Parallel Port Signal Detail
Fig 10-2
Pin #
1
PC Port
Signal name
Driven by
(Source/Sink)
Pin #
1
Mini
1284-II "C"
Pin #
15
Notes
+1.0 as strobe
nStrobe,
HostClk,
nWrite
Host
(4.7k Ohm,OC)
Pulse width min 0.5us. Driven by 7405 inverting open collector buffers,
pulled to +5 volts via 4.7K resistors. Inverted (becomes Strobe) in Status
Input register on PC port.
Must be high until the
Peripheral sends
nAck and releases Busy.
Inline
devices may prevent this signal from being asserted and stop the flow of
data from the Host to the Peripheral by artificially asserting
Busy in any mode.
SPP:
nStrobe: Tells the printer that data
is available on
Data 0-7.
Negotiation phase:
HostClk: Tells the
printer that data is available on
Data 0-7
Nibble:
HostClk: Must remain high
Byte:
HostClk: Becomes
nAck from
Host to
Peripheral
ECP:
HostClk: Tells the printer that
data is available on
Data 0-7 in forward operation. Becomes
nAck from
Host to
Peripheral in reverse
EPP:
nWrite: Set high to allow
the Peripherial to drive the
data bus and read data,
low to assert data and write.
Inline devices must respond
quickly.
2
+0.0
Data D0 to
D7
Host
(2.6 mA, 24 mA, terminated at Periph by 1k Ohm to +5V)
Periph (in byte, ecp, & epp reverse modes)
2
6
Data 0 to
Data 7:
Carry 8 bits of data in all modes. Must be valid 0.5us before leading edge
of nStrobe. Exception: ECP has special RLE compression and multi channel
modes that send 7 bits of either repeat count (if bit 7 is 0) or channel
address (if bit 7 is 1 and not during Device ID) when HostAck is low on forward
data transfers or PeriphAck is low on reverse.
Orginally driven by 74LS374 octal latch, which can source 2.6 mA and sink
24 mA. If peripheral pulls these lines to ground, excessive current may easily
be sourced.
Protect by inserting
resisters inline with the data pins or by using
a buffer like
FcSemi
54AC245. Exception: Some printer
ports allow the driver chip to be tri-stated; PC, Base+2, bit 5 must be set
to enable bi-di.
Host must direct
Peripheral to drive data bus while in:
Byte Reverse
mode by setting HostBusy (pin 14, nAutoFeed) low, in
ECP Reverse mode
by setting nReverseRequest (pin 16-31, nInit) low , or in
EPP Reverse mode
be setting nWrite (pin 1, nStrobe) high.
Negotiation: Carries Extensibility request
value from
Host
Nibble: Not used
3
+0.1
3
7
4
+0.2
4
8
5
+0.3
5
9
6
+0.4
6
10
7
+0.5
7
11
8
+0.6
8
12
9
+0.7
9
13
10
+1.6 as ack
nAck,
PtrClk,
PeriphClk,
Intr
Periph
10
3
Inverted (becomes Ack) in Status Input register on PC port. Pulse width
approx 3us.
Inline devices must respond quickly. nAck is not
a NAck or Nack. It is a negitave logic Acknowledge rather than a Negitive
Acknowledge.
SPP:
nAck is asserted when
Busy goes off or after data is
accepted. May also pulse when
Peripheral
comes on line after power on, reset, or operater selects online mode.
Negotiation: set low with nError, PError and
Select high to acknowledge 1284 support after
Host sets nSelectIn high and nAutoFeed low. At
end of
negotiation, set high to indicate that
the Xflag(Select) and data available flags may be read.
Nibble:
PtrClk is a reverse nStrobe
from
Peripheral to
Host
Byte:
PtrClk is a reverse nStrobe
from
Peripheral to
Host
ECP:
PeriphClk is a reverse nStrobe
from
Peripheral to
Host
EPP:
Intr Interrupts the
Host on a positive edge.
Inline devices should
respond quickly.
11
+1.7 as Ready
Busy,
PtrBusy,
PeriphAck,
nWait
Periph
11
1
Inverted (becomes Ready) in Status Input register on PC port.
