perm filename SERIAL.DOC[KBD,ROB]3 blob
sn#455038 filedate 1979-06-25 generic text, type C, neo UTF8
COMMENT ⊗ VALID 00009 PAGES
C REC PAGE DESCRIPTION
C00001 00001
C00002 00002 Writeup on the Micro Switch Keyboard Serialzers
C00003 00003 Functional description
C00006 00004 Physical characteristics
C00008 00005 Electrical characteristics
C00009 00006 Some notes on EIA and current loop protocols
C00012 00007 Programming considerations
C00016 00008 Signal definitions
C00021 00009 prototype notes:
C00025 ENDMK
C⊗;
Writeup on the Micro Switch Keyboard Serialzers
**** WARNING ****
As of 27-May-79, the prototype is still being built, and the information
contained herein does not neccessarily reflect the way the Keyboard
Serializers will end up.
last update:
25-Jun-79 Spacewar poll character must be a specific character
Functional description
The Keyboard Serialzers take the 10 bits of parallel data from the Stanford
Micro Switch keyboards (11 bits if you count the keyboard strobe), and
transmits this data as two 8-bit bytes on a serial line. Whenever a key
is pressed that generates "KBDSTROBE", a sequence starts which transmits
first a byte containing the "Bucky bits" (Control, Meta, Shift, and Top
keys), followed by a byte that has the actual key data. This data goes
out over a two-wire, electrically floating, passive current loop.
There are times when the host computer wants to know what keys are being
pressed at that moment. To accomplish this, the serializer is built to
receive data as well as transmit it. When the serializer receives the
magic charatacter "***", the seriailzer will transmit a pair of bytes,
exactly as if a key were newly pressed. (This mode of operation is called
"Spacewar Mode" for reasons that date back to the game of the same name.)
Note that if more than one key is pressed, the logical OR of the bits are
read in, just as in the (old) keyboard scanner.
Physical characteristics
The Keyboard Serializers are built on a printed circuit board and mounted
inside of the Micro Switch keyboards. They ride "piggyback" on the
existing circuit boards, and connect to them via a short edge-card
connector jumper cable. The serializer provides power to the Micro Switch
keyboard circtuitry through this jumper.
Another edge connector on the serializer board connects to the I/O cable.
This cable provides power (5 volts) and ground, and two pairs of current
loop signal (on transmit and one receive).
Electrical characteristics
Some notes on EIA and current loop protocols
The two states of an EIA or current loop line are called MARK and SPACE.
An idle keyboard will send continuous MARKS.
An unplugged keyboard (or a keyboard with the BREAK (sic) key pressed) will
send continuous SPACEs. Continuous SPACEs will cause "framing errors" in
the receiving UART.
In EIA land, a MARK is signified by a low voltage. (On the DCA, this is
about -12 volts). A space is signified by a higher voltage (on the DCA,
approching 0? volts).
In current loop land, a MARK is signified by a conducting wire, (as if there
were a closed switch at the far end of the loop). A SPACE is signified by an
open circuit (no current flow).
The first bit of a transmitted byte is the START BIT, which is always a SPACE
(EIA → low voltage, CurLoop → not conducting). The next eight bits are data
bits, where a 1 bit is a MARK and a 0 bit is a SPACE. The last transmitted
is a STOP BIT, which is always a MARK bit. (Note that the line stays in the
MARK condition until the next byte gets transmitted).
When we say that this is the format of the transmitted data,
(first byte)---------------------------¬(second byte)---------------------------
| 00 01 02 03 04 05 06 07| 10 11 12 13 14 15 16 17|
| ON OFF OFF SWRQ KBST META CTRL TOP| OFF SHFT KB5 KB4 KB3 KB2 KB1 KB0|
|_______________________________________|_______________________________________|
We really transmit:
.....idle.........|strt|data...................................|stop|
mrk mrk .... mrk spc mrk spc spc swrq kbst meta ctrl top mrk
|strt|data...................................|stop|idle.......
spc spc shft kb5 kb4 kb3 kb2 kb1 kb0 mrk mrk mrk...
