unidirectional / bidirectional

[evildragon]

Ok, I'm very confused right now, and have tried finding information, even with a book that went to my IBM model 25 PS/2.. No information whatsoever..

IBMMuseum told me ALL PS/2's are bi-directional parallel port equipped, including the 25's.

But, on the three 25's I have used and seen, the 8086 based ones, they are uni-directional.

How have I come to this conclusion? Because all zip drives and external HD's say "Unidirectional Port Detected", and run in nibble mode. Even mbrutmans utility says so too.

So um, malfunction, across THREE 25's? They are of different age across their lifetime.

[Yzzerdd]

Hookup a bi-directional printer to the port and print a page. If it prints on both sweeps(to and fro) it is bidirectional, if only on one sweep then it is uni. Check the BIOS settings(did the PS/2 have a CMOS/BIOS?) and the dip swiches as well.

I couldn't tell ya for sure if the PS/2 is uni or bi, but the printer thing should work. I'd figure it is bidirectional though, as all my 8086s and my 8088 are.

--Ryan

[IBMMuseum]

...IBMMuseum told me ALL PS/2's are bi-directional parallel port equipped, including the 25's...

I've been wrong before, so don't treat it as gospel. Your empirical evidence says otherwise. The Model 25 and Model 30s (at least the 8086-based version) were treated as the low-end (inferior video to VGA level, shipping with 720Kb drives instead of 1.44Mb) of PS/2s.

[mbbrutman]

Hookup a bi-directional printer to the port and print a page. If it prints on both sweeps(to and fro) it is bidirectional, if only on one sweep then it is uni. Check the BIOS settings(did the PS/2 have a CMOS/BIOS?) and the dip swiches as well.

I couldn't tell ya for sure if the PS/2 is uni or bi, but the printer thing should work. I'd figure it is bidirectional though, as all my 8086s and my 8088 are.

--Ryan

Whether the printer head prints both ways or not is completely independent of whether the parallel port can operation in nibble mode or bi-directional mode when reading data in from an external device.

Unless you were joking, then I'm just slow ... ;-0

[Yzzerdd]

I was joking...yes...joking.

OK, but what if the printer is uni/bi capable and you have the parallel port set to "unidirectional"? Wouldnt the printer then only print in uni regardless of it's capability? Or it that setting just for external devices like HDDs and floppy drives?

--Ryan

[mbbrutman]

Uh, now I don't think you are joking anymore ...

Bidirectional/standard when talking about the port talk about it's ability to read data from an external device.

Bidirectional when talking about a print-head just means if the printer is smart enough to print when the head is moving both left and right, as opposed to being able to print when the print head is only moving in one direction.

The two have nothing to do with each other.

[MikeS]

LOL, intentional or not; gotta remember that one for the next time I get that question...

m

[strollin]

In order to support bi-directional printing the cable needs to support it also. I think some of the cheaper parallel cables out there omit certain lines that are required for bi-directional printing.

As stated in an earlier post, bi-directional supports receiving data from the external device. This is how, for instance, the printer can provide data regarding it's paper and ink status.

[mbbrutman]

"Bi-directional printing" refers to the ability of the print head to make characters when moving either to the left or to the right. Older 'uni-directional' printers have less intelligence, and can only print when the head is moving in one direction or the other. (Usually to the right.)

Bi-directional parallel ports are parallel ports that can read and write 8 bits at a time on the data lines. This is an improvement over a standard serial port, which can write 8 bits at a time to a device but only read 4 bits at a time from the device. (And the four bits is read using control and handshaking lines, not the actually data lines.)

The ability of a printer to report status back to the host computer depends on all of the correct wires being in the cable, nothing more. ie: You can have a printer report ink status back even on a standard parallel port. If you can run Ethernet or a Zip drive on it, you certainly can report ink status.

Having a bi-directional printer port just makes the process much faster. Most printers probably don't have the firmware to send status using the control lines of a normal parallel port, hence the requirement for a 'bi-directional' parallel port. But really they are two different issues - the printer could and sent status back using the 4 control lines that make up the 'nibble mode' support, but the manufacturers chose not to build that logic in since most machines have at least a bi-directional parallel port.

[IBMMuseum]

...Check the BIOS settings(did the PS/2 have a CMOS/BIOS?) and the dip swiches as well.

I couldn't tell ya for sure if the PS/2 is uni or bi...

286+ PS/2s had CMOS storage for settings, but either used a "Starter Diskette" (ISA models), "Reference Diskette" (early microchannel models), or "System Partition" (later microchannel models) to configure the system. I know for the ISA Model 25 286 / Model 30 286 (same planar) there was a single bidirectional port, because it has a 16C451 chip (meaning one bi-di parallel, one 16450 UART w/o FIFO). All microchannel systems have at least one bi-di port (and UARTs are 16550-compatible with FIFO) on the planar.

[MikeS]

<snip>
Bi-directional parallel ports are parallel ports that can read and write 8 bits at a time on the data lines. This is an improvement over a standard serial port, which can write 8 bits at a time to a device but only read 4 bits at a time from the device.
<snip>

Hmmm, I haven't run across any standard serial ports like that; is there an RS-xxx, EIA or IEEE spec somewhere?

;-)

[mbbrutman]

Typo duly noted. :-)