Indicates that the Peripheral is
Busy in
SPP,
negotiation,
Byte and
ECP forward
modes. Asserted during
reading of data,
printing, errors, and when offline. Host must not send further data untill
Busy is released.
SPP:
Busy: Released
before, during or after nAck.
Negotiation: not
used.
Nibble:
PtrBusy carrys bit 3 then bit 7 of the reverse data.
ECP reverse: as
PeriphAck, high for
data, low for command bytes.
EPP:
nWait indicates data available
for read by
Host in reverse direction and that
data has been read & may be released by the
Host in forward direction.
Inline devices
should respond quickly.
12
+1.5
PError,
AckDataReq,
nAckReverse,
Periph
12
5
SPP:
Out of Paper or
PError:
High when Peripheral has some problem with paper or feed path. nError is
always low when PError is high.
Negotiation: Set high with Select while nError low after
Host sets nSelectIn high to indicate 1284 support,
then follows nDataAvail(nError).
Nibble:
AckDataReq: Carrys bit 2 then
bit 6 of the reverse data.
Byte:
AckDataReq: Follows
nDataAvail(nError).
ECP:
nAckReverse acks nReverseRequest
allowing
Host to again drive data lines.
Inline devices must respond quickly.
EPP:
User defined 1: Mfgr specific
and not defined by 1284.
13
+1.4 as nSelect
Select,
Xflag,
Periph
13
2
Inverted (becomes OffLine) in Status Input register on PC port.
SPP:
Select: High when Peripheral is online.
Negotiation: Set high with PError while nError
low after
Host sets nSelectIn high to indicate
1284 support, then high if the requested mode is available, or low if not
(except for nibble mode which is implicetly accepted by responding to a request
for negotiation and by then setting Select low, rather than high) after Host
nStrobes the mode request byte to the Peripheral and sets nStrobe and nAutoFeed
high.
Nibble:
Xflag: Carrys bit 1 then bit
5 of the reverse data.
Byte:
Xflag: Not used after
negotiation
ECP:
Xflag: Not used after
negotiation
EPP:
User defined 3: Mfgr specific
and not defined by 1284.
For inline devices: No need to interrupt on change. If all 1284 modes
are supported, can be bypassed.
14
+2.1 as auto feed
nAutoFeed,
HostBusy,
HostAck,
nDStrb,
Host
(4.7k Ohm, OC)
14
17
Driven by 7405 inverting open collector buffers, pulled to +5 volts via
4.7K resistors. Inverted (becomes Auto Feed) in Control output register on
PC port.
SPP:
nAutoFeed or
nAutoFd, Mfgr specific.
Some printers will add Line Feeds to Carrage Returns when this line is low.
Also used as a ninth data, parity, or command/data control bit.
Negotiation: Set low nSelectIn high to request
an 1284 mode. Then set high after peripheral sets PtrClk(nAck) low to cause
Select to be set to Yes/No.
Nibble:
HostBusy set low to indicate
Host ready for each reverse nibble, then high to ack receipt.
Byte:
HostBusy set low to indicate
Host has released the data lines and is ready for each reverse nibble (idle
phase if Peripheral has no data to send), then high to ack receipt (or leave
idle phase after Peripheral pulses nAck low or exit Byte mode if nSelectIn
set low).
Inline devices must respond quickly.
ECP:
HostAck pulsed low to ack data
from Peripheral. Inidicates data or command byte in forward direction.
EPP:
nDStrb denotes a data cycle.
NC
External Oscillator
15
NC
Not used by the PC-Compatible Printer Interface. Not connected to the
PC Parallel Interface. (The Centronics Standard Parallel Interface defined
this lead as the External Oscillator signal.)
NC
NC
Peripheral Logic High,
Periph
18
36
Peripheral Logic High: Set high to indicate (subject to the provisions
of 8.3.5) that all other signals sourced by the peripheral are in a valid
state. Set low to indicate that the peripheral power is off or that
peripheral-driven interface signals are otherwise in an invalid state.