Programming considerations
The serializer sends out two bytes for each keyboard strobe. The order of the
bits are:
(first byte)---------------------------¬(second byte)---------------------------
| 00 01 02 03 04 05 06 07| 10 11 12 13 14 15 16 17|
| ON OFF OFF SWRQ KBST META CTRL TOP| OFF SHFT KB5 KB4 KB3 KB2 KB1 KB0|
|_______________________________________|_______________________________________|
name comments
SWRQ If this bit is on, it means that this data is being sent in response to
a spacewar request to read the keyboard.
KBST If this bit is on, it means that this data is being sent because somebody
typed a key.
META The META key from the keyboard.
CTRL The CONTROL key from the keyboard.
TOP The TOP key from the keyboard.
SHFT The SHIFT key from the keyboard.
KB5-KB0 The keyboard data proper. These are not decoded ASCII, remember.
See the table below for the character codes for each key.
Note that the high order bit of the first byte is always on, and the high order
bit of the second byte is always off. This should make it possible to get back
in sync if somehow we forget which byte we were on.
It is possible for both SWRQ and KBST to be on in the case where a
spacewar request was issued, and a key was concurrently typed. However,
if both these bits are off, there is an error of some sort.
MICROSWITCH KEYBOARD CHARACTER CODES
0 1 2 3 4 5 6 7
00 --- a b c d e f g
10 h i j k l m n o
20 p q r s t u v w
30 x y z CR \ LF --- ---
40 SPC BRK ESC CAL CLR TAB FF VT
50 ( ) * + , - . /
60 0 1 2 3 4 5 6 7
70 8 9 : ; BS ALT --- ---
Signal definitions
Here is a list of the signals used in the keyboard serializers:
BHI Logical HIGH signal used whenever a TTL signal needs to be tied to HIGH.
BSEL Low means first byte selected (bucky bits), high means second byte
selected (data bits).
CTRL "Control" key from keyboard.
DONE Come true when the second byte has been loaded in the UART for
tranmission.
GND Logical LOW.
KB0-KB5 Data bits direct from the keyboard.
KBSTROBE Comes high when any key is pressed, and valid data appears on KBO0-5.
META "Meta" key from keyboard.
MRCLOCK MasteR CLOCK, runs at 16 times the baud rate.
SERRCV E5(20) TIP -1.60 0.01 RI UART TR1602 DATA5 B7
SERXMT E5(25) TO 1.60 -0.20 TRO UART TR1602 DATA5 B7
SHIFT "Shift" key from keyboard.
SPWRQST E3(14) TIP -0.80 0.02 4A 74LS157 LS157 DATA5 B3
SYNC When true, we have just loaded a character into the Transmitter
Holding Register, and better not switch the Byte Select or anything
silly like that.
THRE Transmitter Holding Register Empty, the UART is ready to have a character
loaded into it for transmission.
THRL Transmitter Holding Register Load.
When true, data is latched into the Transmitter Holding Register. On
the transition from true to false, the data is strobed into the
Register (if no character is currently being transmitted).
TOP "Top" key from the keyboard.
VCC Illogical HIGH, +5V, used to power the chips and current loop converter.
XMRQ Means that there is a request to transmit a character, either due
to a keyboard strobe or character received from the host.
prototype notes:
This is where some signals are found
name PC board loc BURNDY loc DEC cable
(note 1) (note 2) (note 3)
+5V A AA2
GND B AC2
KB0(lsb) D AS2
KB1 E AR2
KB2 F AP2
KB3 J AN2
KB4 H AM2
KB5(msb) G AL2
SHIFT K
TOP L
CTRL M
META N
STROBE b
note 2: The BURNDY loc refers to the pin on the 38 pin Burndy
connectors currently in use (5-Jun-79). Their function will
be replaced by the serializer, and thus they will be replaced.
This info is really only for hooking up the prototype.
note 3: The Prototype loc refers to the location on the DEC-style
wire-wrap panel on which the prototype serializer is
built. It will be flushed even sooner than the BURNDY
connectors (I hope).
note 4: Notice that I really mean lower-case "b" for STROBE on the
BURNDY connector.
note 6: Throughout this documentation, the data bits are numbered B0
to B5 (B0 is the least-signigicant bit). The Micro Switch
documentation numbers them B1 through B6. Beware when looking
at the Micro Switch documentation.