baud. Programming the baud rate generator in the 8250 consists
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of loading the baud rate generator on the chip with a "baud rate divisor"
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- which takes the clock frequency and divides it by the number you provide
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to come up with the baud rate itself. (Those of you with Technical Reference
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manuals for the PC can read all about this on pages 2-135 to 2-137). The
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divisor is a two byte value - which is stored into two one byte registers
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within the 8250 chip. First, we need to determine what the baud rate divisor
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should be for 450 baud operation. Take it from me that the value is X'0100'.
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Next, we need to get this value from a BASIC program into the 8250 itself.
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Fortunately, BASIC includes the OUT statement - which allows us to send
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data to the various I/O ports on the system. The 8250 uses several of these
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ports - for the baud rate divisior, modem control, and for actually
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sending and receiving the data across the RS-232-C interface.
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The baud rate divisor ports are X'3F8' for the least significant
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byte and X'3F9' for the most significant byte - when using the COM1 port.
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(COM2 addresses are X'2F8' and X'2F9' respectively. Before we can load
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these divisor ports, however, we must load another value to another port
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to indicate that we are setting the baud rate divisor. This third port is
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the Line Control Register and is port X'3FB' (X'2FB' for COM2). The high
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order bit (X'80') of the LCR controls access to the baud rate divisor
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registers. When this bit is 1, we can load the baud rate divisors. When it
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is 0, the 8250 resumes operation with the baud rate loaded. The following
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sequence of BASIC statements will load the baud rate divisor registers:
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OUT &H3FB,(INP(&H3FB) AND &H80) 'Enable divisor registers.
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OUT &H3F8,0 'Low order byte of X'0100'
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OUT &H3F9,1 'High order byte of X'0100'
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OUT &H3FB,(INP(&H3FB) OR &H80) 'Disable divisor registers.
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Adding this code to a BASIC program will cause the COM1 port to
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immediately switch to 450 baud operation.
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Switching to other speeds is just as easy. The only thing you
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need to know is the baud rate divisor required for that speed. Here are
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a few of the common divisors (see the full table in the Tech. Ref.):
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300 X'0180'
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450 X'0100'
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1200 X'0060'
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I hope this discussion has helped to clarify the subject of 450
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baud operation. Perhaps with this help, you can make some simple changes
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to whatever communications program you're running which will allow you to
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use your Hayes 300 at a speed which will allow up to 50 per cent faster
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communications!
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