Peripheral manufacturers may, but are not required to, use this signal to
provide +5 V of power to an attached device. In any case, the peripheral
shall limit the short-circuit current to a maximum of 1.0 A and shall provide
circuitry to ensure a valid logic low level (as defined in 8.3.5) on this
signal when the peripheral power is off.
15
+1.3
nError or
nFault,
nDataAvail,
nPeriph
Request,
Periph
32
4
SPP:
nError or
nFault, Low when
Peripheral is offline / error / out of paper. Mfgr specific.
Negotiation: Set low with Select and PError
high after
Host sets nSelectIn high to indicate
1284 support then high if a reverse channel is available, or low
if not after Host nStrobes the mode request byte to the Peripheral and sets
nStrobe and nAutoFeed high.
Nibble:
nDataAvail set low to indicate
Peripheral has data ready to send to the host. Then used to send data bit
0 then 4.
Byte:
nDataAvail set low to indicate
Peripheral has data ready to send to the host.
ECP:
nPeriphRequest generates an interrupt
to the host in the forward or reverse directions. Permitted, not required.
EPP:
User defined 2, Mfgr specific
and not defined by 1284.
16
+2.2
nInit,
nReverse Request,
Host
(4.7k Ohm, OC)
31
14
Driven by 7405 inverting open collector buffers, pulled to +5 volts via
4.7K resistors. Double inverted from the PC.
SPP:
nInitialize or
nInit Assert
(low) to reset Peripheral. The IBM/EPSON spec added a requirement for more
than 50uS duration.
Negotiation:
Nibble:
nInit: Always high
Byte:
nInit: Always high
ECP:
nReverseRequest: Set low to allow
Peripheral to drive the data lines once the Peripheral has set Peripheral
Logic High.
Inline devices must respond quickly.
EPP:
nInit: Set low to terminate EPP
and return to SPP. Obviously must be high at end of negotiation for EPP
mode.
17
+2.3
nSelectIn,
IEEE 1284 Active,
nAStrb,
Host
(4.7k Ohm, OC)
36
16
Driven by 7405 inverting open collector buffers, pulled to +5 volts via
4.7K resistors.
SPP:
nSelect Input or
nSelectIn:
Assert to select Peripheral. This line, when low, enables data input into
the printer. When supported by the printer, details vary with the implementation.
In some cases, this line acts as a simple active low enable line. In others,
this signal enables ASCII DC1/DC3 control codes as printer Select/De-select
commands to enable or disable data input. Note that printer pin 33 has been
redefined as a signal ground return lead for this signal.
Negotiation:
IEEE 1284 Active: Set
high with nAutoFeed low to begin negotiation phase.
Nibble:
IEEE 1284 Active: Set high to
indicate that the bus direction is peripheral to host (via status lines).
Set low to terminate nibble mode and to set bus direction to host to
peripheral.
Byte:
IEEE 1284 Active: Set high to
indicate that the bus direction
may be peripheral to host.
Host must direct
Peripheral to drive data bus by setting
HostBusy low. Set low to terminate byte
mode.
ECP:
IEEE 1284 Active: Set high to
indicate that the bus direction
may be peripheral to host.
Host must direct
Peripheral to drive data bus by setting
nReverseRequest low. Set low by the host
to terminate ECP Mode and return the link to the Compatibility Mode.
EPP:
nAStrb: This signal is used to
denote an address cycle. It is active low.
NC
Host
NC
18
Host Logic High?
19
33
Ground Returns
20
24
21
25
22
26
23
27
24
28
25
29
26
30
27
31
28
20,21,23
29
19,22
18-35
30
32,34,35
NC
33
NC
Signal ground connection, added for the return lead of the Select In
signal. (The Centronics Standard Parallel Interface defined this lead as
the Light Detect signal.)
NC
34
NC
Not used by the PC-Compatible Printer Interface. (The Centronics Standard
Parallel Interface defined this lead as the Line Count signal.)
NC
35
NC
This high going signal provides a positive dc level indication that the
printer is connected and powered on. It is a new signal definition; however,
it is not connected to the PC Parallel Interface. (The Centronics Standard
Parallel Interface defined this lead as the Line Count Return signal.)
See also:
Interested:
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