27 Feb
2013
27 Feb
'13
5:02 a.m.
Philipp Hachtmann wrote:
Might be. As I remember the Signetics TTLs had different pinouts compared
to the
TI 74xx series. And now compare the 7438 and the 7439's pinouts: The 7439
doesn't only have a better drive performance (60mA) than the 7438 - its gates
are also in the "Signetics order". I have seen 7439 on a DEC board. Soldered
where other boards have 8881.
Ah, really? Of course NOT! The original driver
was DEC8881 which is a
relabelled 7439.
I think it must have been more than just relablled; the original 8881 was from a
non-7400 Signetics family whose name escapes me at the moment, and may have been
binned. The characteristics were somewhat similar to the 7439
but not identical.
The edge rate and maximum leakage current are different. They may have binned
7439 parts that met the Omnibus specs, but there's no expectation that an
ordinary 7439 (or 7438) would.
That discussion is a completely theoretical thing. Let's fix some facts:
- I have built working hardware
- I use the "wrong" drivers.
- The real drivers are not available anymore.
===>> I find it's a good compromise to use the 7438 as a replacement.
I could easily change the design to use real 8881 drivers. For me it would be
fine - I still have some of them. But I want to supply the board to other people
who might not have those chips. I want to use current hardware for that current
project.
The spare 8881 should be kept for repairs on old hardware.
And by the way I'm always open to suggestions for improvements. But only
complaining is not exactly that productive.
The 7438 I used has very similar specifications.
As the DEC8881 and the 7438 are not available, I have chosen the 7438.
And it does not make a big difference if the machine is more or less loaded.
The load on the drivers is always the same. It's an open collector bus.
The load isn't the same. Unlike modern CMOS parts, the receiver inputs are not
especially high-impedance. The receivers do present both DC and AC loading on
the bus lines. More receivers presents more loading. The AC loading is often
more of an issue than the DC. do NOT work
reliably, because in a heavily loaded bus asserted
signals don't always dip as
low as the threshold of normal TTL parts, or don't
do so in a timely manner.
On a lightly loaded system you can get away with using ordinary TTL receivers
and OC buffers.