Model 100 Serial Interface

Overview

The BASIC ROM provides access to the serial port. BASIC ROM support is well documented elsewhere. This article covers direct use of the IM6402 UART on the Model 100 and Tandy 102.

Configuration

Before the serial port can be used, it must be configured both for serial word format (data bits, stop bits, and parity) and baud rate.

Select RS232 Port

At any given time, either the internal modem or the external RS232 connector may be connected to the UART. This is controllable through software.

For RS232 access, port $B8, bit #3 must be set to 0.

For Modem access, port $B8, bit #3 must be set to 1.

Note that $B8 is shared with other functions, including power control.

Serial Word Format

Port $D8 controls the serial word format.

Actually, any of $D0 → $DF can be used.

Bit	Function	Settings
0	Stop Bit Select	0 → 1 stop bit 1 → 2 or 1.5 stop bits (1.5 if character length is 5, 2 otherwise)
1	Even Parity Enable	0 → Odd parity 1 → Even
2	Parity Inhibit	0 → Parity 1 → No parity
4-3	Character Length Select	Bits: 00 → 5 01 → 6 10 → 7 11 → 8
7-5	Unused

Baud Rate

CPU selection of baud rate is accomplished by loading a divisor into the PIO register through output ports $BC or $B4 and $BD or $B5.

The baud rate selection must be committed by writing $C3 to register $B8

Baud Rate	PIO Divisor	Port $BD	Port $BC
75	2048	72	0
110	1396	69	116
300	512	66	0
600	256	65	0
1200	128	64	128
2400	64	64	64
4800	32	64	32
9600	16	64	16
19200	8	64	8
38400	4	64	4

Here is a practical example of using 38400bps using only BASIC code. This program dumps all RAM contents to the serial port.

 1	DEFINTA-Z
	:OPEN"COM:98N1D"FOROUTPUTAS1
	:D$=""
	:A=FRE("0")
	:L=VARPTR(D$)+1
	:M=L+1
	:POKEL-1,128
	:OUT180,4
	:OUT181,64
	:OUT184,195
	:FORI=0TO255
		:POKEM,128+I/2
		:POKEL,(IMOD2)*128
		:PRINT#1,D$;
	:NEXT

Here's an assembly language version of the same program. It transfers your Model 100's 32K RAM to a serial connection in about 8.5 seconds!

		.org	64704

		; select RS232 port
		MVI	A, $25
		OUT	$BA

		; set up 8N1
		MVI	A, 28
		OUT	$D8

		; set up 38400 bps
		MVI	A, 64
		OUT	$BD
		MVI	A, 4
		OUT	$BC
		MVI	A, $C3
		OUT	$B8

		LXI	H,32768

WAITEMPTY:	IN	$D8
		ANI	$10
		JZ	WAITEMPTY

		MOV	A,M
		OUT	$C8

		INX	H
		MOV	A,H
		ORA	L

		JNZ	WAITEMPTY

		RET

		.END

Data Transmission

Once configured, sending a character is simply

		MVI	'A'
		OUT	$C8

or, in BASIC,

OUT 200,ASC("A")

In the UART, there is room for two characters: the character currently be transmitted, and the next one. This permits you to ensure there is always once character waiting in the wings.

You should never overflow the transmission buffers. Either do some other work during the "downtime" and/or wait until there is room by polling bit 4 (0x10) of register $D8. The code the follows just does a "busy wait."

WAITEMPTY:	IN	$D8
		ANI	$10
		JZ	WAITEMPTY

When deciding whether the other device is ready to receive you should also consider the Clear To Send (CTS) flow control line described later in this document.

Data Reception

(IN PROGRESS)

Flow Control

The Model T has full support in hardware for the CTS and RTS flow control lines. In the BASIC ROM, however, it is not implemented. Instead the BASIC ROM relys on slow, kludgy XON/XOFF character escapes. This makes it difficult to transmit of receive binary files since the XON/XOFF some characters are reserved for flow control.

Since we are discussing direct control of the UART, we can do better and implement full flow control access.

Detect Clear to Transmit (CTS)

The CTS line is an input to the Model 100. It indicates whether the modem or remote computer system is ready for new characters. The foreign system can "flow off" the Model T when its receive buffers are full.

(IN PROGRESS)

Request Peer to Send (RTS)

The RTS line is an output from the Model 100 to the device connected on the serial port. It is a signal "requesting" the device to send (or not to send). From the other point of view, the device should transmit if and only if RTS is high.

(IN PROGRESS)

I/O Map

Name	Direction	Port
TX	Output	$C8
RX	Input	$C8
RTS	Output	$BA, bit 7
CTS	Input	$BB, bit 4
DSR	Input	$BB, bit 5
DTR	Output	$BA, bit 6

Direction indicates both data flow, and whether to use an IN or OUT instruction to read/write to the given pin.